Sex Differences In Contraction-induced Microvascular Oxygen Extraction During Intermittent Maximal-effort Contractions

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 an d 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 ± 0.74vs. 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 reduction s in muscle blood flow from contraction, these data extend the hypothesis of oxygen -dependent sex differences exercise tolerance.