Low-Level Static Exertions - Fundamentals and Assessment Tools for Occupational Ergonomics - page 336

training

injury

Time

rest work 1 work 2 work 3 work 4

FIGURE 14.5 Muscle function in relation to work and recovery (duty circle).

we need guidelines for maximum acceptable time limits for prolonged sustained or repeated muscle con-
tractions. This means that we must focus our attention on how long a time a muscle or group of muscles
can tolerate maintaining or repeating the same low-level exertions. This is especially true when these
same low-level exertions are imposed on a muscle day after day.

14.7 Recommendation for the Practitioner Regarding Job Profile
and Workplace Design

1. The workplace must allow for variation in working postures. This means, for example, that table
and chair are easily adjustable. For instance, shifting frequently between sitting and standing is
recommended and instructions should be given to implement adjustments at frequent time
intervals.
2. The workplace must be designed based on principles of optimization and not minimization of
mechanical workload. Therefore, it is recommended that work cycles include loads ranging from
complete relaxation to moderately high contraction forces and velocities. Workers should be
given the possibility to optimize the phases of a duty cycle according to their capacity. It is of import-
ance that the level or intensity of exertion changes over a wide range continuously over time.
3. The job profile must allow for performing a variety of different tasks. The task variation should
include variation regarding mental load as well as physical (mechanical) load on the musculoske-
letal system. If only specialized work tasks can be performed at each workstation, a job must
include tasks at different workstations. For the use of tools, it is recommended that a variety of
tools with different designs are used interchangeably. In combination, these variations should
cause loading of different body regions and muscle groups regularly over time.

References
1. Rohmert, W. Problems of determination of rest allowances, App. Ergon., 4(3), pp. 158-162, 1973.
2. Bj¨rkst´n, M. and Jonsson, B. Endurance limit of force in long-term intermittent static contractions,
Scand. J. Work. Environ. Health, 3, pp. 23-27, 1977.
3. Jonsson, B. Kinesiology. With special reference to electromyographic kinesiology, in Contemporary
Clinical Neurophysiology., Cobb, W.A. and Van Duijn H., Eds., Elsevier Scientific Publishing
Company, Amsterdam, pp. 417-428, 1978.
4. Jonsson, B. The static load component in muscle work, Eur. J. Appl. Physiol., 57, pp. 305-310, 1988.
5. Veiersted, K.B., Westgaard, R.H., and Andersen, P. Pattern of muscle activity during stereotyped
work and its relation to muscle pain, Int. Arch. Occup. Environ. Health., 62, pp. 31-41, 1990.