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EXAMINATION OF SEX DIFFERENCES IN FATIGABILITY AND ELECTROMYOGRAPHIC RESPONSES DURING SUSTAINED, MAXIMAL, ISOMETRIC LEG EXTENSION

Abstract

BACKGROUND: Despite advances in our understanding of the mechanisms related to neuromuscular performance and fatigability, there remains a paucity of knowledge regarding sex-related differences and subsequent mechanisms related to exercise performance. This study aimed to examine sex-related differences in fatigability and patterns of normalized surface electromyographic amplitude (sEMG AMP) responses during fatiguing, maximal, bilateral, isometric leg extension muscle actions. METHODS: A sample of 20 recreationally active males (n = 10) and females (n=10) performed baseline maximal voluntary isometric contraction (MVIC) testing, followed by a sustained MVIC of the leg extensors until a 50% reduction in force was achieved. Rate of force loss and sEMG AMP for the vastus lateralis on the dominant limb were examined using linear mixed effect models. Specifically, responses were compared between two models where time (continuous) was treated linearly, or as a raw second-order polynomial. RESULTS: The mean ± standard deviation for pre-test force was 193.8 ± 34.4kgf and 142.5 ± 40.2kgf for the males the females, respectively. Statistical analysis revealed that there were differences in the rate of force loss between males and females (β [CI 95%] = 0.53 kgf·s-1 [0.04, 1.02]; t = 2.123; p = 0.034), indicating that males (β [CI 95%] = -1.54 kgf·s-1 [-1.91, -1.17]) experienced a greater rate of reductions in force compared to females (β [CI 95%] = -1.01 kgf·s-1 [-1.34, -0.69]). However, sEMG AMP responses revealed parallel linear decreases between males and females (β [CI 95%] = -0.008 %MVIC [-0.01, -0.005] t = 4.202; p < 0.001). CONCLUSIONS: There are two commonly proposed hypotheses for differences in rate of force loss between males and females; 1) differences in muscle mass, and 2) differences in activation patterns. In accordance with previous data, our data suggest that the differences in rate of force loss may be explained by differences in muscle mass, since differences in rate of force loss were not accompanied by differences in sEMG AMP, a common proxy measure for activation. In addition, greater absolute forces have been shown to increase demand for muscle oxygen, resulting in greater occlusion due to increased compression of tissue. Therefore, it is possible that the absolute force generated by males may have contributed to the differences observed in rate of force loss between males and females.

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