New Findings What is the central question of this study? Does the presence and extent of QO2-to-VO2 heterogeneity across human muscles relate specifically to different muscle activation patterns? What is the main finding and its importance? During ramp incremental knee extension and cycling exercise, the profiles of muscle deoxygenation (deoxy[Hb+Mb]) and diffusive O2 potential (total[Hb+Mb]) in the vastus lateralis corresponded to different muscle activation strategies. However, this was not the case for the rectus femoris where muscle activation and deoxygenation profiles were dissociated and may therefore be determined by other structural and/or functional attributes (e.g. arteriolar vascular regulation and control of RBC flux). Abstract Near-infrared spectroscopy has revealed considerable heterogeneity of O2 delivery-to-uptake (QO2-to-VO2) as identified by disparate deoxygenation (deoxy[Hb+Mb]) values in the exercising quadriceps. However, whether this represents a recruitment phenomenon or contrasting vascular-metabolic control, as seen among fiber types, has not been established. We utilized knee extension (KE) and cycling (CE) incremental exercise paradigms to examine whether differential muscle activation profiles could account for the heterogeneity of deoxy[Hb+Mb] and microvascular hemoconcentration (i.e., total[Hb+Mb]). Using time-resolved near-infrared spectroscopy for the quadriceps femoris (vastus lateralis [VL] and rectus femoris [RF]) during exhaustive ramp exercise in eight participants, we tested the hypotheses that: 1) the deoxy[Hb+Mb] (i.e., fractional O2 extraction) would relate to muscle activation levels across exercise paradigms, and 2) KE would induce greater total[Hb+Mb] (i.e., diffusive O2 potential) at task failure (i.e., VO2peak) than CE irrespective of muscle site. At a given level of muscle activation, as assessed by relative integrated electromyography normalized to maximal voluntary contraction (%iEMGmax), the VL deoxy[Hb+Mb] profile was not different between exercise paradigms. However, at VO2peak and until 20% iEMGmax for CE, RF exhibited a lower deoxy[Hb+Mb] (83.2 ± 15.5 vs. 98.2 ± 19.4 µM) for KE than CE (P < 0.05). The total[Hb+Mb] at VO2peak was not different between exercise paradigms for either muscle site. These data support that the contrasting patterns of convective and diffusive O2 transport correspond to different muscle activation patterns in VL but not RF. Thus, the differential deoxygenation profiles for RF across exercise paradigms may be dependent upon specific facets of muscle architecture and functional hemodynamic events.
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