Pulmonary Vo2 On-kinetic Response To Step Exercise

Abstract

It has been demonstrated that the pulmonary VO kinetic response to a moderate intensity work rate is slower when the transition is initiated from an elevated metabolic rate (Brittain et al., 2001). These data suggest that the metabolic characteristics of the recruited muscle fibers exert an important influence on VO2 kinetics, although differences in heart rate (HR) kinetics (and thus muscle O delivery) in the transition to and from different metabolic rates might also be important. PURPOSE To examine the effect of the initial metabolic rate (unloaded, moderate or heavy) on VO2 and HR kinetics following 'step' transitions to higher metabolic rates (moderate, heavy or severe). We hypothesised that the recruitment of higher-order fibers when the initial and/or exercise metabolic rate was increased would result in a progressive slowing of the VO2 on-kinetics. METHODS Seven healthy young males completed multiple step transitions from unloaded exercise to moderate (80% GET, MOD), heavy (40% of the difference between GET and VO2 max, HVY), and severe (100% VO2 max, SEV) work rates on a cycle ergometer. Multiple step transitions were also performed from a MOD baseline work rate to the HVY work rate, from a MOD baseline work rate to the SEV work rate, and from a HVY baseline work rate to the SEV work rate. HR and pulmonary gas exchange were measured throughout exercise and HR and VO2 kinetics were analysed using non-linear regression techniques. RESULTS ANOVA revealed that the Phase II time constant (T) became progressively longer at higher elevations of baseline and end-exercise metabolic rate (P <0.05): MOD (mean ± SD 25.7 ± 5.9 s), HVY (27.4 ± 4.2 s), SEV (36.9 ± 5.8 s), MOD-HVY (47.7 ±11.3 s), MOD-SEV (59.0 ± 22.7 s), and HVY-SEV (93.1 ± 50.0 s). However, this was not associated with a similar progressive slowing of HR kinetics: MOD (20.3 ± 2.3 s), HVY (35.7 ± 12.8 s), SEV (41.2 ± 16.6 s), MOD-HVY (47.6 ± 12.9 s), MOD-SEV (36.3 ± 10.7 s), and HVY-SEV (45.1 ± 18.1 s). CONCLUSION The progressively slower on-transient VO kinetics observed when the initial and exercise metabolic rates are independently or simultaneously increased, in the absence of a similar progressive slowing of HR kinetics in these transitions, indicates that the recruitment of higher-order muscle fibers exerts an important influence on VO2 kinetics.