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3. ACUMEN
2.3 PHYSICAL CAPABILITIES
2.3.1 Muscular Strength and Endurance
Muscles have limits to their strength and their ability to maintain that strength and resist fatigue (also referred to as muscular endurance).
Strength is defined as the maximum force one can exert voluntarily. It is measured in kilograms (kg) or pounds (lb). The work that muscles may have to do is classified into two types:
• Static work, also referred to as static effort, is characterized by long- term muscular contraction without body motion occurring; for example, when a posture must be held for long periods like sitting, standing in one place, bending, and twisting to perform a maintenance job.
• Dynamic work, also referred to as dynamic effort, is characterized by body motion accompanying muscular tension; for example, repeated muscular contractions with rest periods in between, such as walking.
Most physical activities have both static and dynamic work compo- nents. Take, for example, carrying jars filled with products from a sam- pling point in the plant back to the laboratory. The upper body, especially the arms, shoulders, and back muscles, is under static effort to support the weight and maintain (stabilize) the body in an upright position. However, the lower body, especially the legs, is under dynamic effort to allow the body to move.
Strength is strictly defined as “the maximal force muscles can exert isometrically in a single, voluntary effort” (Kroemer, 1970; Chaffin, 1975). However, strength under dynamic conditions can also be measured (Sanders and McCormick, 1993). Information about isometric muscular strength capability is needed to specify, for example, the minimum valve resistances and the forces required to safely carry bags of chemicals. Most
tasks or activities that operators perform require the integrated exertion of many muscle groups. For example, lifting a bag of chemicals off the floor requires squatting, extending arms, grabbing the bag with both hands, lifting the bag with the arms and shoulders, lifting the body using the legs, and the like. In this complex situation, the maximum force that can be exerted is determined by the weakest link in the different muscle groups involved.
2.3.1.1 Factors Affecting Strength
There is a wide range of muscular strength capabilities among people, from heavy weight lifters to sedentary TV watchers. In addition, many variables play the role of limiting factors and can influence muscular strength:
• Age. Strength increases rapidly in the teens, reaches its peak in the middle to late twenties, levels off, and starts decreasing, slightly in the forties and faster in the fifties and sixties. Most tasks and jobs are designed around the young worker. But, the strength required may exceed the capacity of older groups. This is truly an issue today, since the current workforce contains many older workers.
• Gender. In general, women have less-developed muscles and their overall mean strength is about 67% that of males (Hettinger, 1961;
Roebuck, Kroemer, and Thomson, 1975; Lauback, 1976). However, we must not generalize this figure (Wilmore, 1975; Redgrove, 1979;
Pheasant, 1983; O’Brien, 1985; Chaffin and Andersson, 1991;
Sanders and McCormick, 1993). The reasons are:
1. This 67% is an average value from various groups of muscles. The range is 35 to 85% of male mean strength.
2. For the lower extremities, the average strength of females is com- parable with males. This may reflect the use of lower extremities by both males and females for daily regular activities. But, in general, men use their upper extremities more than women.
3. Trained female athletes are stronger than many untrained men.
In terms of designing manual tasks, such as lifting, the maximum weight that can be lifted is determined by the weakest member of the two genders involved.
• Occupation. Relates to the different levels of muscle use (exertion) by people in different occupations; for example, a process operator versus a desk clerk given that neither is a fitness fanatic.
• Physical training. Physical training, especially weight lifting, tends to increase muscular strength and endurance by as much as 30–50%.
• Others. Other factors that have been associated with influencing mus- cular strength are:
1. Gloves. Wearing gloves, on average, results in a loss of about 20%
in grip strength. Of course, this depends on the thickness of the gloves: the thicker the gloves, the less the strength.
2. Body dimension, such as height and weight. Body weight is an important determinant of arm strength. Likewise, height correlates highly with pulling strength.
3. Body position. For example, the strength of elbow flexion is at a maximum with the elbow at about 90° but drops off to about half this value at extreme angles.
4. Diet. Inadequate and improper nutrition decrease body strength.
5. Other personal factors: genetics, health, fatigue, drugs, motivation.
2.3.1.2 Endurance and Fatigue
As mentioned earlier in this chapter, muscular endurance refers to the ability of the muscle to continue to work. In the case of static work, the length of time (endurance) a force can be maintained depends on the pro- portion of the available strength being exerted. For example, maximum force can be maintained for a very short time (few seconds). However, the smaller the force exerted, the longer it can be maintained (Caldwell, 1963, 1964; Monod, 1985; Deeb, 1988). The applications of these data are important. If we expect an operator to apply and maintain static force to hold an object (i.e., a drill weighing 15 kg or 33 lb) for a period of time, the designer must consider endurance. In the case of dynamic work, endurance is defined in terms of force and frequency of repetition.
However, the endurance time is significantly longer. For example, 25%
maximum voluntary contraction (MVC) can be maintained for about 10 minutes under static contraction compared with over 4 hours under dynamic work.
Fatigue is a product of the work performed, whether dynamic or static.
The degree and severity of fatigue constitutes the level of discomfort, pain, distraction, and reduced performance that may lead to accidents. It is, therefore, very important to design tasks by eliminating factors that induce fatigue.