The purpose of this study was to compare the external oblique and erector spinae muscle activation in the open stance and closed stance tennis two-handed backhands, with the aim to provide objective evidence of the hypothesis of the reliance on greater trunk muscle contribution in the open stance two-handed backhand compared to the closed stance two-handed backhand. Ten highly experienced nationally ranked junior tennis players participated in this study. Surface electromyography was bilaterally collected from the external oblique and erector spinae in open and closed two-handed backhands. Rectified and smooth EMG signals were averaged by using the onset of the ball-racquet impact as time zero, recorded from the strain gauge signal. The averaging process for each trial included a minimum of 25 strokes. A 2 x 3 x 4 repeated measures MANOVA (Stance, Phase, Muscle) showed significant (p<. 05) effects on muscle, phase and several interactions. The non-significant differences in muscle activation between stances did not support the belief of tennis experts that open stance two-handed backhand require greater trunk activation than the closed stance two-handed backhands. However, post-hoc tests indicated that there were significant differences in the activation of specific muscles during the three phases of the stroke. During the back swing phase in the open stance technique the greatest average muscle activation was found in the right external oblique (M= 21.65) and the lowest in the left external oblique muscle (M = 9.38). These findings suggest an imbalance in muscle agonist and antagonist ratios. On the open stance technique the greatest muscle activity was found during the forward swing in the right erector spinae (M=20.02) and the least in the left erector spinae (M=8.68), resulting in an agonist and antagonist imbalance. During the follow through phase the activity of the right external oblique and left external oblique were significantly greater in the closed stance than in the open stance backhand, indicating that the erector spinae may act eccentrically in order to stabilize and decelerate the trunk the. Follow-up Paired Samples t-tests indicated that in the closed stance the trunk extensor muscles activation was significantly greater than the abdominal muscles, which was consistent with previous observations of tennis-specific strength imbalances. Muscle imbalances between the dominant and non-dominant side musculature of the trunk were found in both techniques. The findings of this study should help coach scientists and clinicians to further develop rehabilitation and preventative strength and conditioning programmes, aimed at reducing these imbalances.
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