In-vivo Estimation of Intramuscular and Diffusive Limitations to O2 Flow Limitations in Resistance and Endurance Trained Individuals by Near Infra-red Spectroscopy

Introduction: Compared with resistance training (RT), endurance training (ET) induces structural changes within skeletal muscles that include increased capillary density and increased oxidative enzyme activity. The adaptations in ET of oxidative capacity are thought to be greater than those of capillary density, causing the location of O2 flow limitation to move from the intramuscular (O2 utilization limited) to the extramuscular (O2supply limited) compartment. We recently proposed a new approach to estimate oxidative capacity and O2diffusion limitation in vivo by near-infrared spectroscopy (NIRS), using the premise that diffusion limitation is exposed by an experimentally-imposed reduction in O2 supply. We therefore evaluated whether this technique could distinguish muscular adaptations between ET and RT individuals. We hypothesized that both muscle oxidative capacity and diffusion limitation would be greater in ET than RT. Methods: Twenty-one young male resistance (RT, n=9, 26±4 yrs) and endurance (ET, n=12, 27±7 yrs) trained individuals volunteered. RT or ET status was characterized using: maximal isometric voluntary torque (MVT) of knee extensors; incremental cycle-ergometry to the limit of tolerance for peak oxygen uptake (V̇O2peak); vastus lateralis fractional O2extraction at V̇O2peak (ΔHHbMbpeak), expressed as percentage of ΔHHbMb during sustained occlusion; and by V̇O2 kinetics (τV̇O2) during moderate intensity constant work-rate exercise. Vastus lateralis muscle V̇O2recovery rate constant ( k) following brief knee extensor exercise was assessed using NIRS and repeated transient arterial occlusions. k was assessed twice, with arterial occlusions manipulated to maintain tissue saturation index within a 10% range in either LOW or HIGH conditions. Results: MVT was greater in RT than ET (328±43 vs. 255±40N, P<0.001). V̇O2peak was lower in RT than ET (38.8±4.6 vs. 55.6±3.8ml∙kg-1∙min-1, P<0.001). ΔHHbMbpeak was lower in RT than ET (57±11 vs. 91±6%, P<0.001). τV̇O2 was lower in RT than ET (39.8±7.6 vs. 24.0±4.4s, P<0.001). kHIGH was lower in RT than ET (1.49±0.25 vs. 3.04±0.71min-1, P=0.001) and the difference in k (Δ k = kHIGH – kLOW) was less in RT than ET (0.29±0.16 vs 0.80±0.69 min-1, P=0.045). Conclusions: As hypothesized, non-invasive assessment of muscle functional indices of oxidative and diffusive O2 flow in vivo revealed significantly greater oxidative capacity (greater kHIGH) and a greater diffusion limitation (greater Δ k) in ET individuals compared with RT. The new NIRS method allows independent assessment of limitations in muscle oxidative and diffusive O2 flow in resistance and endurance trained individuals. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.