The influence of muscle length on one-joint shoulder internal and external rotator muscle strength

Abstract

Background and purpose: Kendall suggests testing the rotator cuff muscles in their maximally shortened position, since one-joint muscles are thought to be strongest. We found little evidence to support this concept. The purpose of this study was to determine if the shoulder internal rotator (IR) and external rotator (ER) muscles are strongest when placed in their shortened length position. Methods: Fifty-three subjects participated. Glenohumeral joint internal rotation and external rotation motion was measured. Muscle strength was then tested using a hand-held dynamometer in four positions: (1) end-range ER; (2) neutral 0°; (3) glenohumeral joint mid-range and (4) end-range IR. Data were analyzed using two repeated measures ANOVA’s. Results: The results suggest that rotator muscle strength is dependent on muscle length. IR strength was weakest at end-range IR in its shortest length; ER muscle strength was weakest at end-range ER in its shortest length. Muscle strength of the IR or ER was not significantly different when comparing neutral 0° to the mid-range position and at their most lengthened position. Conclusion: The IR and ER muscles were found to be weakest when placed in a position of shortest muscle length, while the neutral 0° and mid-range positions were the strongest positions.