Barstow, Thomas J.

ABSTRACTExtreme‐intensity exercise is described by W′ext (analogous to J′ for isometric exercise) that is smaller than W′ of severe‐intensity exercise (W′sev) in males. Sex differences in exercise tolerance appear to diminish at near‐maximal exercise, however, there is evidence of greater contributions of peripheral fatigue (i.e. potentiated twitch force; Qpot) in males during extreme‐intensity exercise. Therefore, the current study tested the hypotheses that J′ext would not be different between males and females, however, males would exhibit a greater reduction in neuromuscular function (i.e. maximal voluntary contraction, MVC; Qpot) following extreme‐intensity exercise. Seven males and 7 females completed three severe‐ (Tlim: 2–4 min, S3; 5–8 min, S2; 9–15 min, S1) and three extreme‐intensity (70, 80, 90%MVC) knee‐extension bouts. MVC and Qpot relative to baseline were compared at task failure and at 150 s of recovery. J′ext was significantly less than J′sev in males (2.4 ± 1.2kJ vs 3.9 ± 1.3kJ; p = 0.03) and females (1.6 ± 0.8kJ vs 2.9 ± 1.7kJ; p = 0.05); however, there were no sex differences in J′ext or J′sev. MVC (%Baseline) was greater at task failure following extreme‐intensity exercise (76.5 ± 20.0% vs 51.5 ± 11.5% in males, 75.7 ± 19.4% vs 66.7 ± 17.4% in females), but was not different at 150 s of recovery (95.7 ± 11.8% in males, 91.1 ± 14.2% in females). Reduction in Qpot, however, was greater in males (51.9 ± 16.3% vs 60.6 ± 15.5%) and was significantly correlated with J′ext (r2 = 0.90, p < 0.001). Although there were no differences in the magnitude of J′ext, differences in MVC and Qpot are evidence of sex‐specific responses and highlight the importance of appropriately characterizing exercise intensity regarding exercise domains when comparing physiological responses in males and females.Highlights We have previously shown evidence that extreme‐intensity dynamic exercise is described by W′ext in males and smaller than W′sev. We currently tested for potential sex differences in J′ext (isometric analogue to W′) and neuromuscular responses (i.e. maximal voluntary contraction, MVC; potentiated twitch force, Qpot) during extreme‐intensity exercise. J′ext and extreme‐intensity exercise tolerance was not different between males and females. The reduction in MVC was not different across extreme‐intensity exercise across males and females, whereas the reduction in Qpot was greater in males following all extreme‐intensity exercises, although not after exercise at 90%MVC. Together, although extreme‐intensity exercise tolerance is not different, these data highlight differences in the contributing mechanisms of fatigue during severe‐ and extreme‐intensity exercise between males and females.

We have previously shown that, similar to the severe‐intensity power duration relationship (i.e., W’ sev ), there exists a separate, smaller power‐duration relationship for the extreme‐intensity domain (W’­ ext ) in men. We have also shown that there may be differences in contributions of central and peripheral fatigue between men and women during and immediately following extreme‐intensity exercise. The current study tested the hypotheses that 1) the W’ ext would be significantly less than W’ sev in both sexes, but not different between men and women, 2) absolute Maximum Voluntary Contractions (MVC) would be reduced to greater extent at end‐exercise following severe‐ and extreme‐intensity exercise in men compared to women but not when scaled relative to baseline, and 3) peripheral fatigue would be greater at task failure (T lim ) in men compared to women following severe‐intensity exercise, with no sex difference following extreme‐intensity exercise. Following familiarization, 7 men and 7 women completed three severe‐ (T lim : 2‐4 min, S3; 5‐7 min, S2; 8‐12 min, S1) and three extreme‐intensity (70, 80, 90% MVC) isometric knee‐extension bouts to task failure. MVC and potentiated twitch force (Q pot ) were measured and compared at baseline, task failure, and 150 s of recovery. W’ ext was significantly smaller compared to W’ sev in men (2.4 ± 1.2 kJ vs 3.9 ± 1.3 kJ; p = 0.03) and in women (1.6 ± 0.8 kJ vs 2.9 ± 1.7 kJ; p = 0.05), however there were no sex differences in either W’ ext or W’­ sev . There were no sex differences in MVC at task failure following severe‐intensity exercise. However, MVC was lower in women following extreme‐intensity exercise compared to men, although there were no sex differences in MVC relative to baseline. Further, there were no differences in MVC by the end of recovery compared to baseline in men or women following extreme‐intensity exercise. Q pot was not different at task failure following severe‐ and extreme‐intensity exercise between men and women. However, men showed lower Q pot relative to baseline, suggesting greater peripheral fatigue development, with the exception of exercise at 90% MVC. These data provide evidence that important sex differences exist in the contributions of peripheral fatigue to task failure and in recovery from exercise which depend on exercise intensity. Therefore, practitioners need to consider exercise intensity and the time course of recovery when prescribing exercise to men and women.

