Ergonomics is the discipline that is responsible for the design of workplaces, tools and tasks, so that they match the physiological, anatomical, psychological characteristics and capabilities of the workers who will be involved.[1] It seeks the optimization of the three elements of the system (human-machine-environment), for which it develops methods of the person, the technique and the organization.[2].
Derived from the Greek έργον (ergon, 'work') and νόμος (nomos, 'law'), the term denotes the science of work. It is a systematically oriented discipline, which is now applied to all aspects of human activity with machines.
The Council of the International Ergonomics Association (IEA),[3] which brings together all scientific societies worldwide, established the following definition since 2000, which covers the interdisciplinarity that underpins this discipline:
History and etymology
The foundations of the science of ergonomics appear to have been established within the context of the culture of Ancient Greece. Much of the evidence indicates that Greek civilization in the century BC. C. uses principles of ergonomics in the design of tools in its workplaces.
It can be found in the description that Hippocrates gave of the design of tools and the way in which the workplace should be organized for a surgeon (see Marmaras, Poulakakis and Papakostopoulos, 1999).[4] It is also true that there are archaeological records of the Egyptian dynasties, where it is observed that they manufactured tools, household equipment, among others that illustrate the application of ergonomic principles. Therefore, it is questionable whether the claim by Marmaras, et al., about the origin of ergonomics, can be justified (IG Okorji, 2009). The term ergonomics, from the Greek Έργον, meaning "work", and Νόμος, meaning "natural laws", entered the modern lexicon, when Wojciech Jastrzębowski used the word in his 1857 article «Rys ergonomji czyli Nauki o pracy, opartej na prawdach poczerpniętych z Nauki Przyrody» (“The scheme of ergonomics, the science of work, based on the observations of Natural Sciences”).
Later, in the 19th century, Frederick Winslow Taylor pioneered Scientific Labor Administration (Taylorism), a method that proposes how to find the optimal method to carry out a given task. Taylor discovered that he could, for example, triple the amount of coal that workers were shoveling, gradually enlarging the size and reducing the weight of the coal shovels until the fastest rate of shoveling was achieved. Frank and Lillian Gilbreth expanded on Taylor's methods in the 1900s to develop "The Study of Time and Motion." Their goal was to improve efficiency by eliminating unnecessary steps. By applying this approach, the Gilbreths reduced the number of moves in masonry from 18 to 4.5, allowing bricklayers to increase their productivity from 120 to 350 bricks per hour.
Ergonomic Furniture Design
Introduction
Ergonomics is the discipline that is responsible for the design of workplaces, tools and tasks, so that they match the physiological, anatomical, psychological characteristics and capabilities of the workers who will be involved.[1] It seeks the optimization of the three elements of the system (human-machine-environment), for which it develops methods of the person, the technique and the organization.[2].
Derived from the Greek έργον (ergon, 'work') and νόμος (nomos, 'law'), the term denotes the science of work. It is a systematically oriented discipline, which is now applied to all aspects of human activity with machines.
The Council of the International Ergonomics Association (IEA),[3] which brings together all scientific societies worldwide, established the following definition since 2000, which covers the interdisciplinarity that underpins this discipline:
History and etymology
The foundations of the science of ergonomics appear to have been established within the context of the culture of Ancient Greece. Much of the evidence indicates that Greek civilization in the century BC. C. uses principles of ergonomics in the design of tools in its workplaces.
It can be found in the description that Hippocrates gave of the design of tools and the way in which the workplace should be organized for a surgeon (see Marmaras, Poulakakis and Papakostopoulos, 1999).[4] It is also true that there are archaeological records of the Egyptian dynasties, where it is observed that they manufactured tools, household equipment, among others that illustrate the application of ergonomic principles. Therefore, it is questionable whether the claim by Marmaras, et al., about the origin of ergonomics, can be justified (IG Okorji, 2009). The term ergonomics, from the Greek Έργον, meaning "work", and Νόμος, meaning "natural laws", entered the modern lexicon, when Wojciech Jastrzębowski used the word in his 1857 article «Rys ergonomji czyli Nauki o pracy, opartej na prawdach poczerpniętych z Nauki Przyrody» (“The scheme of ergonomics, the science of work, based on the observations of Natural Sciences”).
The Second World War marked the development of new weapons and complex machines, and new demands on the cognition of operators also arose. The machine operator's decision making, attention, situational awareness, and hand-eye coordination became the key to the success or failure of a task. It was observed that fully operational planes, flown by trained pilots, suffered air accidents. In 1943, Alphonse Chapanis, a lieutenant in the US Army, showed that this so-called "pilot error" could be greatly reduced when controls were replaced by more logical and less confusing layouts in the aircraft cockpit.
In the decades since the war, ergonomics has continued to flourish and diversify. The space age has created new human factors problems, such as weightlessness and G-forces. How far could the human body tolerate these environments in outer space, and what effects would they have on the mind and body? The dawn of the information age has translated into the field of ergonomics as human-computer interaction (HCI).
The coining of long-term ergonomics, however, is widely attributed to British psychologist Hywel Murrell"), at the 1949 meeting at the Navy Office in the United Kingdom, which led to the founding of the Spanish Ergonomics Society[5] created in the late 1980s.[6] He used it to encompass the studies in which they had participated.
Overview
The ergonomist's practice must have a broad understanding of the complete panorama of the discipline, taking into account the physical, cognitive, social, organizational, environmental, among other relevant factors. Ergonomists may work in one or several particular economic sectors or application domains. These application domains are not mutually exclusive and are constantly evolving. Some new ones are created, the old ones take on new perspectives. Within the discipline, domains of specialization represent deep competencies in specific human attributes or characteristics of human interaction.
Ergonomics, as a multidisciplinary science, brings together professionals from various areas: engineers, designers, doctors, nurses, kinesiologists, occupational therapists, psychologists, human resources specialists, architects, and many others.
Domains of ergonomics
Cognitive ergonomics
Cognitive ergonomics (or as it is also called 'cognitive') is interested in how and to what extent mental processes such as perception, memory, reasoning and motor response affect the interactions between human beings and the other elements of a system. Such as the ergonomic triad (human-machine-environment).
Issues that are relevant to you include: mental workload, mental fatigue, decision making, expert functioning, human-computer interaction (e.g., Fitts' law), human reliability, job stress, training and training, to the extent that these factors can be related to the design of human-system interaction.
Information in the everyday sense of the word is the knowledge received about a specific fact. In the technical sense, information is the reduction of uncertainty regarding that fact. Information Theory is measured in bits of information, where a bit is the amount of information required to decide between two equally probable alternatives.
Numerous models have been developed to explain how people process information. Many of these models consist of black boxes that represent the different stages of processing. The figure presents a generic model consisting of four important stages or components; perception, decision, and response selection, response execution, memory and attention resources distributed in the different stages. The decision-making component, combined with working memory and long-term memory, can be considered the central processing unit, while the sensory store is a transitive memory located in the input stage. (Wickens, Giordon and Liu, 1997).
Physical ergonomics
Physical ergonomics deals with the anatomical, anthropometric, physiological and biomechanical characteristics of the user, as they relate to physical activity.
Its most relevant topics include work postures, overexertion, manual handling of materials, repetitive movements, muscle-tendon injuries (MTL) of occupational origin, job design, occupational health and safety.
Visual ergonomics
Visual ergonomics is a concept that is related to the search for the best visual performance of workers in the office. Visual ergonomics, as a domain within the branch of ergonomics, focuses on basic recommendations that must be met by those people who, in the performance of their activity, spend long hours working with screens and monitors. These recommendations include aspects such as the separation between the user and the screen, the need to repeatedly separate the view from the monitor and focus it on a distant point, or the benefits of repeated blinking that hydrates the corneal layers of the eye.
