Shoulder reaction time and muscle-firing patterns in response to an anterior translation force

The primary restraint preventing humeral head translation is the capsuloligamentous system. Muscle forces can also decrease translation; however, the timing and magnitude of muscle response has not been previously reported. Fine wire electromyographic analysis of the biceps long head, anterior deltoid, pectoralis major, latissimus dorsi, and rotator cuff muscles was performed after an anterior translation force was applied to 15 normal shoulders. The reflex response time (time to 5% maximal muscle test), the protection response time (time to 20% maximal muscle test), the duration of the protection response, and the magnitude of the protection response were calculated. The shoulder reaction data showed 2 consistent patterns. Activation of the anteriorly located muscles preceded the posteriorly located muscles, and the rotator cuff muscles fired with greater magnitude than the more peripherally located muscles.