Estimating In Vivo Skeletal Muscle Oxidative And Diffusive Capacity Via Near Infrared Spectroscopy

Recently, Pilotto et al. (2022) established a technique to non-invasively estimate in vivo muscle oxidative and diffusive capacity using near infrared spectroscopy (NIRS), which has great potential to provide insights into the physiological determinants of exercise capacity. However, there is a paucity of data pertaining to the reliability and validity of this method. PURPOSE: This pilot study aimed to determine the reliability of estimating oxidative and diffusive capacity via NIRS and provide further physiological validation of this technique. METHODS: Ten participants (7 males, 3 females, age 34 ± 16 years, maximal oxygen uptake [V̇O2max] 46.6 ± 10.3 ml.min-1.kg-1 and peak power output [PPO] 4.5 ± 1.2 W.kg-1) participated. Oxidative and diffusive capacity were assessed using procedures described by Pilotto et al. (2022), with the exception that an intermediate tissue saturation index (TSI) range was added. The recovery rate constant k in HIGH (kHIGH) reflected mitochondrial oxidative capacity, whereas kLOW/TSILOW was used as a proxy for diffusive capacity. Vastus lateralis muscle biopsies were assessed for mitochondrial oxidative phosphorylation (OXPHOS) capacity via high-resolution respirometry. RESULTS: Participants displayed an ex vivo OXPHOS capacity of 97.4 ± 32.5 pmol.s-1.mg-1. kHIGH (3.43 ± 0.46 min-1) was correlated with V̇O2max (r = 0.89, p < 0.001) and PPO (r = 0.88, p < 0.001), but not with OXPHOS capacity (r = 0.25, p = 0.491). Both kMID (3.29 ± 0.78 min-1) and kLOW (2.22 ± 0.45 min-1) were correlated with V̇O2max (r = 0.68, p = 0.030 and r = 0.77, p = 0.009, respectively) and PPO (r = 0.72, p = 0.019 and r = 0.83, p = 0.003, respectively). kLOW/TSILOW (0.04 ± 0.01 min-1.%-1) was correlated with V̇O2max (r = 0.84, p = 0.003) and PPO (r = 0.83, p = 0.003) but not with OXPHOS capacity (r = 0.52, p = 0.127). Repeated measures ANOVA showed that k-values did not differ between repeated measurements performed on three separate days (kHIGH p = 0.248, kMID p = 0.309, kLOW p = 0.251). The coefficient of variation (CV) of kHIGH was 11.0%, kMID was 8.2%, kLOW was 13.3%, and kLOW/TSILOW was 9.8%. CONCLUSION: These preliminary findings indicate a good within-subject reliability of the occlusion protocol. They also suggest that NIRS-derived measures in vivo oxidative and diffusive capacity correlate well with in vivo aerobic function. Supported by: Amsterdam Movement Sciences, Ageing & Vitality Program Grant 2023