Trunk muscle activation in open stance and square stance tennis forehands.

Electromyography of the trunk muscles were compared between the open and square stance forehand drives of 14 collegiate tennis players. Surface EMG were bilaterally collected from the rectus abdominis (RA), external oblique (EO), and erector spinae (ES) in open and square stance forehand drives. EMG data were transferred by telemetry, 12 bit A/D converted at 1000 Hz, and stored for analysis. Rectified and smoothed EMG data were normalized (NEMG) to maximal isometric voluntary contractions and mean NEMG were calculated during the forward swing and follow through phases of the stroke. A 2 x 2 x 2 x 6 factorial ANOVA (Gender, Stance, Phase, Muscle) with repeated measures on Subject showed significant (p < 0.05) effects of Gender, Muscle, Phase, and several interactions. The nonsignificant differences in muscle activation between stances did not support the belief of tennis experts that open stance forehands require greater trunk activation than square stance forehands. Mean NEMG of the ES were significantly (p < 0.05) larger than EO or RA, which was consistent with observations of tennis-specific strength imbalances and increasing incidence of low back injuries in tennis.