Colin W. Kipper1, Kylie N. Sears1, Petra Kis1, Tony R. Montgomery Jr.1, Joshua L. Keller2, & Shane M. Hammer 1

1 School of Kinesiology, Applied Health, and Recreation, Oklahoma State University, Stillwater, Oklahoma, 2 School of Health, Kinesiology, And Sport, University of South Alabama, Mobile, Alabama

Females are often reported to have greater exercise tolerance at relative intensities than males. Differences in microvascular oxygen extraction across the transition from relaxation to contraction during rhythmic exercise may contribute to sex differences exercise tolerance. PURPOSE: To test the hypothesis that females would have significantly greater contraction-induced oxygen extraction compared to males at the end-test torque plateau during intermittent, maximal-effort contractions. METHODS: 8 males (23 ± 3 yrs) and 8 females (20 ± 1 yrs) performed intermittent (3 s contraction and 3 s relaxation), isometric forearm flexion with maximal effort for 5 minutes. Oxygenated- and deoxygenated-[heme] of the biceps brachii were measured by frequency-domain near-infrared spectroscopy. Changes in [heme] were normalized to the predicted total-[heme] at a hypothetical adipose tissue thickness (ATT) of 0 mm (367 µM) determined by linear regression between ATT and resting total-[heme]. Oxygen extraction was defined as the average difference between oxygenated- and deoxygenated-[heme] from relaxation to contraction ([heme]diff/2) and averaged across three complete contraction cycles at the start of the test and the end of each minute. A two-way ANOVA was used to compare oxygen extraction between males and females at each timepoint. RESULTS: End-test torque was lower in females compared to males (28.9 ± 7.4 vs. 49.5 ± 13.5 Nm; p < 0.01); however, end-test torque relative to maximal torque production was not different between sexes (F: 62 ± 11 vs. M: 58 ± 12%, p > 0.05). No sex differences were detected in contraction-induced oxygen extraction at any timepoint (p > 0.05). After normalizing to maximal-effort torque, contraction-induced microvascular oxygen extraction was greater in females compared to males at end-test (1.19 ± 0.74 vs. 0.53 ± 0.58 µM/Nm; p < 0.05). CONCLUSION: These data suggest that females may extract higher levels of oxygen for a given absolute level of torque production compared to males. Combined with previous data suggesting females incur lower relative reductions in muscle blood flow from contraction, these data extend the hypothesis of oxygen-dependent sex differences exercise tolerance.

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