Influence of work rate on the determinants of oxygen deficit during short-term submaximal exercise: implications for clinical research.

Objective: The effect of increasing work rate was studied on the determinants of the oxygen deficit.

Methods: Exercise testing was performed on a treadmill and gas exchange was measured on a breath-by-breath basis. Eleven healthy subjects, aged 18-25 years, performed three square wave exercise tests of different intensity. Before exercise, gas exchange was measured at rest in the standing position for 3 min, followed by a 6-min square wave exercise test, randomly assigned at 4, 8 or 12% inclination. Immediately after exercise the recovery gas exchange was determined for 3 min. To calculate oxygen deficit, the oxygen uptake (O2) values at onset of exercise were subtracted from the steady-state value, the differences were cumulated and expressed as a percentage of the total oxygen cost for the 6-min exercise.

Results: The oxygen deficit increased significantly (P<0.001) with increasing work rate (6.1 +/- 1.4% for 4%, 8.4 +/- 2.1% for 8% and 9.4 +/- 1.7% at 12% inclination). This resulted from a somewhat slower increase of O2 at the onset of exercise at the highest work rate, reflected by a significantly higher time constant for O2 at 8 and 12% (24.6 +/- 7.3 s at 8% and 24.1 +/- 6.3 s at 12% versus 20.2 +/- 8.1 s at 4%). More importantly a significantly higher steady-state value for O2 was found at the highest exercise level, compared with the other exercise intensities.

Conclusion: The higher oxygen deficit at the highest level of exercise is determined by a slower time constant and a higher asymptote value for O2.