Effects of training status and exercise intensity on phase IIVO2 kinetics

Purpose: To test the hypotheses that: 1) the time constant for the fast component of VO2 kinetics (tau(1)) at exercise onset would be faster in trained than in untrained subjects for both moderate and heavy exercise, and that 2) tau(1) would become progressively slower in untrained subjects at higher power outputs but be invariant in trained subjects. Methods: Eight untrained subjects (VO2peak: 42.9 +/- 5.1 mL(.)kg(-1.)min(-1)) and seven trained cyclists (VO2peak: 66.6 +/- 2.5 mL(.)kg(-1.)min(-1)) completed square-wave transitions to power outputs requiring 60% and 80% of gas exchange threshold (GET), and 50% of the difference between GET and VO2peak (50%Delta) from a baseline of unloaded cycling. VO2 was measured breath-by-breath and individual responses were modeled using nonlinear regression techniques. Results: A repeated measures ANOVA revealed that the tau(1) was significantly smaller (i.e., the kinetics were faster) in the trained compared with the untrained subjects and that tau(1) became significantly greater (i.e., the kinetics were slowed) at higher power outputs both in the untrained (60%GET: 17.8 +/- 3.8 s, 80%GET: 21.5 +/- 6.6 s, and 50%Delta: 23.5 +/- 2.8 s) and the trained (60%GET: 8.9 +/- 1.3 s, 80%GET: 11.7 +/- 2.5 s, and 50%Delta: 15.2 +/- 2.0 s) subjects (P < 0.05). Conclusion: Phase II VO2 kinetics became progressively slower at higher power outputs in both trained and untrained subjects. That a greater tau(1), was evident at a higher power output within the moderate exercise intensity domain (