Organizational ergonomics
Organizational ergonomics or macroergonomics,[7] is concerned with the optimization of socio-technical systems, including their organizational structures, policies and processes.
Relevant topics to this domain are psychosocial factors of work, communication, human resources management, task design, design of working hours and shift work, teamwork, participatory design, community ergonomics, cooperative work, new work paradigms, virtual organizations, teleworking and quality assurance.
Areas of ergonomics
Product design
Ergonomics is a very important factor when designing a product, since it will ensure its usability. By developing a product with the support of ergonomics you achieve:
In order to design an effective ergonomic product, it is very important to carry out a prior study of the target audience, as well as create prototypes that allow its design to be validated.[8].
Job design
Its application to the workplace has traditionally been the most frequent; although it is also very present in product design and in related areas such as home activities, leisure or sports. The design and adaptation of products and environments for people with functional limitations (elderly people, people with disabilities, etc.) is also another field of action of ergonomics.
Any ergonomic design must consider the objectives of the organization, taking into account aspects such as production "Production (economy)"), efficiency, productivity, profitability, innovation and quality of service.
Product ergonomics
The objective of this area is consumers, users and the characteristics of the context in which the product is used. The study of ergonomic factors in products seeks to create or adapt products and elements for everyday or specific use so that they adapt to the characteristics of the people who are going to use them. That is, ergonomics is transversal, but not to all products, but to the users of said product.[9].
The ergonomic design of products tries to ensure that they are: efficient in their use, safe, that contribute to improving productivity, without generating pathologies in humans, that in the configuration of their shape indicate their mode of use and characteristics of use.
To achieve these objectives, ergonomics uses different techniques in the planning, design and evaluation phases. Some of these techniques are: functional, biomechanical analysis, anthropometric data of the user segment targeted by the design, cognitive ergonomics and analysis of the physiological behaviors of the body segments involved in the use of the product.
Strictly speaking, no object is ergonomic by itself, since its quality depends on the interaction with the individual. The characteristics of the object are not enough.
Most people experience some degree of physical limitation at some point in their lives, such as broken bones, sprained wrists, pregnancy, or aging. Others may live with a limitation or impediment every day. When considering product design, designers can recognize the special needs of different users, including people with disabilities.
Issues related to accessibility for people with disabilities are becoming more common, and employers may be required to make accommodations for these people in workplaces and other public spaces.
The Americans with Disabilities Act[10] (ADA) does not specify requirements for its office furniture to accommodate people with disabilities. Therefore, it is incorrect to state that office furniture and products are "ADA compliant."
Designing with all people in mind is a principle known as universal design, which is important to take into account when designing products. In this section we will look at some universal design guidelines.
For common wheelchairs, the seat height is 18" to 22", and the overall width is 22.5"-27.0". These values can assist in furniture design, adjusting work surface height, and ease of wheelchair access. People who work seated in a wheelchair and may require considerations regarding reach in the desk work area.[11].
Some recommendations, regarding what dimensions are appropriate for choosing a wheelchair?; The first thing would be to sit in the wheelchair, adopt a correct posture and proceed to take the dimensions:.
Other dimensions to take into account:.
Important data for a wheelchair; chair sheet:.
A. Seat width
B. Backrest width
C. Backrest-seat distance
D. Footrest-seat distance
E. Overall width
F. Total length.
Some people, when they suffer an accident or a temporary disability, need the help of walking devices, such as crutches, canes or walkers. A minimum width of 36" from wall to wall in a hallway or workplace is necessary to facilitate mobility for these individuals. Studies have shown that 48" is the preferable hallway width for people who use crutches, canes or walkers. It is also important to keep these areas free of obstacles to avoid the risk of a fall and further injury.[12].
Objects that hinder the proper use and maneuverability of pedestrians must be moved and accommodated in a suitable location other than the hallways.
Knobs, handles and controls on products should be easy to use and intuitive. Some people are unable to firmly grasp some types of doorknobs, while others may have prosthetic hands, which make it impossible to easily open doors. An L-shaped handle is preferable to a round one, as it allows access to a greater number of users.
Ergonomic workplace design
The Gilbreth couple introduced the design of manual work through the study of movements, in what is known as Therbligs,[13] and the twenty-one principles of economy of movements. The principles are classified into three basic groups:
Something very important is that the principles are based on anatomical, biomechanical and physiological factors of the human body. These form the scientific basis of ergonomics and work design. The traditional principles of motion economy have been expanded and are now known as principles and guidance for work design:.
Manual work design
The human body is capable of producing movements due to a complex system of muscles and bones, called the musculoskeletal system. There are three types of muscles in the human body: bony or striated muscles, attached to the bone; cardiac muscle, which is found in the heart, and soft muscle, such as that of the internal organs and the walls of the capillaries.[14] It is necessary to know the conformation of the skeletal-muscular system to delve into the analysis of manual work and develop applications that allow reducing the ergonomic risks present in the workplace.
The property of the muscle that allows it to be used with a considerable decrease in muscle strength is known as strength-length relationship. A task that requires considerable force must be performed in an optimal position. For example, the neutral or straight position provides the strongest grip for wrist movements. In elbow flexion, the best position would be with the elbow bent at a little more than 90°. When flexing the soles (such as when pressing a pedal), again the optimal position is a little more than 90°.[15].
The force is sufficient only to move the mass of one segment of the body. This property is known as the force-velocity relationship and is especially important when heavy manual work is involved.[15].
Workstations must allow operators to leave the part in the delivery area while their hands are in motion to pick up another component or tool and start a new cycle.[15].
The capacity of human strength depends on three important factors:
There are three types of muscular effort, defined primarily by the way they are measured. The muscular efforts that result in body movements are the result of a dynamic force. In the case where the movement of the body is restricted, an isometric or static force is obtained. A third type of muscular strength capacity, psychophysics, has been defined for situations in which a demand for strength is required for a prolonged time [15].
Strength in muscles is directly proportional to muscle size, as defined by cross-sectional area (87 psi (60N/cm²) for both men and women.) (Ikai and Fukunaga, 1968). For example, in heavy lifts the leg and trunk muscles should be used, and not weaker muscles.
Muscle fatigue is a very important criterion, but very little used in the adequate design of tasks for the human operator. The human body and muscle tissue rely on two primary types of energy sources, aerobic and anaerobic.
Since anaerobic metabolism can supply energy for only a short period, the oxygen delivered to the muscle fibers via peripheral blood flow becomes critical in determining how long muscle contractions will last. For this reason, any activity that requires the use of force must be estimated at 15% below the maximum force, in order not to completely fatigue the muscle tissues and exhaust the operator. This relationship can be modeled by:
Design of workstations, tools and equipment
Methods Engineering recognizes these concepts by managing to adapt and adjust them to the operator as ergonomics. This approach helps achieve greater production and efficiency in operations and lower injury rates for operators.
The primary guideline is to design the workplace[15] to fit the majority of individuals in terms of the structural size of the human body. The science of measuring the human body is known as anthropometry, which uses caliper-type devices to determine structural dimensions, such as height, forearm length, and others.
Designing for extremes implies that a specific characteristic is a limiting factor in determining the maximum and minimum value of a population variable that will be adjusted, for example, gaps, such as a door or the entrance to a storage tank, should be designed for the maximum case, that is, for the height or shoulder width corresponding to the 95th percentile. This way 95% of men and almost all women will be able to pass through the gap. The reach for things like a brake pedal or a control knob is designed for the minimum individual, that is, for legs or arms of women in the 5th percentile, so 95% of women and almost all men will have a greater reach and will be able to activate the pedal or control.
