(5) SEX DIFFERENCES IN MAXIMAL ISOMETRIC STRENTGH FOLLOWING HIGH OR LOW LOAD EXERCISE TO FAILURE

Purpose: The purpose of this study was to investigate differences in fatigue responses between the sexes to high and low load lower body exercise taken to failure.

Methods: Twenty-five resistance trained college aged volunteers (11M/14F; age (mean ± SD): 22 ± 4 yrs.) with no recent history of lower extremity injury volunteered to participate in this randomized, cross-over study, completing 80% (HL) or 30% (LL) of their 1 repetition maximum unilateral leg extension to volitional failure. Each visit was separated by 24 hours (±1 hr.) and the leg utilized and load utilized was randomized and counterbalanced across visits. All visits begun with the establishment of a unilateral 1RM on a plate-loaded leg extension. Following a brief period of rest and several submaximal warm-ups, maximum voluntary isometric contraction (MVIC) strength was measured via a load cell attached to the base of the leg extension machine. After 5-min. rest period, subjects completed 3 sets of unilateral leg extension to volitional failure with their assigned load. Immediately after every failure set, MVIC strength was assessed followed by 2 minutes of rest. A time (Pre/Post Set 1/Post Set 2/Post Set 3) × condition (30%/80%) × sex (M/F) repeated measures ANOVA with Bonferroni post-hoc comparisons was run for utilizing SPSS and the alpha was set a-priori at 0.05.

Results: There was a significant sex × condition × time interaction for MVIC strength (p=0.029). Post-hoc analysis revealed that during LL in males, MVIC strength significantly declined from Pre (694.2 ± 158.2 N) to Post Set 1 (548.4 ± 160.2 N; p< 0.001), before further declining at Post Set 3 (463.7 ± 114.3; p = 0.001). In HL, males MVIC strength significantly declined from Pre (673.1 ± 175.7 N) to Post Set 1 (581.7 ± 141.7 N; p = 0.007) but did not decline any further during the exercise bout (p = 0.410-0.999). Consequently, MVIC strength was significantly greater at Post Set 3 during HL (561.2 ± 159.7) when compared to LL (p< 0.001). In females during LL, MVIC strength significantly declined from Pre (434.1 ± 91.0 N) to Post Set 1 (337.2 ± 91.7 N; p = 0.020) but did not decline any further throughout the exercise bout (p = 0.999). Conversely, during HL, females MVIC strength did not significantly decline from Pre (444.3 ± 103.4 N) until Post Set 3 (361.6 ± 90.0 N; p = 0.002). There were no differences in MVIC strength between HL and LL at the same timepoints in females (p = 0.078-0.670). Finally, males exhibited great MVIC strength than females in both conditions and at all timepoints (p< 0.001-0.003).

Conclusions: The present data indicate that the LL protocol elicited greater fatigue across both sexes when compared to HL. Importantly, there were sex-related differences in time-course of changes in MVIC strength across loading conditions. Overall, our data underscore the existence of sex-related differences in fatiguability of the lower body. PRACTICAL APPLICATIONS: The present data suggest that biological sex should be considered when writing load prescriptions for resistance training, particularly in the lower body musculature. Based on our data, women exhibit greater fatigue resistance, particularly during the later sets of low-load exercise. Finally, our data indicate that females may need additional sets of high-load exercise to achieve similar levels of fatigue to males.

Acknowledgements: None