Noninvasive measurements demonstrate that indices of microvascular oxygen transport to skeletal muscle plateau at submaximal work rates during incremental exercise, despite continued increases in limb blood flow and muscle recruitment.
Microvascular O2 delivery plateaus submaximally despite rising limb flow; hypothesis-generating for limits during incremental exercise in health and disease.
Limb blood flow increases linearly with exercise intensity; however, invasive measurements of muscle microvascular blood flow during incremental exercise have demonstrated submaximal plateaus. We tested the hypotheses that 1) brachial artery blood flow (Q̇ BA ) would increase with increasing exercise intensity until task failure, 2) blood flow index of the flexor digitorum superficialis (BFI FDS ) measured noninvasively via diffuse correlation spectroscopy would plateau at a submaximal work rate, and 3) muscle oxygenation characteristics (total-[heme], deoxy-[heme], and percentage saturation) measured noninvasively with near-infrared spectroscopy would demonstrate a plateau at a similar work rate as BFI FDS . Sixteen subjects (23.3 ± 3.9 yr, 170.8 ± 1.9 cm, 72.8 ± 3.4 kg) participated in this study. Peak power (P peak ) was determined for each subject (1.8 ± 0.4 W) via an incremental handgrip exercise test. Q̇ BA , BFI FDS , total-[heme], deoxy-[heme], and percentage saturation were measured during each stage of the exercise test. On a subsequent testing day, muscle activation measurements of the FDS (RMS FDS ) were collected during each stage of an identical incremental handgrip exercise test via electromyography from a subset of subjects ( n = 7). Q̇ BA increased with exercise intensity until the final work rate transition ( P < 0.05). No increases in BFI FDS or muscle oxygenation characteristics were observed at exercise intensities greater than 51.5 ± 22.9% of P peak . No submaximal plateau in RMS FDS was observed. Whereas muscle activation of the FDS increased until task failure, noninvasively measured indices of perfusive and diffusive muscle microvascular oxygen delivery demonstrated submaximal plateaus. NEW & NOTEWORTHY Invasive measurements of muscle microvascular blood flow during incremental exercise have demonstrated submaximal plateaus. We demonstrate that indices of perfusive and diffusive microvascular oxygen transport to skeletal muscle, measured completely noninvasively, plateau at submaximal work rates during incremental exercise, even though limb blood flow and muscle recruitment continued to increase.
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Hammer et al. (2018) studied this question.
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