Effect of work rate on the functional 'gain' of Phase II pulmonary O2 uptake response to exercise

It has recently been reported that the 'gain' of Phase II increase in pulmonary oxygen uptake (i.e. the 'fundamental' increase in V(O(2)) per unit increase in work rate; G(p)) does not attain the anticipated value of approximately 10 ml min(-1)W(-1) following the onset of high-intensity exercise. In the present study, we hypothesised that G(p) would fall significantly below 10 ml min(-1)W(-1) only when the work rate exceeded the so-called 'critical power' (CP). Seven healthy males completed several 'square-wave' transitions from 'unloaded' cycling to work rates requiring 60 and 90% of the gas exchange threshold (GET), 40 and 80% of the difference between the GET and V(O(2)) peak (i.e. below and above the CP, respectively), and 100, 110 and 120% of V(O(2)) peak. Pulmonary V(O(2)) was measured breath-by-breath and V(O(2)) kinetics were determined using non-linear regression techniques. The asymptotic G(p) was significantly lower at work rates above (7.2-8.6 ml min(-1)W(-1)) compared to work rates below (9.3-9.7 ml min(-1)W(-1)) the CP (P < 0.05). We conclude that the gain of Phase II increase in V(O(2)) becomes significantly reduced when the work rate exceeds the CP.