NIRS-Estimated Muscle Oxidative Capacity And Capillary-To-Fiber Ratio Differs By Sex And Muscle In Endurance Trained Athletes

Abstract

The relationship between muscle oxidative capacity and muscle diffusing capacity (DmO2), and how this differs among muscles or by sex in endurance trained athletes in vivo is unknown. Estimates of muscle oxidative capacity and capillary-to-fiber ratio (C:F; a mediator of DmO2) can be assessed by near infra-red spectroscopy (NIRS): muscle oxidative capacity is proportional to the recovery rate constant (k) of muscle V̇O2, and C:F is proportional to the change in k (∆k) when tissue saturation (TSI) is reduced. PURPOSE: To determine the role of sex on k and ∆k in the vastus lateralis (VL) and medial gastrocnemius (GS) in endurance trained athletes. METHODS: Twenty-six competitive college-level endurance athletes volunteered: 14 women (20 ± 1 yrs) and 12 men (27 ± 7 yrs). The NIRS assessments were administered using controlled arterial occlusions to maintain TSI in ±5% bands at each of HIGH (averaging 87% of functional range) and LOW (24%) TSI. k was measured in HIGH and ∆k was calculated from the difference in k between HIGH and LOW. RESULTS: k was lower in females than males (mean difference [95% confidence interval]: -0.61[-0.12,-1.08] min-1, p = 0.014, partial ηp2 = 0.13) and lower in VL than GS (-0.83[-0.35,-1.31] min-1, p = 0.001, ηp2 = 0.22), but there was no muscle-by-sex interaction (p = 0.860) (Table). ∆k tended to be greater in females (-0.43[-0.98,0.13] min-1, p = 0.131, ηp2 = 0.05) and in GS (-0.40[0.95,0.16] min-1, p = 0.159, ηp2 = 0.05), but there was no interaction (p = 0.676). CONCLUSIONS: Overall, both GS and VL muscle oxidative capacity was lower in female than male endurance trained athletes with similar training history. We also found that oxidative capacity in medial GS was greater than VL in these endurance-trained athletes. In females, there was a trend towards greater NIRS-derived estimates of C:F, a mediator of DmO2, suggesting potential differential training adaptations in muscle oxidative capacity and DmO2 to achieve maximal myocellular oxidation rates between sexes.