Design to fit is generally used for equipment or facilities that must fit a wide variety of individuals. Chairs, tables, desks, vehicle seats, a gear shift, and tool holders are devices that fit a population of workers between the 5th percentile for women and the 95th percentile for men. It is obvious that designing to fit is the most convenient method of design, but there is a trade-off with the cost of implementation.
Design for average is the least expensive but least preferred approach. Although there is no individual with all average dimensions, there are certain situations in which it would be impractical or too costly to include adjustment possibilities for all characteristics. It is useful, practical and cost-effective to build a one-to-one model of the equipment or facility being designed and have users evaluate it.
The height of the work surface (with the worker either sitting or standing) should be determined by a comfortable working posture for the operator. In general, this means that the forearms are in the natural downward position and the elbows are flexed at 90°, so that the arm is parallel to the ground. The elbow height becomes the proper operating or work surface height. If it is too high, the forearms shrink and cause shoulder fatigue, if it is too low, the neck or back bends and causes shoulder fatigue.
There are exceptions to this first principle. For heavy assembly with lifting of heavy parts, it is more advantageous to lower the work surface by up to 20 cm, to take advantage of the stronger trunk muscles. For fine assembly that includes small visual details, it is more advantageous to raise the work surface by 20 cm, to bring the details closer to the optimal 15° line of sight. Alternatively, it is perhaps better to tilt the surface around 15°, this way both principles are satisfied. However, the rounded parts have a tendency to roll off the surface.
Ergonomics in Spain
Contenido
La ergonomía ha cobrado relevancia en España en los últimos años, especialmente en el contexto laboral y académico. A continuación, se presentan algunos aspectos clave sobre este tema.
Rules and Regulations
Spain has various regulations that regulate ergonomics in the workplace. Some of the most important are:
Ergonomics is applied in various sectors, such as:.
Spanish universities offer programs and courses on ergonomics and related disciplines, such as work engineering and work psychology.") This is essential to train professionals in the implementation of ergonomic solutions. You can find more details about academic programs in the Ministry of Universities.
There are various institutions and research centers in Spain dedicated to the study of ergonomics. The National Institute of Safety and Health at Work (INSST) is one of them.
Ergonomics in Spain is an expanding field that seeks to improve working conditions and the quality of life of workers, adapting to changes in the contemporary work environment.
References
[1] ↑ Vern, Putz-anderson (1992). Cumulative trauma disorders: A manual for musculoskeletal diseases of the upper limbs. London: Taylor & Francis.
[6] ↑ «Asociación Española de Ergonomía». www.ergonomos.es. Consultado el 20 de septiembre de 2024.: http://www.ergonomos.es
[7] ↑ Montero Martínez, Ricardo (2000). Un paso hacia el futuro: el desarrollo de la Macroergonomía. España: Factores Humanos, 23.
[8] ↑ 3DALIA (15 de marzo de 2021). «Diseño ergonómico. La ergonomía en el diseño de producto». Consultado el 11 de marzo de 2022.: https://3dalia.com/diseno-ergonomico/
[10] ↑ ADA’s Accessibility Guidelines for Buildings and Facilities (ADAAG), «ADA’s Accessibility Guidelines for Buildings and Facilities (ADAAG)» (en inglés). Consultado el 8 de agosto de 2011.: https://www.ada.gov
[11] ↑ Tilley, Alvin R. & Henry Dreyfuss Associates (1993, 2002), The Measure of Man & Woman: Human Factors in Design: A human factors design manual.
[12] ↑ Openshaw, Scott. Taylor, Erin: Ergonomics and Design: A Reference Guide, página 36. Allsteel Inc, 2006.
[13] ↑ Niebel, Benjamin W. Freivalds, Andris: Ingeniería Industrial; Métodos, estándares y diseño del trabajo, pp. 148-150. The McGraw-Hill companies, Inc, 2005.
[14] ↑ Drake, Richard L., A. Wayne Vogl y Adam W. M. Mitchell: Gray's anatomy for students, Churchill Livingstone; 2 edition, Feb 11, 2009.
[15] ↑ a b c d e f g h Niebel, Benjamin W. Freivalds, Andris: Ingeniería Industrial; Métodos, estándares y diseño del trabajo. The McGraw-Hill companies, Inc, 2005, 11 Edición.
[16] ↑ a b Freiwald, Jürgen et al. En forma mediante el entrenamiento muscular, página 21. Editorial Paidotribo, 2002.
[17] ↑ Conglenton, J. J. (1983). The design and evaluation of the neutral posture chair, (tesis doctoral). Lubbock, TX: Texas Tech University.
[20] ↑ «BOE-A-1997-1853 Real Decreto 39/1997, de 17 de enero, por el que se aprueba el Reglamento de los Servicios de Prevención.». www.boe.es. Consultado el 20 de septiembre de 2024.: https://www.boe.es/buscar/act.php?id=BOE-A-1997-1853
Later, in the 19th century, Frederick Winslow Taylor pioneered Scientific Labor Administration (Taylorism), a method that proposes how to find the optimal method to carry out a given task. Taylor discovered that he could, for example, triple the amount of coal that workers were shoveling, gradually enlarging the size and reducing the weight of the coal shovels until the fastest rate of shoveling was achieved. Frank and Lillian Gilbreth expanded on Taylor's methods in the 1900s to develop "The Study of Time and Motion." Their goal was to improve efficiency by eliminating unnecessary steps. By applying this approach, the Gilbreths reduced the number of moves in masonry from 18 to 4.5, allowing bricklayers to increase their productivity from 120 to 350 bricks per hour.
The Second World War marked the development of new weapons and complex machines, and new demands on the cognition of operators also arose. The machine operator's decision making, attention, situational awareness, and hand-eye coordination became the key to the success or failure of a task. It was observed that fully operational planes, flown by trained pilots, suffered air accidents. In 1943, Alphonse Chapanis, a lieutenant in the US Army, showed that this so-called "pilot error" could be greatly reduced when controls were replaced by more logical and less confusing layouts in the aircraft cockpit.
In the decades since the war, ergonomics has continued to flourish and diversify. The space age has created new human factors problems, such as weightlessness and G-forces. How far could the human body tolerate these environments in outer space, and what effects would they have on the mind and body? The dawn of the information age has translated into the field of ergonomics as human-computer interaction (HCI).
The coining of long-term ergonomics, however, is widely attributed to British psychologist Hywel Murrell"), at the 1949 meeting at the Navy Office in the United Kingdom, which led to the founding of the Spanish Ergonomics Society[5] created in the late 1980s.[6] He used it to encompass the studies in which they had participated.
Overview
The ergonomist's practice must have a broad understanding of the complete panorama of the discipline, taking into account the physical, cognitive, social, organizational, environmental, among other relevant factors. Ergonomists may work in one or several particular economic sectors or application domains. These application domains are not mutually exclusive and are constantly evolving. Some new ones are created, the old ones take on new perspectives. Within the discipline, domains of specialization represent deep competencies in specific human attributes or characteristics of human interaction.
Ergonomics, as a multidisciplinary science, brings together professionals from various areas: engineers, designers, doctors, nurses, kinesiologists, occupational therapists, psychologists, human resources specialists, architects, and many others.
Domains of ergonomics
Cognitive ergonomics
Cognitive ergonomics (or as it is also called 'cognitive') is interested in how and to what extent mental processes such as perception, memory, reasoning and motor response affect the interactions between human beings and the other elements of a system. Such as the ergonomic triad (human-machine-environment).
