THE RELATIONSHIP BETWEEN PASSIVE GLENOHUMERAL TOTAL ROTATION AND THE STRENGTH OF THE INTERNAL AND EXTERNAL ROTATOR MUSCLES, A PRELIMINARY STUDY.

BACKGROUND: There is little research on how the amount of shoulder joint range of motion, specifically glenohumeral rotation, may be related to the muscle strength of the rotator cuff muscles. A long held belief is that a joint with excessive range of motion needs sufficient muscular strength for stability. However, no studies have examined this concept. PURPOSE: The purpose of this study was to see if total arc of glenohumeral joint rotation (External rotation [ER]+Internal rotation [IR]) could predict peak isometric muscle strength of the IR or ER muscles of the shoulder. STUDY DESIGN: Cross-sectional study design. METHODS: Fifty-three participants (41 females, 12 males) participated in the study. Passive glenohumeral joint internal rotation and external rotation motion was measured for each participant with a standard goniometer. Isometric muscle force of the ER and IR muscles were tested using a handheld dynamometer in three positions: end range ER, neutral 0°, and end range IR. Data were analyzed using a non-parametric tree based regression method (CART) and then cross-validated. RESULTS: The results showed that those with an increased total arc of motion of glenohumeral rotation (greater than 165.0°) had less muscle isometric muscle strength in all tests positions than those with less glenohumeral rotation. CONCLUSION: Decreased force of the ER and IR muscles of the shoulder was noted in those with increased total arc glenohumeral rotation ( > 165.0°), specifically those with increased glenohumeral internal rotation ( > 80.0°) when compared to those with glenohumeral rotation ( < 165.0°) and glenohumeral internal rotation ( < 80.0°). Future studies should include more males and attempt to develop strategies to assist those with larger excursions of shoulder rotation who may be at risk of developing shoulder problems. LEVEL OF EVIDENCE: Level 2.

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

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.