Investigating the role of the shoulder musculature during maximum unilateral isometric exertions.

Abstract

The extent to which specific muscles may limit maximum isomet ric force production is largely unknown. This study investigated shoulder muscle activity in six muscles and maximum force generation at the hand in three directions, while in eight different working positions. Ten right hand dominant, university-aged female participants completed twenty-four maximal isometric force hand exertions against a handle positioned by a robot arm within a 3-dimensional simulated workspace. A multivariate, full factorial ANOVA indicated a reach distance main effect where 11% greater force production occurred at the lesser reach distance. A target handle elevation and force direction interaction effect on maximum force production also existed. These findings add to normative static strength data for hand locations typical of an operating work envelope. Although hand force was significantly influenced by position in each direct ion (p < 0.05), muscle EMG was not influenced in any of the six muscles measured. No muscle achieved 100% MVE in any of the tested conditions, with the highest total muscle activity recorded at 86% MVE for the Pectoralis Major - sternal origin. Collectively, these data better demonstrate that the loca tion and direction of work presentation influences force outputs mor e than specific muscle demands (of those measured), and shoul d be considered in evaluating workstations.