Introduction Age related declines in oxidative metabolic capacity have been attributed to slowing of O 2 delivery and utilization. While these declines are pronounced in postmenopausal women, it is unclear how these changes compare with age‐matched men. This study aimed to phenotype adjustments of muscle oxy‐ and‐ deoxygenation during transitions of low intensity fixed‐load exercise in postmenopausal women and age‐matched men. Methods Healthy postmenopausal women (PW) and men (M) (N = 7 and 6; age 57 ± 7 and 61 ± 10 years; BMI 26 ± 2 and 27 ± 4 kg/m 2 , respectively, all P >0.05) completed 3 identical 8 min transients of fixed pedal rate (=35) and load (=20W) ergometry wearing bilateral thigh cuffs. After 5 min of rest, exercise began with 3 min without thigh cuff inflation followed by 5 min of exercise+cuff inflation at 90 mm Hg (to provoke locomotor congestion). Deoxy (HHb)‐ and total (Hb tot )‐ hemoglobin were measured by near‐infrared spectroscopy (NIRS) at the right calf. Data were sampled at 1 s intervals and ensemble averaged across 3 transients and binned to 10 s epochs for each participant prior to mono‐exponential modeling: Y B + A [1 − e −(t‐TD)/τ ]; where Y B = baseline value of Y for the 30 s prior to the step transition; τ = time constant; A = asymptote; TD = time delay. Mean response time (MRT) = TD + τ. Kinetics were modelled for the transition from exercise without to exercise+cuff inflation. Results Following the exercise on‐transient, TD was 9±4 vs. 7±3 s (P=0.35) and 8±4 vs. 6±3 s (P=0.28) for PW vs. M prior to the rise in ΔHHb or ΔHb tot , respectively. For PW vs. M, the A representing the rise in ΔHHb was 8±7 vs. 15±7 μM (P=0.04) and for ΔHb tot it was 13±8 vs. 18±8 μM (P=0.17), respectively. The kinetics of HHb following the exercise+cuff onset were similar for PW and M (τ=53±38 vs. 34±9 s, respectively, P=0.36) as well as for Hb tot (τ=43±29 vs. 29±16 s, respectively, P=0.43). The overall time course for the change in HHb (MRT=62±35 vs. 41±8 s, P=0.27) or Hb tot (MRT=51±27 vs. 35±14 s, P=0.31) did not differ for PW and M, respectively. Conclusions Despite age‐matched men demonstrating greater ΔHHb compared with PW, these data do not support the hypothesis that following exercise onset, the rate of locomotor hemodynamic adjustment and O 2 extraction for a given metabolic demand differ between these groups. Thus, as opposed to gross factors of blood flow and O 2 delivery, effects such as slowing of the rate of activation and/or processes regulating oxidative phosphorylation and mitochondrial respiration may play a stronger role in explaining differences in oxidative metabolic capacity between PW and M. Support or Funding Information Funding for this work was supported by National Institutes of Health [RO1‐HL126638 to TPO]; and American Heart Association [16POST30260021 to EHV] and National Institutes of Health [HL138814 to EHV]. This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .