Abstract
To date our knowledge of skeletal muscle deoxygenation as measured by near‐infrared spectroscopy (NIRS) is predicated almost exclusively on sampling of superficial muscle(s), most commonly the vastus lateralis (VL‐s). Recently developed high power NIRS facilitates simultaneous sampling of deep (i.e., rectus femoris, RF‐d) and superficial muscles of RF (RF‐s) and VL‐s. Because deeper muscle is more oxidative with greater capillarity and sustains higher blood flows than superficial muscle, we used time‐resolved NIRS to test the hypotheses that, following exercise onset, the RF‐d has slower deoxy[Hb+Mb] kinetics with reduced amplitude than superficial muscles. Thirteen participants performed cycle exercise transitions from unloaded to heavy work rates. Within the same muscle (RF‐s vs. RF‐d) deoxy[Hb+Mb] kinetics (mean response time, MRT) and amplitudes were not different. However, compared with the kinetics of VL‐s, deoxy[Hb+Mb] of RF‐s and RF‐d were slower (MRT: RF‐s, 51 ± 23; RF‐d, 55 ± 29; VL‐s, 18 ± 6 s; P < 0.05). Moreover, the amplitude of total[Hb+Mb] was greater for VL‐s than both RF‐s and RF‐d (P < 0.05). Whereas pulmonary V˙O2 kinetics (i.e., on vs. off) were symmetrical in heavy exercise, there was a marked on‐off asymmetry of deoxy[Hb+Mb] for all three sites i.e., MRT‐off > MRT‐on (P < 0.05). Collectively these data reveal profoundly different O2 transport strategies, with the RF‐s and RF‐d relying proportionately more on elevated perfusive and the VL‐s on diffusive O2 transport. These disparate O2 transport strategies and their temporal profiles across muscles have previously been concealed within the “global” pulmonary V˙O2 response.
Highlights
Much of our understanding of muscle deoxygenation dynamics following exercise onset and offset in humans is predicated on nearinfrared spectroscopy (NIRS) measurements in superficial muscle(s)
Physiological Reports published by Wiley Periodicals, Inc. on behalf of The Physiological Society and the American Physiological Society
High power TRSNIRS resolves substantially slower deoxy[Hb + Mb] kinetics in the superficial and deep rectus femoris muscle compared with the superficial vastus lateralis muscle
Summary
Pulmonary V_ O2 kinetics following the onset and offset of heavy intensity cycling exercise are characterized typically as biexponential processes with primary and slow components reflecting the changes in O2 consumption in the active leg muscles (Barstow and Mole 1991; Grassi et al.1996; Rossiter et al 2002; Koga et al 2005; Krustrup et al 2009; Jones et al 2012; Poole and Jones 2012).these responses represent a vast consortium of muscle and microvascular units each with its own dynamic O2 delivery to O2 consumption (Q_ O2-to-V_ O2)a 2017 The Authors. Pulmonary V_ O2 kinetics following the onset and offset of heavy intensity cycling exercise are characterized typically as biexponential processes with primary and slow components reflecting the changes in O2 consumption in the active leg muscles (Barstow and Mole 1991; Grassi et al.1996; Rossiter et al 2002; Koga et al 2005; Krustrup et al 2009; Jones et al 2012; Poole and Jones 2012). Much of our understanding of muscle deoxygenation dynamics following exercise onset and offset in humans is predicated on NIRS measurements in superficial muscle(s) (e.g., vastus lateralis, VL, up to ~1.5 cm depth; e.g., DeLorey et al 2003; Grassi et al, 2003; Ferrari and Quaresima 2012; Grassi and Quaresima 2016)
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