Issues that are relevant to you include: mental workload, mental fatigue, decision making, expert functioning, human-computer interaction (e.g., Fitts' law), human reliability, job stress, training and training, to the extent that these factors can be related to the design of human-system interaction.
Information in the everyday sense of the word is the knowledge received about a specific fact. In the technical sense, information is the reduction of uncertainty regarding that fact. Information Theory is measured in bits of information, where a bit is the amount of information required to decide between two equally probable alternatives.
Numerous models have been developed to explain how people process information. Many of these models consist of black boxes that represent the different stages of processing. The figure presents a generic model consisting of four important stages or components; perception, decision, and response selection, response execution, memory and attention resources distributed in the different stages. The decision-making component, combined with working memory and long-term memory, can be considered the central processing unit, while the sensory store is a transitive memory located in the input stage. (Wickens, Giordon and Liu, 1997).
Physical ergonomics
Physical ergonomics deals with the anatomical, anthropometric, physiological and biomechanical characteristics of the user, as they relate to physical activity.
Its most relevant topics include work postures, overexertion, manual handling of materials, repetitive movements, muscle-tendon injuries (MTL) of occupational origin, job design, occupational health and safety.
Visual ergonomics
Visual ergonomics is a concept that is related to the search for the best visual performance of workers in the office. Visual ergonomics, as a domain within the branch of ergonomics, focuses on basic recommendations that must be met by those people who, in the performance of their activity, spend long hours working with screens and monitors. These recommendations include aspects such as the separation between the user and the screen, the need to repeatedly separate the view from the monitor and focus it on a distant point, or the benefits of repeated blinking that hydrates the corneal layers of the eye.
Organizational ergonomics
Organizational ergonomics or macroergonomics,[7] is concerned with the optimization of socio-technical systems, including their organizational structures, policies and processes.
Relevant topics to this domain are psychosocial factors of work, communication, human resources management, task design, design of working hours and shift work, teamwork, participatory design, community ergonomics, cooperative work, new work paradigms, virtual organizations, teleworking and quality assurance.
Areas of ergonomics
Product design
Ergonomics is a very important factor when designing a product, since it will ensure its usability. By developing a product with the support of ergonomics you achieve:
In order to design an effective ergonomic product, it is very important to carry out a prior study of the target audience, as well as create prototypes that allow its design to be validated.[8].
Job design
Its application to the workplace has traditionally been the most frequent; although it is also very present in product design and in related areas such as home activities, leisure or sports. The design and adaptation of products and environments for people with functional limitations (elderly people, people with disabilities, etc.) is also another field of action of ergonomics.
Any ergonomic design must consider the objectives of the organization, taking into account aspects such as production "Production (economy)"), efficiency, productivity, profitability, innovation and quality of service.
Product ergonomics
The objective of this area is consumers, users and the characteristics of the context in which the product is used. The study of ergonomic factors in products seeks to create or adapt products and elements for everyday or specific use so that they adapt to the characteristics of the people who are going to use them. That is, ergonomics is transversal, but not to all products, but to the users of said product.[9].
The ergonomic design of products tries to ensure that they are: efficient in their use, safe, that contribute to improving productivity, without generating pathologies in humans, that in the configuration of their shape indicate their mode of use and characteristics of use.
To achieve these objectives, ergonomics uses different techniques in the planning, design and evaluation phases. Some of these techniques are: functional, biomechanical analysis, anthropometric data of the user segment targeted by the design, cognitive ergonomics and analysis of the physiological behaviors of the body segments involved in the use of the product.
Strictly speaking, no object is ergonomic by itself, since its quality depends on the interaction with the individual. The characteristics of the object are not enough.
Most people experience some degree of physical limitation at some point in their lives, such as broken bones, sprained wrists, pregnancy, or aging. Others may live with a limitation or impediment every day. When considering product design, designers can recognize the special needs of different users, including people with disabilities.
Issues related to accessibility for people with disabilities are becoming more common, and employers may be required to make accommodations for these people in workplaces and other public spaces.
The Americans with Disabilities Act[10] (ADA) does not specify requirements for its office furniture to accommodate people with disabilities. Therefore, it is incorrect to state that office furniture and products are "ADA compliant."
Designing with all people in mind is a principle known as universal design, which is important to take into account when designing products. In this section we will look at some universal design guidelines.
For common wheelchairs, the seat height is 18" to 22", and the overall width is 22.5"-27.0". These values can assist in furniture design, adjusting work surface height, and ease of wheelchair access. People who work seated in a wheelchair and may require considerations regarding reach in the desk work area.[11].
Some recommendations, regarding what dimensions are appropriate for choosing a wheelchair?; The first thing would be to sit in the wheelchair, adopt a correct posture and proceed to take the dimensions:.
Other dimensions to take into account:.
Important data for a wheelchair; chair sheet:.
A. Seat width
B. Backrest width
C. Backrest-seat distance
D. Footrest-seat distance
E. Overall width
F. Total length.
Some people, when they suffer an accident or a temporary disability, need the help of walking devices, such as crutches, canes or walkers. A minimum width of 36" from wall to wall in a hallway or workplace is necessary to facilitate mobility for these individuals. Studies have shown that 48" is the preferable hallway width for people who use crutches, canes or walkers. It is also important to keep these areas free of obstacles to avoid the risk of a fall and further injury.[12].
Objects that hinder the proper use and maneuverability of pedestrians must be moved and accommodated in a suitable location other than the hallways.
Knobs, handles and controls on products should be easy to use and intuitive. Some people are unable to firmly grasp some types of doorknobs, while others may have prosthetic hands, which make it impossible to easily open doors. An L-shaped handle is preferable to a round one, as it allows access to a greater number of users.
Ergonomic workplace design
The Gilbreth couple introduced the design of manual work through the study of movements, in what is known as Therbligs,[13] and the twenty-one principles of economy of movements. The principles are classified into three basic groups:
Something very important is that the principles are based on anatomical, biomechanical and physiological factors of the human body. These form the scientific basis of ergonomics and work design. The traditional principles of motion economy have been expanded and are now known as principles and guidance for work design:.
Manual work design
The human body is capable of producing movements due to a complex system of muscles and bones, called the musculoskeletal system. There are three types of muscles in the human body: bony or striated muscles, attached to the bone; cardiac muscle, which is found in the heart, and soft muscle, such as that of the internal organs and the walls of the capillaries.[14] It is necessary to know the conformation of the skeletal-muscular system to delve into the analysis of manual work and develop applications that allow reducing the ergonomic risks present in the workplace.
The property of the muscle that allows it to be used with a considerable decrease in muscle strength is known as strength-length relationship. A task that requires considerable force must be performed in an optimal position. For example, the neutral or straight position provides the strongest grip for wrist movements. In elbow flexion, the best position would be with the elbow bent at a little more than 90°. When flexing the soles (such as when pressing a pedal), again the optimal position is a little more than 90°.[15].
The force is sufficient only to move the mass of one segment of the body. This property is known as the force-velocity relationship and is especially important when heavy manual work is involved.[15].
Workstations must allow operators to leave the part in the delivery area while their hands are in motion to pick up another component or tool and start a new cycle.[15].
The capacity of human strength depends on three important factors:
There are three types of muscular effort, defined primarily by the way they are measured. The muscular efforts that result in body movements are the result of a dynamic force. In the case where the movement of the body is restricted, an isometric or static force is obtained. A third type of muscular strength capacity, psychophysics, has been defined for situations in which a demand for strength is required for a prolonged time [15].
