Low-Level Static Exertions
William S. Marras · 2006
Low-level static exertions have been identified as a risk factor for the development of cumulative trauma disorders or repetitive strain injuries from epidemiological studies. The exposure in terms of static exer- tions in the workplace has been assessed for different jobs and tasks based on electromyographic record- ings from specific muscle groups (Table 14.1). Jobs characterized by relatively high static levels in neck and shoulder showed health outcomes in terms of musculoskeletal disorders in these body regions (Table 14.2). In the 1970s, static contractions of 15% MVC (maximum voluntary contraction) were con- sidered to be tolerated for an “unlimited” period of time for a muscle.1 However, later studies showed that if a contraction is to be maintained for just 1 h, it may have to be as low as 8% MVC.2 A permissible level of static muscle load of 2 to 5% MVC was then suggested.3 However, it was observed that muscu- loskeletal disorders were frequent even in jobs with static levels of this magnitude, and it was suggested to reduce the acceptable static level, for example, by job rotation.4 Static levels as low as 0.5 to 1% MVC may relate to troubles in the shoulder region,5,6 and most recently, statements have been brought forward that static loads are not acceptable at all if sustained frequently or over a long period of time. Actually, “working hours as a risk factor in the development of musculoskeletal complaints” has been proposed.7 Such continuous revision of recommendations can be foreseen if we do not understand why low-level static exertions cause disorders. The acceptable limits or interventions recommended in the workplace will only reduce cumulative trauma disorders if the true risk factors that elicit adverse health outcome are eliminated or minimized. Therefore, it is important to identify which aspect of these so-called low-level static exertions may be the risk factors. In this context, plausibility also plays an important role in risk identification, that is, possible physiological mechanisms of tissue degradation, which may be causally related to the identified risk aspect. The term “low-level static exertions” will be discussed, fol- lowed by a presentation of possible short-and long-term physiological responses. Based on this, preven- tive strategies are presented.