Muscle activation in coupled scapulohumeral motions in the high performance tennis serve.

OBJECTIVE To evaluate muscle activation patterns in selected scapulohumeral muscles in the tennis serve. These patterns of muscle activation have not been evaluated in other studies of the tennis serve. Fine wire and surface EMG was used to calculate onset and offset timing of muscle activation. DESIGN Controlled laboratory study. SETTING Biomechanical laboratory. SUBJECTS 16 tennis players (age 18-40) with rated skills (National Tennis Rating Program (NTRP) rating 4.5-6.5; club tournament level or higher) were subjects. MAIN OUTCOMES MEASURE Dependent variables of muscle activation onset and offset as well as sequencing of the stabilising muscles of the scapula (upper trapezius, lower trapezius, serratus anterior; the muscles that position the arm) anterior deltoid and posterior deltoid; and the muscles of the rotator cuff muscles (supraspinatus, infraspinatus, teres minor) during the tennis serve motion. RESULTS Patterns of muscle activation were observed during the tennis serve motion. The serratus anterior (-287 ms before ball impact) and upper trapezius (-234 ms) were active in the early cocking phase, while the lower trapezius (-120 ms) was activated in the late cocking phase just before the acceleration phase. The anterior deltoid (-250 ms) was activated in early cocking, while the posterior deltoid (-157 ms) was activated later. The teres minor (-214 ms) was activated early in the cocking phase. The supraspinatus (-103 ms) was activated in late cocking. The infraspinatus (+47 ms after ball impact) was activated in follow-through. All muscles except infraspinatus were activated in duration of more than 50% of the service motion. CONCLUSIONS This study demonstrates that there are patterns of activation of muscles around the scapulohumeral articulation in the normal accomplished tennis serve. Rehabilitation and conditioning programs for tennis players should be structured to restore and optimise the activation sequences (scapular stabilisers before rotator cuff), task specific functions (serratus anterior as a retractor of the scapula, lower trapezius as a scapular stabiliser in the elevated rotating arm) and duration of activation of these muscles.