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Blood properties influence aerobic exercise performance. While vascular volumes and hemoglobin mass (Hbmass) are elevated in trained individuals, evidence of sex differences in vascular volumes is equivocal due to inadequate matching of aerobic fitness between males and females. This cross-sectional study aimed to compare hematological values normalized to body mass (BM) and fat-free mass (FFM) between males (n = 45) and females (n = 34) matched for aerobic fitness (V̇O2max) normalized to FFM (mL∙kg FFM−1∙min−1). Data included body composition measured by dual-energy X-ray absorptiometry (DXA), V̇O2max from an incremental test, and hematological values derived from a CO rebreathe test. Fat mass was unrelated to blood volume (BV; R2 = 0.02, P = 0.26) and Hbmass (R2 = 0.03, P = 0.16), while FFM was the strongest predictor of both (R2 = 0.75 and R2 = 0.83, respectively, P < 0.001). Females exhibited higher FFM-normalized BV (+4%, P < 0.05) and plasma volume (PV) (+14%, P < 0.001) and lower red blood cell volume (RBCV) (−8%, P < 0.001) and Hbmass (−8%, P < 0.001) compared to males. Positive correlations between aerobic fitness and relative Hbmass and BV were observed in both sexes when normalized to BM and FFM (0.48 < r < 0.71; P < 0.003). Stepwise multiple regression models, including FFM, V̇O2max, height, and [Hb], provided accurate predictions of Hbmass (R2 = 0.91) and BV (R2 = 0.85). Overall, sex differences persist in relative Hbmass, BV, PV, and RBCV after matching of aerobic fitness, though relative BV and PV were greater in females. These findings suggest sex-specific strategies in oxygen delivery and/or extraction, and they underscore the importance of carefully selecting normalization practices when assessing sex-based differences in hematological variables. [ABSTRACT FROM AUTHOR]

Few studies have explored the kinetics of performance and perceived fatigability during high‐intensity interval training, despite its popularity. We aimed to characterize the kinetics of fatigability and recovery during an 8 × 4‐min HIIT protocol, hypothesizing that most muscle function impairment would occur during the initial four intervals. Fifteen healthy males and females (mean ± standard deviation; age = 26 ± 5 years, V̇O2max = 46.8 ± 6.1 mL·kg−1·min−1) completed eight, 4‐min intervals at 105% of critical power with 3 min of rest. Maximal voluntary knee extension contractions (MVCs) coupled with electrical nerve stimulation were performed at baseline and after the first, fourth, and eighth intervals. MVC, potentiated twitch force (Pt), and Db10:100 ratio all declined throughout HIIT (p < 0.05). MVC sharply declined after interval 1 (−15 ± 9% relative to baseline; p < 0.05) and had only further declined after interval 8 (−26 ± 11%; p < 0.05), but not interval 4 (−19 ± 13%; p > 0.05). Pt and Db10:100 also sharply declined after interval 1 (Pt: −18 ± 13%, Db10:100: −14 ± 20%; p < 0.05) and further declined after interval 4 (Pt: −35 ± 19%, Db10:100: −30 ± 20%; p < 0.05) but not interval 8 (Pt: −41 ± 19%; Db10:100: −32 ± 18%; p > 0.05). Voluntary activation did not significantly change across the HIIT protocol (p > 0.05). Evoked force recovery was significantly blunted as more intervals were completed: after interval 1, Pt recovered by 7 ± 11% compared to −6 ± 7% recovery after interval 8 (p < 0.05). Ratings of perceived effort, fatigue, and leg pain rose throughout the session (p < 0.05 for each) and were greater (effort and fatigue) for females (p < 0.05). Otherwise, males and females exhibited similar performance fatigability kinetics, with contractile function declines blunted in response to additional intervals. [ABSTRACT FROM AUTHOR]