Maximum Voluntary Contraction as a Function of Upper Extremity Posture in Females

Abstract

Background: Little has been published on the maximum strength of the shoulder girdle as a function of varying postures. Such information may help aid in job design by better assessing the capabilities of workers.

Methods: Female subjects (n=12) were required to hold a weight in place for 4 seconds in one of 6 different functional postures: 0/90 (shoulder flexion angle = 0° and included elbow angle = 90°), 30/90, 60/90, 90/120, 120/150 and 150/180. Postures and weights were chosen at random and measurements were repeated. Final weight adjustments were in increments of 0.1 lb. At least 3 minutes of rest between lifts was provided. Maximum voluntary contractions (MVCs) were required to be reproduced within 2 lbs. Dynamic lifting of the weight and holding in place MVCs (“lift and hold”), as well as only holding the weight in place MVCs (“hold only”) were assessed separately. Maximum voluntary torques were also computed and adjusted for arm weight. Ratings of perceived exertion (Borg CR-10) and pain ratings (0–10 scale) were obtained.

Results: Maximum voluntary contractions varied greatly, with the strongest to weakest measurements approximating 2:1 ratios. For the 90/120 posture, lift and hold MVC was 14.6±3.0 lb. (range 9.5–19.0). The hold only MVC for 90/120 was 18.1±2.9 lb. (range 13.5–25.3). Similar variability occurred in all postures. Adjusted for arm weight, MVC for lift and hold at 90/120 was 18.1±3.3 lb, while hold only was 21.7±3.2 lb. Across all postures, hold only MVCs were greater than lift and hold MVCs by 4.3 lb. (26%). Lift and hold MVC torque at 120/150 was 217.2±39.3 in-lb. (range 157.5–266.8), while hold only was 280.6±38.0 in-lb. Across all postures, hold only maximum voluntary torques were greater than lift and hold torques by 25%. Except for the 0/90 and 150/180 postures, posture had little effect on hold only maximum voluntary torques. Borg CR-10 ratings averaged 6.6 for lift and hold and 6.9 for hold only (p>0.05). Posture had no effect on pain ratings, which averaged 0.8.

Conclusions: Variability in female shoulder girdle lifting strength is substantial. Removing the lifting portion of job tasks likely substantially increases the percentage of workers capable of performing the tasks. Weakest postures are 90/120 and 120/150 and the female lift and hold MVC for those postures is 15 lb. (hold only 19 lb.). 30/90, 60/90 and 150/180 postures are equally strong; MVCs for lift and hold in those postures are 18 lb. (hold only 23 lb.). When overhead work is required, 150/180 posture appears preferable to 90/120 or 120/150. Some screening or selection process may be necessary on some jobs due to the degree of variability in strength.