Near‐infrared spectroscopy of superficial and deep rectus femoris reveals markedly different exercise response to superficial vastus lateralis

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 pulmonaryV ˙ O 2kinetics (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 O 2 transport strategies, with the RF ‐s and RF ‐d relying proportionately more on elevated perfusive and the VL ‐s on diffusive O 2 transport. These disparate O 2 transport strategies and their temporal profiles across muscles have previously been concealed within the “global” pulmonaryV ˙ O 2response.