No sex differences in involuntary contractile properties after fatiguing isometric and isotonic exercise for the plantar flexors

Introduction: Females tend to be more fatigue resistant during isometric contractions, however, little research has investigated sex differences using dynamic contractions. Further, the plantar flexors may not be as prone to sex differences in fatigability compared to the knee extensors. The purpose of the study was to investigate potential sex differences in contractile properties after a sustained maximal voluntary isometric contraction (MVIC) and isotonic contractions. Methods: Twenty-seven healthy (18-30 years) males (n = 14) and females (n = 13) completed a sustained 2-min MVIC and 120 isotonic contractions for the plantar flexors on separate days. Before and after fatigue, electrical stimulation of the tibial nerve was performed to obtain contractile properties. Peak twitch and the time taken to achieve peak twitch (TTP) were calculated from a dynamometer torque signal. Results: After isometric fatigue, peak twitch (males: -27.2 ± 13.6%; females: -30.4 ± 16.1%; p=0.58) and time to peak twitch (TPT) (males: +33.5 ± 29.2%; females: +44.8 ± 36.9%; p=0.64) demonstrated similar changes between sexes. Similarly, changes in peak twitch (males: -20.0 ± 15.2%; females: -18.1 ± 11.5%; p=0.72) and TPT (males: +27.1 ± 28.8%; females: +33.8 ± 24.0%; p=0.13) were not different between sexes after isotonic fatigue. Conclusions: Males and females showed similar changes in contractile properties after isometric and isotonic contractions signifying a similar degree of peripheral fatigue between sexes. The greater diversity of muscle fiber types, compared to other muscle groups, for the plantar flexors may at least partially explain our findings