Abstract
CONTEXT: The abdominal-bracing maneuver, a volitional preemptive abdominal contraction (VPAC) strategy, is commonly used during resisted shoulder exercises. How VPAC affects shoulder-muscle function during resisted shoulder exercise is unknown. OBJECTIVE: To identify the effects of VPAC on selected parascapular and glenohumeral muscles during specific shoulder exercises with or without resistance. DESIGN: Cross-sectional study. SETTING: Clinical biomechanics research laboratory. PATIENTS OR OTHER PARTICIPANTS: Twenty-two asymptomatic volunteers between 18 and 40 years of age. INTERVENTION(S): Participants performed arm elevation in scaption and D1 shoulder-flexion (D1F) patterns with and without resistance and VPAC. MAIN OUTCOME MEASURE(S): Electromyography was used to test the muscle-contraction amplitudes and onset timing of the anterior deltoid, posterior deltoid, upper trapezius, lower trapezius, and serratus anterior. Muscle-response amplitudes were quantified using root mean square electromyography. Shoulder-muscle relative-onset timing was quantified in reference to kinematic elbow-movement initiation. RESULTS: The VPAC increased serratus anterior amplitude during D1F ( P < .001) and scaption ( P < .001) and upper trapezius amplitude ( P < .001) in scaption. All muscle amplitudes increased with resistance. The VPAC decreased muscle-onset latencies for the anterior deltoid ( P < .001), posterior deltoid ( P = .008), upper trapezius ( P = .001), lower trapezius ( P = .006), and serratus anterior ( P = .001) during D1F. In addition, the VPAC decreased muscle-onset latencies for the anterior deltoid ( P < .001), posterior deltoid ( P = .007), upper trapezius ( P < .001), lower trapezius ( P < .001), and serratus anterior ( P < .001) during scaption. CONCLUSIONS: The VPAC affected only the parascapular muscles that had the greatest scapular-stabilizing roles during the specific open chain movement we tested. It decreased latencies in all muscles. These neuromuscular changes may enhance the stability of the shoulder during D1F and scaption exercises.