Sex difference in force generation capacity during repeated maximal knee extensions.

The force generation capacity, during 50 repeated maximal knee extensions with a constant velocity of 3.14 rad.s-1, and cross-sectional area (CSA) of the quadriceps femoris muscles were determined for untrained women (n = 36) and men (n = 27) aged from 18 to 25 years. As force scores in the maximal repetitions, the mean values of force (F) of every 5 consecutive and all trials and the percentage of decline of F (%D) with 50 contractions were calculated. The CSA was measured by using a B-mode ultrasound technique at the midpoint of the thigh length. The F decreased from 303 (SEM 13) N at the 1st-5th trial to 155 (SEM 9) N at the 46th-50th trial for the men, and from 202 (SEM 9) N to 94 (SEM 4) N for the women. The F of every 5 consecutive and all trials were significantly correlated to muscle CSA: for the men r = 0.552-0.872 (P < 0.01) and for the women r = 0.609-0.857 (P < 0.01). The men showed significantly higher F at every 5 consecutive trials than the women even when calculated per unit muscle CSA (F.CSA-1). There were significant correlations between %D and F.CSA-1 at the 1st-5th trial: r = 0.538 (P < 0.01) for the men and r = 0.631 (P < 0.01) for the women, respectively. The average values of %D were almost the same in both sexes: for the men 48 (SEM 3) % and for the women 52 (SEM 2) %, respectively. However, an ANCOVA calculation on %D, using F.CSA-1 at the 1st-5th trial as the covariate, indicated that the women had significantly higher %D than the men. Thus, the force output during the maximal repetitions was significantly correlated to the CSA of quadriceps femoris in both sexes. The force output was lower in the women than in the men even when the difference in the muscle CSA was allowed for. The women had higher %D than the men when force output per unit muscle CSA during the initial 5 repetitions was compared.