Strength in muscles is directly proportional to muscle size, as defined by cross-sectional area (87 psi (60N/cm²) for both men and women.) (Ikai and Fukunaga, 1968). For example, in heavy lifts the leg and trunk muscles should be used, and not weaker muscles.
Muscle fatigue is a very important criterion, but very little used in the adequate design of tasks for the human operator. The human body and muscle tissue rely on two primary types of energy sources, aerobic and anaerobic.
Since anaerobic metabolism can supply energy for only a short period, the oxygen delivered to the muscle fibers via peripheral blood flow becomes critical in determining how long muscle contractions will last. For this reason, any activity that requires the use of force must be estimated at 15% below the maximum force, in order not to completely fatigue the muscle tissues and exhaust the operator. This relationship can be modeled by:
Design of workstations, tools and equipment
Methods Engineering recognizes these concepts by managing to adapt and adjust them to the operator as ergonomics. This approach helps achieve greater production and efficiency in operations and lower injury rates for operators.
The primary guideline is to design the workplace[15] to fit the majority of individuals in terms of the structural size of the human body. The science of measuring the human body is known as anthropometry, which uses caliper-type devices to determine structural dimensions, such as height, forearm length, and others.
Designing for extremes implies that a specific characteristic is a limiting factor in determining the maximum and minimum value of a population variable that will be adjusted, for example, gaps, such as a door or the entrance to a storage tank, should be designed for the maximum case, that is, for the height or shoulder width corresponding to the 95th percentile. This way 95% of men and almost all women will be able to pass through the gap. The reach for things like a brake pedal or a control knob is designed for the minimum individual, that is, for legs or arms of women in the 5th percentile, so 95% of women and almost all men will have a greater reach and will be able to activate the pedal or control.
Design to fit is generally used for equipment or facilities that must fit a wide variety of individuals. Chairs, tables, desks, vehicle seats, a gear shift, and tool holders are devices that fit a population of workers between the 5th percentile for women and the 95th percentile for men. It is obvious that designing to fit is the most convenient method of design, but there is a trade-off with the cost of implementation.
Design for average is the least expensive but least preferred approach. Although there is no individual with all average dimensions, there are certain situations in which it would be impractical or too costly to include adjustment possibilities for all characteristics. It is useful, practical and cost-effective to build a one-to-one model of the equipment or facility being designed and have users evaluate it.
The height of the work surface (with the worker either sitting or standing) should be determined by a comfortable working posture for the operator. In general, this means that the forearms are in the natural downward position and the elbows are flexed at 90°, so that the arm is parallel to the ground. The elbow height becomes the proper operating or work surface height. If it is too high, the forearms shrink and cause shoulder fatigue, if it is too low, the neck or back bends and causes shoulder fatigue.
There are exceptions to this first principle. For heavy assembly with lifting of heavy parts, it is more advantageous to lower the work surface by up to 20 cm, to take advantage of the stronger trunk muscles. For fine assembly that includes small visual details, it is more advantageous to raise the work surface by 20 cm, to bring the details closer to the optimal 15° line of sight. Alternatively, it is perhaps better to tilt the surface around 15°, this way both principles are satisfied. However, the rounded parts have a tendency to roll off the surface.
Ergonomics in Spain
Contenido
La ergonomía ha cobrado relevancia en España en los últimos años, especialmente en el contexto laboral y académico. A continuación, se presentan algunos aspectos clave sobre este tema.
Rules and Regulations
Spain has various regulations that regulate ergonomics in the workplace. Some of the most important are:
Ergonomics is applied in various sectors, such as:.
Spanish universities offer programs and courses on ergonomics and related disciplines, such as work engineering and work psychology.") This is essential to train professionals in the implementation of ergonomic solutions. You can find more details about academic programs in the Ministry of Universities.
There are various institutions and research centers in Spain dedicated to the study of ergonomics. The National Institute of Safety and Health at Work (INSST) is one of them.
Ergonomics in Spain is an expanding field that seeks to improve working conditions and the quality of life of workers, adapting to changes in the contemporary work environment.
References
[1] ↑ Vern, Putz-anderson (1992). Cumulative trauma disorders: A manual for musculoskeletal diseases of the upper limbs. London: Taylor & Francis.
[6] ↑ «Asociación Española de Ergonomía». www.ergonomos.es. Consultado el 20 de septiembre de 2024.: http://www.ergonomos.es
[7] ↑ Montero Martínez, Ricardo (2000). Un paso hacia el futuro: el desarrollo de la Macroergonomía. España: Factores Humanos, 23.
[8] ↑ 3DALIA (15 de marzo de 2021). «Diseño ergonómico. La ergonomía en el diseño de producto». Consultado el 11 de marzo de 2022.: https://3dalia.com/diseno-ergonomico/
[10] ↑ ADA’s Accessibility Guidelines for Buildings and Facilities (ADAAG), «ADA’s Accessibility Guidelines for Buildings and Facilities (ADAAG)» (en inglés). Consultado el 8 de agosto de 2011.: https://www.ada.gov
[11] ↑ Tilley, Alvin R. & Henry Dreyfuss Associates (1993, 2002), The Measure of Man & Woman: Human Factors in Design: A human factors design manual.
[12] ↑ Openshaw, Scott. Taylor, Erin: Ergonomics and Design: A Reference Guide, página 36. Allsteel Inc, 2006.
[13] ↑ Niebel, Benjamin W. Freivalds, Andris: Ingeniería Industrial; Métodos, estándares y diseño del trabajo, pp. 148-150. The McGraw-Hill companies, Inc, 2005.
[14] ↑ Drake, Richard L., A. Wayne Vogl y Adam W. M. Mitchell: Gray's anatomy for students, Churchill Livingstone; 2 edition, Feb 11, 2009.
[15] ↑ a b c d e f g h Niebel, Benjamin W. Freivalds, Andris: Ingeniería Industrial; Métodos, estándares y diseño del trabajo. The McGraw-Hill companies, Inc, 2005, 11 Edición.
[16] ↑ a b Freiwald, Jürgen et al. En forma mediante el entrenamiento muscular, página 21. Editorial Paidotribo, 2002.
[17] ↑ Conglenton, J. J. (1983). The design and evaluation of the neutral posture chair, (tesis doctoral). Lubbock, TX: Texas Tech University.
[20] ↑ «BOE-A-1997-1853 Real Decreto 39/1997, de 17 de enero, por el que se aprueba el Reglamento de los Servicios de Prevención.». www.boe.es. Consultado el 20 de septiembre de 2024.: https://www.boe.es/buscar/act.php?id=BOE-A-1997-1853
For example, a worker will be able to withstand a force level of 50% of maximum force for only about a minute:.
Whether repeated static contractions are performed (such as holding a load with a flexed elbow) or a series of dynamic work elements (such as moving a lever with arms or legs), work and recovery must be assigned in short and frequent cycles (Active Micro Pauses).[16] This is due, first of all, to a rapid initial recovery period, which then tends to level out. Thus, most of the benefit is obtained in a relatively short period.
A program of carrying out Active Breaks within the worker's working hours allows improving motor skills, increasing speed, coordination and, above all, aerobic capacity. Its objectives are:
Active pauses allow:
If the worker reaches complete muscle (or whole-body) fatigue, full recovery will require a longer time, perhaps several hours.[15].
