The Adaptation Of Pulmonary O2 Uptake, Limb Blood Flow, And Muscle Deoxygenation During The Off-transient Of Moderate-intensity Exercise

Abstract

During recovery from moderate-intensity exercise the rate of adaptation (i.e., decline) of pulmonary O2 uptake (VO2p) has been shown to be faster than cardiac output suggesting that O2 availability is not a limitation. However, the adequacy of O2 delivery during exercise recovery has not been examined at the level of the working muscle. PURPOSE Off-transient kinetics of VO2p, limb blood flow (LBF) and local muscle deoxygenation (HHb) were determined to investigate the regional balance between muscle O2 delivery and O2 utilization during recovery from moderate-intensity kneeextension (KE) exercise. METHODS Seven adults (27±5 yrs; mean ± SD) performed 3 repeated step-transitions (8 min) from moderate-intensity (MOD; 80% θL) alternateleg KE exercise to passive exercise. VO2p was measured breath-by-breath; LBF was measured via Doppler ultrasound at the femoral artery; and deoxy- (HHb), oxy- (HbO2), and total Hb/Mb (Hbtot) of the vastus lateralis were measured continuously by nearinfrared spectroscopy (NIRS; Hamamatsu NIRO-300). Phase 2 VO2p, LBF and HHb data were fit with a mono-exponential model using non-linear regression techniques. HHb data were fit from the first time point of recovery indicative of an effective τ or mean response time. RESULTS Off-transient phase 2 τVO2p (32±5 s) and τLBF (25±5 s) were not different, however both were faster than the effective τHHb (91±26 s; p < 0.01). CONCLUSIONS The adaptation of conduit artery blood flow and VO2p to a step decrease in work rate at end-exercise were not different. However, the NIRS-derived deoxygenation signal suggests that within the muscle, the decrease in local muscle blood flow relative to local muscle O2 utilization was greater resulting in a very slow return of the HHb signal back to pre-transition levels.