Prior Heavy Exercise Speeds VO2 Kinetics By Improving Local O2 Delivery-to-Utilization Matching

Abstract

Priming bouts of heavy-intensity exercise (HVY) have been used to investigate the mechanism(s) responsible for regulating pulmonary O2 uptake kinetics (tVO2p) at the onset of subsequent moderate-intensity exercise (MOD). Insights into the balance between local muscle O2 delivery and O2 utilization (VO2m) can be gleaned from the ratio of NIRS-derived muscle deoxygenation (Δ[HHb])-to-VO2m (represented by phase II VO2p) to determine whether O2 delivery may constrain VO2p kinetics. PURPOSE: To compare the Δ[HHb]-to-VO2p ratio in MODs performed with and without prior HVY. METHODS: VO2p and Δ[HHb] kinetics were examined in 18 males during repeated repetitions of a MOD1-HVY-MOD2 leg-cycling protocol where each of the 6 min (MOD or HVY) exercise bouts were separated by 6 min of 20W cycling; the work rates corresponded to 90% estimated lactate threshold (qL; MOD) and 50% of the difference between qL and VO2peak (HVY). VO2p and Δ[HHb] responses from MOD were modeled as a mono-exponential using non-linear regression, and later scaled to a relative % of the response (0-100%). The Δ[HHb]-to-VO2p ratio for each individual was calculated as the average Δ[HHb]/VO2 relative response during the 20s to 120s period of the exercise on-transient. RESULTS: HVY priming exercise reduced tVO2p in MOD2 (tVO2p = 21 ± 5 s) compared to MOD1 (tVO2p = 27 ± 10 s). An O2 delivery-to-utilization mismatch (Δ[HHb]/VO2 "overshoot") (p < 0.05) was identified between 35-80s of MOD1, resulting in a greater mean Δ[HHb]/VO2 value (1.07 ± 0.09) than in MOD2 (1.02 ± 0.07; p < 0.05), where no mismatch was observed. A positive correlation between the decrease in tVO2p and the decrease in Δ[HHb]/VO2 from MOD1 to MOD2 also was identified (r = 0.80; p < 0.05). CONCLUSIONS: The elimination of the small but significant transient Δ[HHb]/VO2 "overshoot" that was observed in MOD1, along with the accompanying reduction in tVO2p following HVY, suggests that local muscle O2 delivery plays an integral role in the adjustment of VO2m during the on-transient of exercise. Supported by NSERC