Females Experience Greater Relative but Similar Absolute Neuromuscular Fatigue than Males During a Ramp-Incremental Exercise Test

Sex differences in pulmonary, cardiovascular, or neuromuscular function may affect the development of neuromuscular fatigue and the primary mechanism of exercise intolerance. Unlike the standard cardiopulmonary exercise test (CPET), the musculo-cardio-pulmonary exercise test (mCPET) uses isokinetic power (Piso) to directly assess neuromuscular performance and fatigue. PURPOSE: : To examine sex differences in neuromuscular performance and fatigue during mCPET. We hypothesize that compared to females, males will have greater power (baseline Piso), lower capacity to aerobically support their available muscle power (lower aerobic power index; API), greater reduction in Piso per watt of work performed (higher fatigue index; FI), be similarly limited by neuromuscular performance at the point of exercise intolerance (no power reserve), and have a similar rate of fatigue during the test. METHODS: Participants with a V̇O2peak of 80-120% predicted (FRIEND nomogram) performed an mCPET with and without interleaved Piso measurements. The mCPET measures Piso from brief (5 s) maximal efforts at baseline and the limit of ramp-incremental (RI) exercise and can include interleaved Piso measures to characterize the rate of RI fatigue. Data are presented as mean [SD] and analyzed with two-tailed t-tests and two-way ANOVA. RESULTS: Five males (23 [2] yr; 25 [3] kg/m2; 90 [7] % predicted V̇O2peak) and six females (21 [2] yr; 22 [2] kg/m2; 101 [12] % predicted V̇O2peak) completed both visits. Baseline Piso was greater in males (703 [159] vs 569 [63] W, p=0.09). The API was similar between sexes (40 [4] vs 38 [4] %, p=0.49). The reduction in Piso per RI-watt was lower in males (FI (0.14 [0.06)] vs 0.26 [0.08] %/W, p=0.03); however, both sexes were similarly limited by neuromuscular performance (no power reserve at RI-intolerance: 152 [41] vs 119 [18] % RI-WRpeak, p=0.12). During interleaved mCPET the rate of fatigue was similar between sexes (main effect of time (F(1.3, 11.9) = 30.5, p<0.001), no main effect of sex (F(1, 9) = 2.6, p=0.14), no interaction between sex and time (F(3,27) = 1.1, p=0.35)). CONCLUSION: Despite lower baseline Piso and the decline in neuromuscular performance per watt being greater in women, the dynamics of the decline with time were similar between the sexes.