Slow upward drift of $$\dot V_{{\text{O}}_{\text{2}} } $$ during constant-load cycling in untrained subjects

The oxygen uptake kinetics during constant-load exercise when sitting on a bicycle ergometer were determined in 7 untrained subjects by measuring breath-by-breath \(\dot V_{{\text{O}}_{\text{2}} } \) during continuous exercise to volitional exhaustion (mean endurance time=1160±172 s) at a pedal frequency of 70 revolutions · min⁻¹. The power output, averaging 189,5 W, was set at 82.5% of that eliciting the individual \(\dot V_{{\text{O}}_{{\text{2}} {\text{max}}} } \) during a 5 min incremental exercise test. Throughout the exercise period, the \(\dot V_{{\text{O}}_{\text{2}} } \) kinetics could be appropriately described by a two-component exponential equation of the form: $$\dot V_{{\text{O}}_{\text{2}} } (t) = Y_a [1 - \exp ( - k_a t)] + Y_b [1 - \exp ( - k_b t)]$$ where \(\dot V_{{\text{O}}_{\text{2}} } \) is net oxygen consumption andt the time from work onset. \(\dot V_{{\text{O}}_{\text{2}} } \) measured at the end of exercise was close to \(\dot V_{{\text{O}}_{{\text{2}} {\text{max}}} } \) (98% \(\dot V_{{\text{O}}_{{\text{2}} {\text{max}}} } \) ) and the mean values ofY _a ,k _a ,Y _b andk _b amounted to 1195 ml O₂ · min⁻¹, 0.034s⁻¹, 1562 ml O₂ · min⁻¹, and 0.005 s⁻¹ respectively. The initial rate of increase in \(\dot V_{{\text{O}}_{\text{2}} } \) predicted from the above equation is slower than that calculated, for the same work intensity, on the basis of the data obtained by Morton (1985) in trained subjects. For t>480 s, however, the two models yield substantially equal results.