For a given group of muscles, there is a considerable range of strength in the normal, healthy adult population, with the strongest being five to eight times stronger than the weakest. The difference is greater for the strength of the upper extremities and less in the lower extremities. However, the primary cause of this effect is body size (i.e., total muscle mass) and not just sex; The average woman is considerably smaller and lighter than the average man. Furthermore, with the wide distribution for the strength of a given muscle, there are many women stronger than many men. In terms of age, muscle strength appears to peak around age 25 and then decrease linearly by 20 to 25% by age 60. The decrease in strength is due to the reduction of muscle mass and the loss of muscle fibers.[15].
Muscle contractions are initiated by neuronal innervation from the brain and spine, which together form the central nervous system. The electrical activity of muscles, called electromyogram (EMG), is a useful measure of local muscle activity. A typical motor neuron or nerve cell that reaches the muscle from the central nervous system may have connections with several hundred muscle fibers. The innervation rate of the number of fibers per neuron ranges from less than 10 in small eye muscles to more than 1000 in large muscles and can vary considerably even within the same muscles. This functional arrangement is called motor unit and has important implications in the control of movement.[15].
Lifting containers with heavy parts requires targeting small motor units as well as large ones to generate the necessary muscle forces. During lifting and refueling, some motor units become fatigued and others are selected to compensate. When the operator finishes replenishing containers and returns to precise assembly work, some motor units, including small precision ones, are not available. That is, using large muscles in the first instance to perform heavy tasks at the workstation will cause that when fine control movements are used to execute precision tasks, the muscular response will not be correct because there is already greater prior fatigue.
Cross innervation of agonists and antagonists always occurs through spinal reflexes. This minimizes unnecessary conflict between the muscles, as well as the resulting excessive energy expenditure.
That is, it is preferable to use movements that describe a ballistic or parabola-shaped trajectory, from the center to the outside and from the outside to the center, than inaccurate movements with sudden and abrupt changes.
When the right hand works in its normal area to the right of the body and the left hand works in its normal area, to the left of the body, the feeling of balance tends to induce a rhythm in the operator's performance, which leads to maximum productivity. The left hand on right people can be as effective as the right and should be used. Both hands should not be left idle, except during rest periods.
It is natural for both hands to move in symmetrical patterns. Deviations from the symmetry of a two-handed workstation lead to uncomfortable movements of the operator. Many people are familiar with the difficulty of tapping the stomach with the left hand and rubbing the top of the head with the right. Another experiment that illustrates the difficulty of performing non-symmetrical operations is trying to draw a circle with your left hand and a square with your right.
Spinal reflexes that excite or inhibit muscles also lead to natural rhythms in the movement of body segments that can be compared to second-order mass-spring-damper systems, where the body segments provide the mass and the muscle has internal resistance and damping.
Natural frequency is essential for the smooth and automatic performance of a task. Drillis (1963) studied a variety of very common manual tasks and suggested optimal work times, as follows:.
Due to the nature of the ligaments that join body segments (approximating dowel joints), it is easier for people to produce curved movements, that is, pivoting around a joint. Straight-line movements that involve sharp, sudden changes in direction take more time and are less precise. This law is easily demonstrated by moving either hand in a rectangular pattern, and then in a circular pattern of approximate magnitudes.
Continuous curved movements do not require deceleration and, consequently, are performed faster per unit of distance.
This classification of movements finally ends up becoming a fundamental law of the economy of movements, to execute an adequate study of methods.
Class one moves require the least effort and time, while class five moves are considered the least efficient. Thus, the minor grading motion should always be used to do adequate work.
Since the hands are more dexterous than the feet, it would be unwise to make the feet work while the hands are still. Devices such as pedals can often be arranged to allow for clamping, ejection or feeding, freeing up the hands for other more useful work and consequently decreasing cycle time. When the hands move, the feet should not move, since simultaneous movement of hands and feet is difficult; but the feet may be applying pressure on something like a pedal. Furthermore, the operator must be seated, since it is not easy to operate a foot pedal, and support the entire weight of the body on the other foot.
These principles also apply to the seated work station. Many tasks, such as writing or light assembly, are best performed at resting elbow height. If the work requires the perception of fine detail, it may be necessary to elevate the work so that it is closer to the eyes. Seated work stations should have adjustable chairs and footrests. Ideally, once the operator is seated comfortably with both feet on the ground, the work surface is positioned at the appropriate elbow height to adjust the operation. Thus, the workstation also needs to be adjustable. Short operators, whose feet do not reach the floor even after adjusting the seat, should use a foot rest that provides appropriate support.
Sitting posture[17] is important from the point of view of reducing both stress on the feet and overall energy expenditure. Because comfort is an individual response, it is quite difficult to establish strict principles for sitting well. Furthermore, few chairs will accommodate the comfort of many possible sitting postures. It is very important to provide lumbar support through a bump on the back of the chair or with a lumbar cushion placed at belt height. provide easy adjustment for specific seat parameters. Height is the most critical, where the ideal is determined by the person's popliteal height. A seat that is too high will uncomfortably compress the lower thighs, decrease the angle of the trunk and, again, increase the pressure on the discs. In addition, elbow pads are recommended to provide support for shoulders, arms and footrests in the case of shorter individuals. In general, chairs should have a soft contour, a padded seat and covered with a fabric that allows air to pass through to prevent moisture from sweat. A seat with a cushion that is too soft restricts posture and can restrict circulation in the legs.
The height of the workstation should be adjusted so that it is possible to work efficiently whether standing or sitting. The human body is not designed to sit for long periods of time. The discs between the vertebrae do not have a blood supply on their own; they depend on pressure changes that result from movement to receive nutrients and eliminate waste. Stiff posture also reduces blood flow to the muscles and induces muscle fatigue and cramps.
Different researchers[18] report that more than a third of all workers have to work standing or walking for periods greater than four hours a day. Prolonged standing, defined as standing for more than two hours a day, has been linked to different health problems, such as:
People who stand for 45 to 50% of their work day have foot and leg discomfort and those who stand for more than 25% of their work day have low back pain (Rys 1994).
It is tiring to stand for long periods on a cement floor. Operators should be provided with elastic anti-fatigue mats that allow small muscle contractions in the legs, forcing blood to move and prevent it from pooling in the lower extremities.
A distance intervenes in each movement. The greater the distance, the greater the muscular effort, control and time. Therefore, it is important to minimize distances. The normal working area of the right hand in the horizontal plane includes the area circumscribed by the forearm when moved in an arc with a pivot at the elbow. This area represents the most convenient zone within which the hand performs movements with a normal expenditure of energy. The normal area of the left hand is established in a similar way. Since the movements are made in the third dimension, as well as in the horizontal plane, the normal work area also applies to the vertical plane.
When driving a car, we are all familiar with the short time it takes to apply your foot to the brake. The reason is obvious: since the brake pedal has a fixed position, it does not take time to decide where it is located. The body responds instinctively and applies pressure to the area where the driver knows the brake pedal is located. If its location changed, the driver would need much more time to stop the car. Likewise, providing fixed locations for all tools and materials at the workstation eliminates, or at least minimizes, the minor guesswork required to search for and select the objects needed to do the job.
Gravity chutes make a clean work area possible, as the finished material is sent outside, rather than piled up around it. A container elevated above the work surface (so that the hand can slide material under it will also decrease the time required to perform this task by 10 to 15 percent. Gravity chutes allow finished parts to be sent within the normal area and eliminate the need for distant movements.
The optimal arrangement depends on many characteristics, both human (strength, reach, senses) and task (loads, repetition, orientation). It is obvious that not all factors can be optimized. The designer must establish priorities in the distribution of the work area. Once the location has been determined for a group of components, that is, the parts most frequently used for assembly, the principles of functionality and sequence of use must be taken into account. Functionality refers to the grouping of components according to the similarity of their function, for example, all fasteners in one area, all gaskets and rubber components in another area. It is very important to place the components or subassemblies in the order in which they are assembled, as this will have a great effect on reducing wasteful movement.
