(3) SEX-SPECIFIC DIFFERENCES IN FATIGABILITY DURING BILATERAL VS UNILATERAL MAXIMAL ISOMETRIC EXERCISE TO TASK FAILURE

Women are typically less fatigable than men during submaximal isometric exercise to task failure. However, the prevailing mechanisms and functional consequences related to differences in sex-specific fatigability have not been fully elucidated. Some evidence suggests that differences in fatigability between men and women depend on the intensity of contraction during isometric exercise. Further, independent of sex, it has been suggested that the total muscle mass utilized (i.e., bilateral vs unilateral) during exercise influences fatigability.

Purpose: The purpose of this study was to identify the effects of unilateral versus bilateral contractions on time to task failure (TTF) between men and women during fatiguing maximal isometric voluntary contractions (MVIC) of the leg extensors.

Methods: 16 recreationally trained men (mean±SD; age=24.38±3.62, n=8) and women (age=22.88±2.70, n=8) were recruited for this study. On separate visits, participants performed, either bilateral or unilateral (dominant limb) pre-test MVICs of the leg extensors at a joint angle of ~110 degrees, followed by a maximal (100% of pre-test MVIC), isometric hold to task failure under either bilateral (mean±SD; Men=203.37±66.98kg; Women=127.75±56.52kg) or unilateral conditions (Men=102.35±28.76kg; Women=77.22±22.32kg). Task failure was determined by a force loss threshold that corresponded with 50% of the participants pre-test MVIC force. The conditions were performed in a randomized order, separated by a minimum of 48 hours. A sex (men vs women) x condition (bilateral vs unilateral) mixed analysis of variance (mixed-model ANOVA) was used to examine differences in TTF. To supplement null-hypothesis testing, eta- (η²), partial eta-squared (ηp²), and Cohen’s d (d) effect sizes were calculated. Significance was set at p< 0.05.

Results: There was no sex x condition interaction (p=0.844; ηp²=0.003) or main effect of condition (p=0.519; η²=0.003, mean diff=2.33±7.56s; d=0.145 [95%CI: -0.314, 0.604]). However, significant differences in TTF were observed between men and women (Figure 1; p=0.003; η²=0.421). The significant main effect for sex indicated that the TTF was longer for the women (TTF=67.60±11s) than the men (TTF=41.76±11s) (mean diff=25.84±15.56s; d=1.602 [95%CI: 0.439, 2.764]).

Conclusion: Similar to previous findings for fatiguing submaximal isometric exercise, these data indicated that women exhibited greater fatigue resistance compared to men; perhaps due to differences in the contractile properties of muscle (i.e., proportional area of slow-twitch fibers). Interestingly, unlike previous reports for submaximal exercise, TTF for maximal isometric exercise was not different between bilateral and unilateral conditions. PRACTICAL APPLICATIONS: Sex may contribute to differences in TTF during fatiguing, maximal isometric exercise. Thus, practitioners should consider the potential influence of sex when designing any testing or training protocol involving isometric exercise.

Acknowledgements: None