The effect of maximum voluntary contraction on endurance times for the shoulder girdle

Abstract

This study investigated endurance times as percentages of maximum voluntary contractions (MVCs) in 12 healthy females (mean 25.8±4.3 years) in 5 postures and at 7% MVCs. The shoulder postures utilized were 30/90 (shoulder forward flexion=30° and included elbow angle=90°), 60/90, 90/120, 120/150, and 150/180. The %MVCs were 5%, 15%, 30%, 45%, 60%, 75%, and 90% of MVC at each of these postures. Outcome measures included: endurance times, ratings of perceived exertion, fatigue ratings, pain ratings, and surface electromyography (trapezius and mid-deltoid). As expected the endurance time decreased non-linearly with an increase in %MVC. However, the relationship between endurance time and %MVC differed significantly from Rohmert's curve and suggests it considerably overestimates endurance times for %MVCs <45% and it underestimates endurance times for %MVCs >45%. This study's curve did not become asymptotic even at 5% MVC. Shoulder posture (shoulder forward flexion angle) had a significant effect on endurance time. In general, endurance time decreased with an increase in shoulder flexion angle up to 120° and then it increased. Thus, overhead work (hands above the head) would appear to be better designed with a greater degree of forward flexion, rather than in front of the worker at lower degrees of forward flexion. Relevance to industry Published data on endurance times for the shoulder girdle are scant, yet shoulder injuries are neither rare nor inexpensive. This study completely redefines the relationship between endurance time and %MVC for the shoulder girdle. It will allow for a better definition of worker capabilities for the design of overhead work.