The most efficient advanced production planning for manufacturing includes making multiple cuts with combination tools and simultaneous cuts with different tools. Of course, the type of work to be processed and the number of parts to be produced determines whether it is desirable to combine the cuts, as in the case of cuts with a square and hexagonal tower.
If either hand is used for holding during processing of a part, then the hand is not doing useful work. A device can always be designed to hold work satisfactorily, and allow both hands to do useful work. The devices not only save part processing time, but also allow work to be held more accurately and firmly. Many times, foot-operated mechanisms allow both hands to do productive work.
Many machine tools and other devices are perfect in the mechanical sense, but do not provide effective operation, because the designer of the installation did not take into account the different human factors. Handwheels, cranks and levers must be of adequate size and position so that the operator can manipulate them with maximum skill and minimum fatigue. Controls that are often used should be placed between elbow and shoulder height. Seated operators can apply maximum force to the levers at elbow level; standing operators, to levers that are shoulder height. The diameter of the steering wheels and handlebars depends on the torque that must be applied and the mounted position.
Shape codes, with two- or three-dimensional geometric configurations, allow for both tactile and visual identification. It is useful, especially in low light conditions, or in situations where redundancy or duplicate quality in identification is desired, to help minimize errors. Multi-rotation knobs are used for continuous controls where the adjustment range is greater than one full turn. Fractional rotation knobs are used for continuous controls with intervals less than one turn, while positioning knobs are used for discrete adjustments.
In their work assignments, operators use various types of controls and control designs all the time. The three parameters that have a great impact on performance are:.
A control that is too small or too large cannot be activated efficiently.
Compatibility is defined as the relationship between controls and displays that is consistent with human expectations. The basic principles include:
so that the operator knows that the function has been achieved.
For example, a good performance is a door with a pull-open handle or a door with a push-open plate. Space mapping is observed in well-designed stoves. Movement compatibility is provided with direct action, reading scales that increase from left to right, and clockwise movements that increase adjustment. For circular displays, the best compatibility is achieved with a fixed scale and moving pointers or needles.
In horizontal or vertical screens the Warrick principle is used, which says that the closest pointers on the screen and the control movement in the same direction provide the best compatibility. (Sanders and McCormick, 1993).
The noise dose that is above 80 dBA causes anyone listening to that amount to be affected by a partial dose. If said total daily exposure consists of several partial exposures to different noise levels, the partial doses are added to obtain a combined exposure:.
D = 100
Where:
D = noise dose
C = time spent under the effects of a specific noise level (h)
T = time allowed under the effects of a specific noise level (h).
The total exposure to different noise levels cannot be exceeded at a dose of 100%.
Noise exposures allowed.
When daily noise exposure is composed of two or more periods of noise exposure of different levels, their combined effect should be considered rather than the independent effects of each of them. If the sum of the following fractions C1/T1 + C2/T2 + … + Cn/Tn exceeds unity, the combined exposure must be considered to exceed the maximum value. Cn indicates the total exposure time to a specific noise level, while Tn is equal to the total exposure time allowed during a work day.
Exposure to impact noise should not exceed the peak sound pressure level of 140 dB.
For example, a worker will be able to withstand a force level of 50% of maximum force for only about a minute:.
Whether repeated static contractions are performed (such as holding a load with a flexed elbow) or a series of dynamic work elements (such as moving a lever with arms or legs), work and recovery must be assigned in short and frequent cycles (Active Micro Pauses).[16] This is due, first of all, to a rapid initial recovery period, which then tends to level out. Thus, most of the benefit is obtained in a relatively short period.
A program of carrying out Active Breaks within the worker's working hours allows improving motor skills, increasing speed, coordination and, above all, aerobic capacity. Its objectives are:
Active pauses allow:
If the worker reaches complete muscle (or whole-body) fatigue, full recovery will require a longer time, perhaps several hours.[15].
For a given group of muscles, there is a considerable range of strength in the normal, healthy adult population, with the strongest being five to eight times stronger than the weakest. The difference is greater for the strength of the upper extremities and less in the lower extremities. However, the primary cause of this effect is body size (i.e., total muscle mass) and not just sex; The average woman is considerably smaller and lighter than the average man. Furthermore, with the wide distribution for the strength of a given muscle, there are many women stronger than many men. In terms of age, muscle strength appears to peak around age 25 and then decrease linearly by 20 to 25% by age 60. The decrease in strength is due to the reduction of muscle mass and the loss of muscle fibers.[15].
Muscle contractions are initiated by neuronal innervation from the brain and spine, which together form the central nervous system. The electrical activity of muscles, called electromyogram (EMG), is a useful measure of local muscle activity. A typical motor neuron or nerve cell that reaches the muscle from the central nervous system may have connections with several hundred muscle fibers. The innervation rate of the number of fibers per neuron ranges from less than 10 in small eye muscles to more than 1000 in large muscles and can vary considerably even within the same muscles. This functional arrangement is called motor unit and has important implications in the control of movement.[15].
Lifting containers with heavy parts requires targeting small motor units as well as large ones to generate the necessary muscle forces. During lifting and refueling, some motor units become fatigued and others are selected to compensate. When the operator finishes replenishing containers and returns to precise assembly work, some motor units, including small precision ones, are not available. That is, using large muscles in the first instance to perform heavy tasks at the workstation will cause that when fine control movements are used to execute precision tasks, the muscular response will not be correct because there is already greater prior fatigue.
Cross innervation of agonists and antagonists always occurs through spinal reflexes. This minimizes unnecessary conflict between the muscles, as well as the resulting excessive energy expenditure.
That is, it is preferable to use movements that describe a ballistic or parabola-shaped trajectory, from the center to the outside and from the outside to the center, than inaccurate movements with sudden and abrupt changes.
When the right hand works in its normal area to the right of the body and the left hand works in its normal area, to the left of the body, the feeling of balance tends to induce a rhythm in the operator's performance, which leads to maximum productivity. The left hand on right people can be as effective as the right and should be used. Both hands should not be left idle, except during rest periods.
It is natural for both hands to move in symmetrical patterns. Deviations from the symmetry of a two-handed workstation lead to uncomfortable movements of the operator. Many people are familiar with the difficulty of tapping the stomach with the left hand and rubbing the top of the head with the right. Another experiment that illustrates the difficulty of performing non-symmetrical operations is trying to draw a circle with your left hand and a square with your right.
Spinal reflexes that excite or inhibit muscles also lead to natural rhythms in the movement of body segments that can be compared to second-order mass-spring-damper systems, where the body segments provide the mass and the muscle has internal resistance and damping.
Natural frequency is essential for the smooth and automatic performance of a task. Drillis (1963) studied a variety of very common manual tasks and suggested optimal work times, as follows:.
Due to the nature of the ligaments that join body segments (approximating dowel joints), it is easier for people to produce curved movements, that is, pivoting around a joint. Straight-line movements that involve sharp, sudden changes in direction take more time and are less precise. This law is easily demonstrated by moving either hand in a rectangular pattern, and then in a circular pattern of approximate magnitudes.
Continuous curved movements do not require deceleration and, consequently, are performed faster per unit of distance.
This classification of movements finally ends up becoming a fundamental law of the economy of movements, to execute an adequate study of methods.
Class one moves require the least effort and time, while class five moves are considered the least efficient. Thus, the minor grading motion should always be used to do adequate work.
