Reduction of [Image: see text](O2) slow component by priming exercise: novel mechanistic insights from time-resolved near-infrared spectroscopy

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 V̇(O2) slow component (V̇(O2SC)) (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 V̇(O2) kinetics consequent to a substantial reduction in the V̇(O2SC) (0.27 ± 0.12 vs. 0.11 ± 0.09 L·min(−1), P < 0.05) with an unchanged primary V̇(O2) 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 V̇(O2SC) 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 V̇(O2SC) reduction. These results support a mechanistic link between priming exercise-induced increase in muscle [Hb] volume and the reduced V̇(O2SC) that serves to speed overall V̇(O2) kinetics. However, reduction in the heterogeneity of muscle deoxygenation kinetics does not appear to be an obligatory feature of the priming response.