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Low Level Static Muscle Contractions in the Workplace

What is the Significance of Low Level Static Muscle Contractions? 

There is a phenomena referred to as Low Level Static Muscle Contractions resulting in muscle fibrosis that is found in many workers today. The effect is becoming prevalent in jobs requiring precision movements and static postures. 

Increasing mechanization of our labor has given rise to this unique form of muscle fatigue. The effect is generalized muscle pains otherwise known as myalgias. If this condition goes unheeded it can cause permanent damage to the muscle. 

What is Muscle Fatigue? 

Muscle fatigue is an important factor in any work situation where physical activity takes place. This activity usually involves a combination of static and dynamic exertions. We can define muscle fatigue as a state in which there is decreased muscle function induced through work overload. 

Low Level Static vs. Dynamic Muscle Contractions? 

Static exertions involve sustained high or low forced contractions of muscles where there is no movement. Regardless of the forces required for any activity, injury may occur if the contraction interval is sufficiently sustained. 

Activities involving dynamic contractions may be repetitive in nature and there is usually loading alternating with unloading of the muscle.  

With the emergence in the work force of high-tech computer jobs we began to see Cumulative Trauma Disorders or repetitive strain injuries with work that involve “low level static exertions.” These exertions are defined in terms of the workers ability to maintain them over several hours. In the work setting they include intermittent static activity or some activity where there are some dynamic contractions of low intensity and slight movements but the activity is prolonged for several hours. 

The Physiology of the Low Level Static Muscle Contraction 

The functional component of a muscle known as the motor unit is composed of the muscle and connected tendon, and the controlling nerve. When the motor unit reaches a point of fatigue there is a rest period required for full recovery. The rest period depends on many variables such as the type of work involved, the intensity of the load and the duration of the sustained muscle contraction.  

Generally, the longer is the contraction the longer is the necessary recuperation time. With fatigue there is a drop off of muscle efficiency and precision that can cause falls and accidents. The muscle overload itself can also damage the muscle. 

Permanent Damage Due to Necrosis of Muscle Cells 

The danger of the low level static exertions is that normal signals of fatigue may be suppressed or ignored and the overloaded muscle fibers are deprived of their corresponding rest cycle. 

As a result there is a damaging effect of circulatory and metabolic mechanisms leading to the death of muscle cells. The long-term result is the replacement of muscle tissue by fibrotic or scar tissue.  

Prevention and Ergonomics 

These findings lead us to consider some workstation recommendations for avoiding these undesirable effects:  

   1) The workstation must allow for changes in postures over frequent time intervals.

   2) The workload should be designed systematically with an optimized work/rest cycle tailored to the individual and the workload should vary continuously over time.

   3) The job profile should include a variety of tasks requiring different tools that shifts the workload to different muscle groups regularly over time.

   4) There should be planned exercise breaks for stretching of the muscles.

   5) Using ergonomic furniture and workstation set-ups that allow for relaxed

Sources:
1) Kroemer, K. H. E and E. Grandjean. “Muscular Work.” Fitting the Task to the Human: A Textbook of Occupational Ergonomics. London: Taylor and Francis, 1997
2) Sjogaard, Gisela and Bente Rona Jensen. “Low Level Static Exertions.” The Occupational Ergonomics Handbook. Ed. Waldemar Karwowski and William S. Marras. Boca Raton: CRC Press, LLC, 1999