Since the hands are more dexterous than the feet, it would be unwise to make the feet work while the hands are still. Devices such as pedals can often be arranged to allow for clamping, ejection or feeding, freeing up the hands for other more useful work and consequently decreasing cycle time. When the hands move, the feet should not move, since simultaneous movement of hands and feet is difficult; but the feet may be applying pressure on something like a pedal. Furthermore, the operator must be seated, since it is not easy to operate a foot pedal, and support the entire weight of the body on the other foot.
These principles also apply to the seated work station. Many tasks, such as writing or light assembly, are best performed at resting elbow height. If the work requires the perception of fine detail, it may be necessary to elevate the work so that it is closer to the eyes. Seated work stations should have adjustable chairs and footrests. Ideally, once the operator is seated comfortably with both feet on the ground, the work surface is positioned at the appropriate elbow height to adjust the operation. Thus, the workstation also needs to be adjustable. Short operators, whose feet do not reach the floor even after adjusting the seat, should use a foot rest that provides appropriate support.
Sitting posture[17] is important from the point of view of reducing both stress on the feet and overall energy expenditure. Because comfort is an individual response, it is quite difficult to establish strict principles for sitting well. Furthermore, few chairs will accommodate the comfort of many possible sitting postures. It is very important to provide lumbar support through a bump on the back of the chair or with a lumbar cushion placed at belt height. provide easy adjustment for specific seat parameters. Height is the most critical, where the ideal is determined by the person's popliteal height. A seat that is too high will uncomfortably compress the lower thighs, decrease the angle of the trunk and, again, increase the pressure on the discs. In addition, elbow pads are recommended to provide support for shoulders, arms and footrests in the case of shorter individuals. In general, chairs should have a soft contour, a padded seat and covered with a fabric that allows air to pass through to prevent moisture from sweat. A seat with a cushion that is too soft restricts posture and can restrict circulation in the legs.
The height of the workstation should be adjusted so that it is possible to work efficiently whether standing or sitting. The human body is not designed to sit for long periods of time. The discs between the vertebrae do not have a blood supply on their own; they depend on pressure changes that result from movement to receive nutrients and eliminate waste. Stiff posture also reduces blood flow to the muscles and induces muscle fatigue and cramps.
Different researchers[18] report that more than a third of all workers have to work standing or walking for periods greater than four hours a day. Prolonged standing, defined as standing for more than two hours a day, has been linked to different health problems, such as:
People who stand for 45 to 50% of their work day have foot and leg discomfort and those who stand for more than 25% of their work day have low back pain (Rys 1994).
It is tiring to stand for long periods on a cement floor. Operators should be provided with elastic anti-fatigue mats that allow small muscle contractions in the legs, forcing blood to move and prevent it from pooling in the lower extremities.
A distance intervenes in each movement. The greater the distance, the greater the muscular effort, control and time. Therefore, it is important to minimize distances. The normal working area of the right hand in the horizontal plane includes the area circumscribed by the forearm when moved in an arc with a pivot at the elbow. This area represents the most convenient zone within which the hand performs movements with a normal expenditure of energy. The normal area of the left hand is established in a similar way. Since the movements are made in the third dimension, as well as in the horizontal plane, the normal work area also applies to the vertical plane.
When driving a car, we are all familiar with the short time it takes to apply your foot to the brake. The reason is obvious: since the brake pedal has a fixed position, it does not take time to decide where it is located. The body responds instinctively and applies pressure to the area where the driver knows the brake pedal is located. If its location changed, the driver would need much more time to stop the car. Likewise, providing fixed locations for all tools and materials at the workstation eliminates, or at least minimizes, the minor guesswork required to search for and select the objects needed to do the job.
Gravity chutes make a clean work area possible, as the finished material is sent outside, rather than piled up around it. A container elevated above the work surface (so that the hand can slide material under it will also decrease the time required to perform this task by 10 to 15 percent. Gravity chutes allow finished parts to be sent within the normal area and eliminate the need for distant movements.
The optimal arrangement depends on many characteristics, both human (strength, reach, senses) and task (loads, repetition, orientation). It is obvious that not all factors can be optimized. The designer must establish priorities in the distribution of the work area. Once the location has been determined for a group of components, that is, the parts most frequently used for assembly, the principles of functionality and sequence of use must be taken into account. Functionality refers to the grouping of components according to the similarity of their function, for example, all fasteners in one area, all gaskets and rubber components in another area. It is very important to place the components or subassemblies in the order in which they are assembled, as this will have a great effect on reducing wasteful movement.
The most efficient advanced production planning for manufacturing includes making multiple cuts with combination tools and simultaneous cuts with different tools. Of course, the type of work to be processed and the number of parts to be produced determines whether it is desirable to combine the cuts, as in the case of cuts with a square and hexagonal tower.
If either hand is used for holding during processing of a part, then the hand is not doing useful work. A device can always be designed to hold work satisfactorily, and allow both hands to do useful work. The devices not only save part processing time, but also allow work to be held more accurately and firmly. Many times, foot-operated mechanisms allow both hands to do productive work.
Many machine tools and other devices are perfect in the mechanical sense, but do not provide effective operation, because the designer of the installation did not take into account the different human factors. Handwheels, cranks and levers must be of adequate size and position so that the operator can manipulate them with maximum skill and minimum fatigue. Controls that are often used should be placed between elbow and shoulder height. Seated operators can apply maximum force to the levers at elbow level; standing operators, to levers that are shoulder height. The diameter of the steering wheels and handlebars depends on the torque that must be applied and the mounted position.
Shape codes, with two- or three-dimensional geometric configurations, allow for both tactile and visual identification. It is useful, especially in low light conditions, or in situations where redundancy or duplicate quality in identification is desired, to help minimize errors. Multi-rotation knobs are used for continuous controls where the adjustment range is greater than one full turn. Fractional rotation knobs are used for continuous controls with intervals less than one turn, while positioning knobs are used for discrete adjustments.
In their work assignments, operators use various types of controls and control designs all the time. The three parameters that have a great impact on performance are:.
A control that is too small or too large cannot be activated efficiently.
Compatibility is defined as the relationship between controls and displays that is consistent with human expectations. The basic principles include:
so that the operator knows that the function has been achieved.
For example, a good performance is a door with a pull-open handle or a door with a push-open plate. Space mapping is observed in well-designed stoves. Movement compatibility is provided with direct action, reading scales that increase from left to right, and clockwise movements that increase adjustment. For circular displays, the best compatibility is achieved with a fixed scale and moving pointers or needles.
In horizontal or vertical screens the Warrick principle is used, which says that the closest pointers on the screen and the control movement in the same direction provide the best compatibility. (Sanders and McCormick, 1993).
The noise dose that is above 80 dBA causes anyone listening to that amount to be affected by a partial dose. If said total daily exposure consists of several partial exposures to different noise levels, the partial doses are added to obtain a combined exposure:.
D = 100
Where:
D = noise dose
C = time spent under the effects of a specific noise level (h)
T = time allowed under the effects of a specific noise level (h).
The total exposure to different noise levels cannot be exceeded at a dose of 100%.
Noise exposures allowed.
When daily noise exposure is composed of two or more periods of noise exposure of different levels, their combined effect should be considered rather than the independent effects of each of them. If the sum of the following fractions C1/T1 + C2/T2 + … + Cn/Tn exceeds unity, the combined exposure must be considered to exceed the maximum value. Cn indicates the total exposure time to a specific noise level, while Tn is equal to the total exposure time allowed during a work day.
Exposure to impact noise should not exceed the peak sound pressure level of 140 dB.