Reduction of Vo2 slow 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 VO2 slow component (VO2SC) (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 VO2 kinetics consequent to a substantial reduction in the VO2SC (0.27 ± 0.12 vs. 0.11 ± 0.09 L·min-1, P < 0.05) with an unchanged primary VO2 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 VO2SC 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 VO2SC reduction. These results support a mechanistic link between priming exerciseinduced increase in muscle [Hb] volume and the reduced VO2SC that serves to speed overall VO2 kinetics. However, reduction in the heterogeneity of muscle deoxygenation kinetics does not appear to be an obligatory feature of the priming response.