INFLUENCE OF SEX ON LOWER LIMB SKELETAL MUSCLE OXIDATIVE CAPACITY AND MUSCLE DIFFUSION IN ENDURANCE TRAINED INDIVIDUALS

Near infra-red spectroscopy (NIRS) is a noninvasive methodology that allows in vivo measurement of the skeletal muscle tissue metabolism. Recently, a novel, of short duration, NIRS-based approach, based on a brief set of muscle contractions followed by a series of repeated intermittent arterial occlusions, has been validated to determine the skeletal muscle oxygen consumption (mV̇ O2) recovery rate constant (k, min-1), which is proportional to the muscle oxidative capacity. However, since its relatively recent introduction, the validity of this approach in investigating the difference in lower limb muscle oxidative capacity between female and male individuals is still unclear. Aim: The overall aim of the current study was to determine whether sex plays a major role in determine k when comparing young individuals exposed to a long-term endurance-based training. Methods: We administered two NIRS-based protocols: 1) a traditional test, where after a ~10 s muscle contraction a series of 15 arterial occlusion is administered to measure the mV̇ O2 recovery kinetics and estimate k; and, 2) a newly designed, protocol (called experimental), modified version of 1, to inquiry about changes in k at three different levels of tissue oxygenation (high, medium, low). Both protocols were administered at the level of the vastus lateralis (VL) and the medial gastrocnemius (GS) skeletal muscles, during four separate visits (1 for the traditional and 3 for the experimental NIRS-based protocols). Results: A group of 17 females and 12 males successfully completed the study. The traditional protocol showed a high test-retest reproducibility in both muscle sites under investigation (VL: CV = 9.0%, ICC = 0.76; GS: CV = 6.0%, ICC = 0.91). The experimental protocol showed a good to very good reproducibility ICC (range 0.80 – 0.92) and CV (range 8.70 – 16.30%) in both muscles (ICC range: 0.80 to 0.92), despite reporting a larger variability than the traditional independently of the muscle tested. Factorial analysis on the traditional protocol revealed that sex had a statistically significant effect on the participants k value (P = 0.008), and that on average k was significantly lower in the VL muscle in females than males. In the experimental protocol, the 3-way ANOVA showed no interaction between (sex) x (muscle site) x (TSI oxygenation level) but confirmed a statistically significant effect when these factor are considered independently. Furthermore, independently of VL or GS, post hoc analysis reported a significant difference in k values (P \u3c 0.001) when this is determined on a low oxygenation level than the high and medium ones, which was no different from each other. Conclusion: the NIRS-based protocol is a valid and reproducible methodology to assess the skeletal muscle oxidative capacity in two representative locomote muscles in young individuals. Our findings suggest that female endurance trained subjects have a lower muscle oxidative capacity in the locomotor muscles compared to peer males. Therefore, our results seem to suggest that sex is an important factor in determining one of the functional ‘properties’ of the muscle metabolism