Shoulder muscle activation patterns and levels differ between open and closed-chain abduction

Open and closed-chain abduction of the shoulder are commonly used in rehabilitation and exercise programs to assess and/or improve shoulder muscle function. However, it is not known if shoulder muscle activation patterns differ between these two exercises. Therefore the purpose of this study was to compare muscle activation patterns during closed-chain shoulder abduction performed using a shoulder press machine with open-chain abduction using free weights.Experimental study.Open and closed-chain abduction were performed by 15 and 14 subjects respectively at low (25%), medium (50%) and high (75%) load. Surface and indwelling electrodes were used to record the activation pattern of seven shoulder muscles during the concentric phase of each exercise. Data were normalised to maximum voluntary contractions (MVC), time normalised and compared over the common range of motion (40°-140° abduction).Only the activation pattern of middle deltoid had a strong positive correlation between exercises (r≥0.65, p<0.05) with similar activation levels at all loads (35%, 50% and 60% MVC, p=1.0). All other muscles tested had inconsistent, low or negative correlations between exercises. Significantly lower average activation levels were recorded during closed-chain abduction for subscapularis at all loads, upper trapezius at medium and high loads and infraspinatus and lower trapezius at high load (p <0.05).Open-chain abduction is required to facilitate the stabilising role of the rotator cuff and axioscapular muscles, in response to middle deltoid activity. Closed-chain exercises may enable full range shoulder abduction earlier in rehabilitation programs, with an inherent stability and less demand on the rotator cuff.