Reduction of V̇_O2slow component by priming exercise: novel mechanistic insights from time-resolved near-infrared spectroscopy

Abstract

Novel time-resolved near-infrared spectroscopy (TR-NIRS), with adipose tissue thickness correction, was used to test the hypotheses that heavy priming exercise reduces the V̇O2 slow component (V̇O2SC) (1) by elevating microvascular [Hb] volume at multiple sites within the quadriceps femoris (2) rather than reducing the heterogeneity of muscle deoxygenation kinetics. Twelve subjects completed two 6-min bouts of heavy work rate exercise, separated by 6 min of unloaded cycling. Priming exercise induced faster overall V̇O2 kinetics consequent to a substantial reduction in the V̇O2SC (0.27 ± 0.12 vs. 0.11 ± 0.09 L·min−1, P < 0.05) with an unchanged primary V̇O2 time constant. An increased baseline for the primed bout [total (Hb + Mb)] (197.5 ± 21.6 vs. 210.7 ± 22.5 μmol L−1, P < 0.01), reflecting increased microvascular [Hb] volume, correlated significantly with the V̇O2SC reduction. At multiple sites within the quadriceps femoris, priming exercise reduced the baseline and slowed the increase in [deoxy (Hb + Mb)]. Changes in the intersite coefficient of variation in the time delay and time constant of [deoxy (Hb + Mb)] during the second bout were not correlated with the V̇O2SC reduction. These results support a mechanistic link between priming exercise-induced increase in muscle [Hb] volume and the reduced V̇O2SC that serves to speed overall V̇O2 kinetics. However, reduction in the heterogeneity of muscle deoxygenation kinetics does not appear to be an obligatory feature of the priming response.

The effect of dietary nitrate supplementation on the spatial heterogeneity of quadriceps deoxygenation during heavy-intensity cycling

“…An additional novel feature of this study was the application of TRS-NIRS within an increased number of active muscle sites (other than the VL) following PLA and BR supplementation. In line with previous studies (Koga et al 2007(Koga et al , 2011Chin et al 2011;Fukuoka et al 2015), this revealed inter-muscular differences in deoxy [Hb + Mb] profiles which may be explained by disparities in muscle fiber type (Johnson et al 1973;Lexell et al 1983) and/or activity level (Chin et al 2011) between skeletal muscles. However, there were no significant differences in the inter-site CV of the deoxy [Hb + Mb] MRT p or A p between PLA and BR supplementation.…”

“…; Fukuoka et al. ), this revealed inter‐muscular differences in deoxy [Hb + Mb] profiles which may be explained by disparities in muscle fiber type (Johnson et al. ; Lexell et al.…”

“…It is also presently unclear whether dietary NO 3 À supplementation might alter the muscle spatial heterogeneity in deoxy [Hb + Mb] kinetics (and, by extension, _ QO 2 -to-_ VO 2 matching) which has previously been reported during cycling exercise (Koga et al 2007(Koga et al , 2011Chin et al 2011;Bowen et al 2013;Spencer et al 2014;Fukuoka et al 2015;Okushima et al 2015Okushima et al , 2016. Given that NO 2 À reduction to NO is potentiated in low O 2 environments (Lundberg and Weitzberg 2009), and that NO 2 À itself can promote vasodilation in hypoxic tissues (Gladwin et al 2000), NO 3 À treatment (and the associated increase in circulating plasma [NO 2 À ]) may be particularly effective at improving vascular function within muscle regions where a low _ QO 2 / _ VO 2 ratio is manifest, for example, within type II muscle or muscle fibers (see (Jones et al 2016) for review).…”

“…Alternatively, an increased potential for O 2 -independent NO bioavailability after BR supplementation, via its effect on increasing the number of red blood cells (RBCs) adjacent to active myocytes, may also attenuate the _ VO 2Sc amplitude. For instance, the decreased _ VO 2Sc following an initial bout of high-intensity "priming" exercise has been reported to correlate inversely with an increased baseline total [Hb + Mb] across active muscle sites (Fukuoka et al 2015). These findings suggest that development of the _ VO 2Sc during high-intensity constant work rate exercise (indicative of a progressive decline in efficiency as continuous high-intensity exercise proceeds) is inversely linked to the skeletal muscle hyperemia and the associated improvement in muscle O 2 diffusing capacity (D m O 2 ) (Federspiel and Popel 1986;Groebe and Thews 1990).…”

Prior exercise speeds pulmonary oxygen uptake kinetics and increases critical power during supine but not upright cycling

“…Hence, in addition to an improved P O 2 mv , an alternative mechanism by which priming exercise might have resulted in the independent improvement of τV O 2 and critical power in the supine position is via a reduced heterogeneity of muscle deoxygenation. Such a hypothesis remains to be tested; however, previous research has indicated no relationship between the change in muscle deoxygenation heterogeneity and the slow component of pulmonaryV O 2 kinetics with priming exercise (Saitoh et al 2009;Fukuoka et al 2015).…”