Human muscle activity related to non-biomechanical factors in the workplace

Abstract

This paper presents current knowledge on low-level, long-lasting work-related muscle activity, focusing on the shoulder and the upper part of the trapezius muscle, and on mental, rather than biomechanical reasons for the muscle activation. The paper identifies three sources of vocational muscle activity: the biomechanical need for force production in order to perform movements or maintain postures against the force of gravity, the biomechanical need to stabilise body parts as a reference for performing movements and securing a stable visual field, and finally, muscle activity without obvious biomechanical purposes. This last category has been labelled non-biomechanical muscle activity in this review. Non-biomechanical muscle activity is related to the mental load, the emotional load and the individual characteristics of the subject, and is identified as having a low-level and a low second-to-second variability, resembling a static muscle contraction. Recent research has indicated that the size principle for motor unit recruitment order puts a strain on a limited number of low-threshold motor units which might be heavily taxed despite the overall low level of this muscle activity. However, the paper also cites a recent report showing that motor unit substitution may occur in prolonged low-level muscle activation (longer than a few minutes). Evaluations of muscle load at work usually omit the possibility of extra muscle activation due to nonbiomechanical factors, and thus may often give estimates of the muscle load that are too low, or misinterpret nonbiomechanical muscle activity as biomechanical muscle load.

Physiological responses to four hours of low-level repetitive work

“…Previously observed attention-related muscle activity (14) was also shown for forearm muscles in our study. This finding indicates that the effect of mental demands on muscle activity is a more general effect on all the active muscles, as observed in a recent study (15).…”

“…Hence, our study revealed no straightforward relationship between muscle activity and external force during combined attention-demanding low-level physical work. Therefore the study supported the hypothesis that the biomechanical need to stabilize body parts and factors that are not directly linked to biomechanical needs (eg, mental load, emotional load, and trait characteristics of the participant) also contribute to muscle activity (14). Thus assessments of workloads based primarily on external force measurements during a low-level repetitive task including demands for attention may lead to the wrong conclusions.…”

“…Several studies have shown increased muscle activity in response to mental demands, as reviewed by Waersted (14). It has been debated in the literature whether some muscles are more sensitive to mental demands during work than others.…”

Effects of precision demands and mental pressure on muscle activation and hand forces in computer mouse tasks

“…Previous studies have also shown effects of work pressure (Waersted 2000, Bansevicius et al 2001, Lundberg 2002. It should be noted that a wide range of stressors, some related to the actual task and others additional to the task, were used in these studies.…”

Repetitive and Monotonous Work Among Women: Psychophysiological and Subjective Stress Reactions, Muscle Activity and Neck and Shoulder Pain

“…The former method is widely used in ergonomic field studies evaluating work-related muscle activity through ambulatory monitoring of EMG, while the latter is confined to laboratory research and clinical applications (Sandsjö, 2004; For a review of electromyographic techniques, see Basmajian & De Luca, 1985) 6 PSYCHOLOGICAL FACTORS, STRESS AND MUSCLE ACTIVITY Obviously, muscle activity is needed to perform different work tasks. This muscle activity is related to biomechanical needs for force production (that enable movements) and stabilization of body parts (i.e., postural stabilization) (Waersted, 2000). However, besides this muscle activity related to biomechanical needs, vocational muscle activity also includes another type of muscle activity, namely one that is suggested to be related to mental load, emotional load and individual characteristics.…”