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.