Sex differences in quadriceps and hamstrings EMG-moment relationships.

Purpose: To evaluate sex differences in quadriceps and hamstrings muscle EMG-moment relationships when the muscles were acting as agonists and antagonists across the range of contraction intensity.

Methods: Twenty-two age- and activity-level-matched young people (11 females, 11 males) with no history of serious lower extremity injuries participated in this study. Muscle-specific EMG-moment relationships were determined for the quadriceps and hamstrings muscles when acting as agonists and antagonists during isometric target matching at 10 loads ranging from 10% to 100% peak moment. Sex differences in quadriceps and hamstrings muscle activation were assessed across the normalized moment spectrum.

Results: Females had significantly higher vastus medialis activity than males during knee extension trials at 10%, 20%, and 30% peak moment (P < or = 0.05). Significant sex differences were broadly observed in the subjects' quadriceps muscle EMG-moment relationships (females displayed higher activity) during knee flexion trials (P < 0.05). Conversely, no sex differences were observed in the subjects' hamstrings muscle EMG-moment relationships. The shape of the EMG-moment relationships in agonist contractions were variable with linear patterns observed in the rectus femoris, semitendinosus, and biceps femoris muscles, and nonlinear patterns observed in the vastus medialis and vastus lateralis muscles. Antagonistic muscle activity increased with increases in moment magnitude.

Conclusions: The results of this study provide evidence of some sex differences in quadriceps muscle EMG-moment relationships. Conversely, the activation patterns for the hamstrings muscles were similar between the sexes. The consistent association between antagonist activity patterns and moment magnitudes supports the idea that the control of agonist-antagonist activity in the thigh muscles is linked.