Isolated Activation of the Infraspinatus Muscle in Four Manual Muscle Testing Positions

Isolated Activation of the Infraspinatus Muscle in Four Manual Muscle Testing Positions

Background To normalize infraspinatus muscle activity among different exercises or positions, it is necessary to isolate the infraspinatus from the surrounding musculature. Purpose We sought to determine the best position in which to obtain an isolated infraspinatus electromyography (EMG) signal for normalization of the infraspinatus muscle in four selected positions. Study design Comparative, repeated measures design. Methods To measure infraspinatus maximal voluntary muscle contraction (MVIC), 13 healthy participants performed four test positions (side-lying with the shoulder adducted, prone, shoulder abducted, and side-lying wiper external rotation [SWE]). EMG activity was measured in the infraspinatus, middle trapezius, and posterior deltoid muscles. Results The EMG ratio was significantly higher for SWE than for the other test positions. To maximize the EMG ratio, high infraspinatus activity and low surrounding muscle (middle trapezius and posterior deltoid) activity are required. Conclusions We recommend switching from prone external rotation to SWE to normalize the infraspinatus EMG signal.