Contraction mode and intensity effects on elbow antagonist muscle co-activation

Introduction: Agonist muscle contraction mode and intensity effects on antagonist co-activation was examined between men and women. Methods: Fifteen healthy young men (mean +/- standard deviation age = 24.9 +/- 4.2 years, body mass index = 23.17 +/- 2.34) and 15 women (mean +/- standard deviation age = 21.8 +/- 2.3 years, body mass index = 24.16 +/- 2.91) performed five isokinetic concentric and eccentric maximal-effort elbow flexor/extensor contractions to establish their peak torque. Participants then performed a series of randomly ordered sub-maximal (10-90% of peak torque, 10% increments) elbow flexor contractions over two separate experimental sessions. All sub-maximal elbow flexor contractions were concentric during the first session, while eccentric contractions were performed during the second session. Antagonist co-activation was quantified as the elbow extensor surface electromyogram (EMG) magnitude during all flexor contractions, normalized to its' respective MVC level, when acting as an agonist. Results: The results demonstrated significant contraction intensity (p < 0.001), gender (p < 0.001) and contraction mode (p < 0.001) main effects, indicating that co-activation: (1) increased from 10- 90% MVC (5.40% to 12.01%), (2) was greater in women than men (12.06% vs 3.68%), and (3) was greater during concentric than eccentric contractions (9.82% vs 5.92%), without correcting for skinfold thickness. A significant gender by contraction intensity interaction demonstrated that women displayed greater increases in co-activation, as compared to the men, across 10-90% MVC. Following correction for skinfold thickness, the gender difference was not found to be statistically significant. Discussion The major findings demonstrated that antagonist muscle co-activation demonstrated a dependency on contraction intensity and mode; however, a gender difference was not observed when corrected for skinfold thickness.