Physical ergonomics deals with the physical load on the human body when performing activities like work, sports, jobs at home or dealing with products. With regard to the exposure to physical loads and its potential effects on the human body, the presented framework is helpful.
In this article we explain more about the backgrounds of physical ergonomics, the risk assessment, the types of physical load: heavy work, repetitive work, and sedentary work.
Ergonomics (or human factors) is the scientific discipline concerned with the understanding of the interactions among humans and other elements of a system, and the profession that applies theoretical principles, data and methods to design in order to optimise human well-being and overall system performance (definition International Ergonomics Association (IEA). Within this discipline or profession, physical ergonomics is regarded as one of the domains of specialization, beside organizational ergonomics, and cognitive ergonomics.
Within the discipline of ergonomics, physical ergonomics is regarded as one of the domains of specialisation, beside organisational ergonomics and cognitive ergonomics. Although the primary focus can be on any one of these, interventions should pay attention to all three aspects and their interactions.
Physical ergonomics deals with the physical load on the human body. With regard to the exposure to physical loads and its potential effects on the human body, the presented framework is helpful. This framework distinguishes external and internal exposures. The human at work is (externally) exposed to a work situation (work demands, working environment) and working methods (the work activities to be performed). These result in the adoption of specific human body postures and the execution of movements, as well as some external forces on the human body. Furthermore, the exposure to posture, movement and force leads to the exposure to internal forces on body structures and to elevated levels of energy expenditure (i.e. internal exposure). Mechanical and physiological responses occur in the short term (i.e. acute responses). Musculoskeletal disorders are among the potential longer term effects that follow acute responses. Musculoskeletal disorders are among the potential longer term effects that follow acute responses. The responses and depend also on the individual worker’s capacities (including body dimensions, physical fitness, condition) as illustrated in Figure 1.
Next to health protection, ergonomics focusses on system performance. In today’s ergonomics it is obvious that the aims for health protection and enhanced system performance can very well be achieved together. .
Risk assessment and prevention of risks are among the main issues in physical ergonomics. Risk assessment is about the assessment of the risks for developing a specific type of musculoskeletal disorders associated with the exposure to a specific type physical load. Relevant aspects of the exposure in this respect are: the level or intensity of the exposure  exposure duration  and the frequency of the exposure. 
Of course, risk assessment is only the first step towards good working conditions. The assessment needs to be evaluated and, where applicable, in following steps, improvements developed, tested and implemented. Finally an end evaluation should prove that the enhanced situation indeed meets the requirements.
Prevention is another main focus within physical ergonomics. It entails the definition and implementation of measures that may reduce the risk for developing musculoskeletal disorders. These measures may include all those interfering in the work situation and working methods that potentially decrease the level, duration or frequency of the external exposure.
The main European legislation relevant to risk assessment and prevention (from a broader perspective than physical load) is Framework Directive 89/391 . This Directive offers a framework, with “general principles concerning the prevention of occupational risks as well as general guidelines for the implementation of the said principles" (Art. 1.2.).
Framework Directive 89/391  offers a long list of hazards and risks that may occur in occupational settings. Here we address the four major types of physical load, in perspective of prevalence of risks in the contemporary:
- heavy work,
- repetitive work,
- static work,
- sedentary work,
For each of these, we systematically address the potential risks, the risk assessment methods, and preventive measures.
Scope and prevalence
Heavy work includes the type of work that is characterized by high external forces on the body. These may result from:
- lifting of heavy loads,
- carrying of heavy loads,
- pushing of loads,
- pulling of loads.
Despite the increased mechanization and automation in many sectors of industry, the proportion of workers exposed to heavy work have remained relatively stable over the past decennia. In the Netherlands the percentage of workers exposed to heavy work is about 20% . Across Europe, carrying or moving heavy loads is part of the work of 25-40% of the workers (Eurofound, data can be found for each country . In industrially developing countries that percentage may be much higher. Sectors with high incidences of heavy work are: nursing, construction[http://en.wikipedia.org/wiki/Occupational_safety_and_health#Construction], metal, agriculture [http://en.wikipedia.org/wiki/Occupational_safety_and_health#Agriculture], transport and logistics.
There is strong evidence that the frequent lifting of heavy loads is related to the occurrence of lower back pain. Many epidemiological studies have investigated this relationship. Estimates for the relative risk associated with occupational lifting and lower back injury ranges from 1.3 to 4.2. It was also established that the risk of developing injury increases with the frequency of lifting. 
However, there is no evidence that the frequent pushing and pulling of loads is related to the occurrence of lower back pain. Some studies suggest a relationship of pulling/pushing with shoulder complains, but the number of these studies is quite limited.
Risk assessment method
The risk associated with the lifting of loads depends on several factors, among others the weight of the load, the vertical travel distance, the horizontal distance between the load and the body, and the frequency of lifting. These risk factors are addressed in the so-called NIOSH http://en.wikipedia.org/wiki/National_Institute_for_Occupational_Safety_and_Health equation which can be considered as the most complete and applied risk assessment method for lifting. The method is relatively simple to use, but one needs to follow the manual carefully (CDC website).
