Low-Level Static Exertions - Fundamentals and Assessment Tools for Occupational Ergonomics - page 332

sufficient recovery periods are allowed during such work tasks. A more informative term for the related
risk factor would be prolonged sustained or repeated muscle contractions. According to the endurance
curve, it is possible to sustain low-level exertions for a longer time than high-level exertions. It is
likely, that this time factor is the risk. This hypothesis is supported by the physiological responses to
such exertions, which constitute the plausibility. Standardized muscle contractions have been studied
in combination with detailed physiological responses. In the following discussion, focus will be on mech-
anisms, which may induce muscle damage.
An example of a standardized setup for studying muscle contractions is shown in Figure 14.3. The test
chair can be regulated for the subject to adopt any working posture and the force transducers connected
to the handles allow for three-dimensional recordings. During specific work tasks, biomechanical calcu-
lations may then assess the relative load on various muscles or muscle parts groups based on maximum

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contractions performed in identical postures and directions. 33
Intramuscular Pressure and Blood Flow With each muscle contraction, the tissue pressure (hydrostatic
pressure) in the muscle increases in proportion to the force development. The absolute level in terms

FIGURE 14.3 Experimental chair, where arm posture can be adjusted in any position of abduction (a) and flexion
(b). The hands are grasping handles connected to three-dimensional force transducers (c). Professor Bjørn Quistorff,
University of Copenhagen, is acknowledged for the design.