Vo2 And Muscle Deoxygenation Kinetics In Moderateintensity Exercise

Abstract

Hyperventilation-induced respiratory-alkalosis (R-ALK) was shown to slow the adaptation of pulmonary O2 uptake (VO2) during moderate-intensity exercise (MOD; Leigh, MSc thesis (UWO), 2003). Also prior heavy-intensity exercise (HVY) was shown to speed VO2 kinetics in young adults exhibiting “slow” VO2 kinetics without warm-up (Gurd, MSc thesis (UWO), 2003). PURPOSE To examine the combined effects of R-ALK and HVY on VO2 kinetics at the onset of MOD. METHODS Young male adults (n = 7; age, 23±3 yrs; mean ± SD) completed 4 repetitions of leg cycling exercise during normal breathing (CON; PETCO2 ∼40 mmHg) and controlled hyperventilation (R-ALK; PETCO2 ∼20 mmHg). The protocol consisted of two step transitions in work rate (MOD1 and MOD2) from 20W to 80% estimated lactate threshold, with MOD2 preceded by HVY (Δ50%); each transition was preceded and followed by 20W exercise. VO2 and heart rate (HR) were measured breath-by-breath and beat-by-beat, respectively. Changes in deoxy- (HHb), oxy- (O2Hb) and total hemoglobin (HbTOT) of the vastus lateralis were measured continuously by nearinfrared spectroscopy (NIRS; Hamamatsu, NIRO 300). Adaptation of VO2, HR and HHb were modeled using a mono-exponential equation and non-linear regression techniques. RESULTS The phase 2 VO2 time constant (τVO2) during MOD1 was greater (p < 0.05) in R-ALK (45±24 s) than CON (28±17 s). After HVY, τVO2 was reduced (p < 0.05) in R-ALK only, such that there was no difference between conditions (R-ALK: 24±7 s; CON: 20±8 s). The HHb mean response time (TD + τHHb) was not different amongst conditions (R-ALK: MOD1, 20±3 s, MOD2, 22±10 s; CON: MOD1, 23±2 s, MOD2, 22±7 s). Prior to exercise transitions, baseline HbTOT was elevated (p < 0.05) after (R-ALK: 5±7 μM; CON: 6±6 μM) compared to before HVY (R-ALK: −7±4 μM; CON: −6±3 μM), with no difference between conditions. CONCLUSION In MOD1 the slower VO2 kinetics, but similar HHb kinetics, in R-ALK compared to CON is consistent with a blood flow limitation within the muscle during R-ALK. After HVY, improved muscle perfusion and O2 delivery was seen in both conditions, however the speeding of VO2 kinetics seen in only R-ALK, is consistent with a removal of a blood flow limitation in R-ALK but not CON. Supported by NSERC, Canada.