The Effect of Shoulder Abduction Angle on Trapezius Muscle Activity During Scapular Retraction Exercise.

CONTEXT Scapular retraction exercises are often prescribed to enhance scapular stabilization. OBJECTIVES To investigate the upper trapezius (UT), middle trapezius (MT), lower trapezius (LT) activities, and UT/MT and UT/LT ratios during scapular retraction exercise with elastic resistance at different shoulder abduction angles. DESIGN Descriptive laboratory study. SETTING Biomechanical analysis laboratory. PATIENTS OR OTHER PARTICIPANTS Thirty-five asymptomatic individuals. MAIN OUTCOME MEASURES Surface electromyography was used to evaluate UT, MT, and LT activities during the scapular retraction exercise at 0o, 45o, 90o, and 120o shoulder abductions. RESULTS The mean muscle activity ranged from 15.8%-54.7% maximum voluntary isometric contraction (MVIC) for UT, 30.5%-51.6% MVIC for MT, 21.4%-25.5% MVIC for LT. A significant "muscle×angle" interaction was found (p<0.001). Post hoc analysis revealed that the MT was significantly more activated than UT and LT during both retraction at 0o (p<0.001; p=0.01, respectively) and 120o (p=0.03; p=0.002, respectively). During retraction at 45o and 90o, the LT generated significantly lower activity than the UT (p=0.02; p=0.03, respectively) and MT (p<0.001; p=0.002, respectively). Besides, UT/MT and UT/LT ratios during retraction at 0o were significantly lower than 45o (p=0.03; p=0.001, respectively) and 90o (p<0.001; p<0.001, respectively). Retraction at 90o resulted significantly higher UT/LT ratio than 45o (p=0.004) and 120o (p=0.004). CONCLUSIONS Due to lower UT activity relative to MT, retraction at 0o, 45o, and 120o can be preferred in early shoulder training or rehabilitation. Additionally, retraction at 90o was the most effective exercise in activating entire trapezius muscle parts.