The primary product of the NIOSH lifting equation is the Recommended Weight Limit (RWL), which defines the maximum acceptable weight (load) that nearly all healthy employees could lift over the course of an 8 hour shift, without increasing the risk of musculoskeletal disorders (MSD) to the lower back. In the NIOSH equation:
RWL (kg) = 23 x HM x VM x DM x AM x FM x CM, where 23 reflects the number of kgs considered to be safe in ideal lifting conditions, and HM up to CM represent the multipliers M associated with the following risk factors:
- H = Horizontal location of the object relative to the body
- V = Vertical location of the object relative to the floor
- D = Distance the object is moved vertically
- A = Asymmetry angle or twisting requirement
- F = Frequency and duration of lifting activity
- C = Coupling or quality of the workers grip on the object.
Another method for risk assessment is the KIM method . This method not only addresses lifting, but also holding, carrying, pushing and pulling. The method was developed by the Federal Institute for Occupational Safety and Health (BAuA) and Committee of the Laender for Occupational Safety and Health (LASI) (European Agency for Safety and Health at Work website).
The NIOSH calculation allows identification of the factor(s) that are the most promising for intervention. As loads are usually not easy to change, in many cases workplace improvements are the best option. Reducing the horizontal distance is important, and can be attained by both workplace design and good work instruction.
Scope and prevalence
Repetitive work is the type of work that involves repetitive movements of the arms and hands. This type of work is prevalent in many occupations, like assembly, packing, computer work, hair dressing, etc. For the Dutch population of workers it has been established that 35% are exposed to ‘repetitive movements of the arms and hands’ .
Computer work is a specific form of repetitive work involving repetitive movements mainly of the hands and fingers. Obviously, the prevalence of this type of work has increased dramatically over the past decennia. The Eurofound website provides detailed information for European countries about the percentage of workers that are involved in computer work . In many countries about 40% of the employees work (almost) all the time with computers.
The type of musculoskeletal disorders associated with repetitive work mainly concern neck, shoulder, elbow and wrist complaints. Several terms are used to group these type of disorder, for instance Repetive Strain Injury or Cumulative Trauma Disorder or Complaints or Arms, neck and Shoulders (CANS).
Epidemiological evidence for the association between repetitive work and repetitive strain injury has been provided: relative risks ratios have been estimated in a range from 2,3 to 8,8 
Typical computer work with keyboard and mouse is characterized by a combination of small repetitive movements of fingers, precise hand operations and static muscles in the neck-shoulder region. It has been reported that the task duration is a main determinant of risk in this kind of work. The risk of developing health complaints particularly increases at a task duration above 6 hours per day . Other extra risk factors are:
- too little recovery time (minimum 5 minutes per hour),
- no option to take micro breaks (minimum 20 seconds every 10 minutes),
- no individual control of work pace,
- mental work strain.
Risk assessment method
The OCRA method  can be used for the assessment of the risk of repetitive movements of elbows, wrists and hands in cases where:
- one or both upper extremities moves in cycles of less than 30 seconds,
- these or similar cycles are performed for more than 50% of the working time,
- the cycles are frequent and are identical or similar in nature.
As with the NIOSH formula, the OCRA method calculates the risk with the Recommended Technical Actions, the maximum permissible number of actions per minute:
RTA = 30 x Pf x Rf x Af x Ff x (Rc x Dc) where the risk factors are:
- Constant of Frequency" of technical actions per min = 30
- Posture (Pf)
- Repetition (Rf)
- Additional factors (Af): cold, vibrations, noise, gloves
- Force (Ff)
- Recovery options (Rc)
- Duration (Dc)
The OCRA index = real number of movements / RTA and is judged according to the following traffic light model: Green <2,2 no risk Orange 2,3 – 3,5 low risk Red >3,5 risk, action must be taken.
The OCRA calculation allows identification of the most promising factor(s) for intervention.
Static work is the type of work that involves prolonged standing and prolonged postures of the back, neck and arms.
Prolonged standing (more than 4 hours a day) without regular intermittent walking is prevalent in many occupational sectors like healthcare, catering, retail and security. The type of musculoskeletal disorders associated with prolonged standing mainly concern chronic venous insufficiency and musculoskeletal pain of the lower back and feet. .
As for repetitive work, prolonged postures (holding times longer than 4 seconds) are prevalent in many occupations like assembly, packing, computer work, hair dressing, etc. Exposure and risk are much the same as for repetitive work, and the risk assessment methods for upper body movements and postures are similar. . .
Scope and prevalence
Many workers perform their work seated for prolonged periods. In the Netherlands the mean worker sits 5,8 hours per day at work plus 3,5 hours in private life . Jobs with high incidence of sedentary work are: office work, security (control rooms), transport (drivers), services, cashier work.
Sedentary work is related to overweight, lack of physical activity and poor and prolonged postures. In addition to MSDs, it has been linked to CVD, digestive cancers, reproductive disorders and reduced mental health.
A good workplace design and frequent changes in postures are required in order to prevent strains. Some desks allow alternate sitting and standing, and if one does not have such a desk, one should walk regularly. As a minimum it is advised to move semi-intensively during 30 minutes per day. Mini breaks will contribute to the reduction of overloads. Walking during lunch breaks is an option for office workers. The newest designs in ergonomic seating allow for more dynamic seating, with postural changes. However, even the best furniture cannot overcome the strains on the body from excessive periods of sitting. Good instruction on how to use adjustable furniture is also important.
Work has changed over the ages. Where physical effort used to be the basis for human performance, today equipment and tools allow much lower levels of physical exertion in many trades, and ergonomics has become more concerned with the elimination of risks related to repetitive work and static postures. Postural issues and lack of physical activity associated with sedentary work, have resulted in sedentary work gaining importance in physical ergonomics. The development of new knowledge remains important.
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