Aspects of shoulder function in relation to exposure demands and fatigue -a mini review

Objective. To discuss the literature on control and function of multiple mu scle systems with special focus on shoulder and upper extremity under subma ximal conditions. Design. The paper is a mini review based on 31 studies. Background. Control mechanisms underlying the recruitment and gradation of muscle activity in complex multiple muscle systems during various voluntary exertions is still not fully understood. Load sharing principles may be in fluenced by several factors like work demand, fatigability, metabolic facto rs etc. Methods. Several methods, e.g., electromyography, intramuscular pressure, a nd tissue oxygenation are used. The definition of fatigue is discussed. Results and conclusion. A relatively fixed load sharing of the shoulder mus cles has been found at low load levels, submaximal speeds and with a limite d range of movements of the arms for healthy subjects. However, the load sh aring of the shoulder muscles can be changed to some extent in response to fatigue, mental demands, visual feedback and in patients suffering from dis orders in the upper extremities. It is hypothesized that lack of ability to redistribute muscle activation pattern in the shoulder region or the upper extremity increases the risk of development of work related symptoms. (C) 2000 Elsevier Science Ltd. All rights reserved.