The effect of exercise duration on the fast component of exercise hyperpnoea at work rates below the first ventilatory threshold.

We examined the effect of exercise duration on the fast component of exercise hyperpnoea for light and moderate work rates [mean oxygen uptakes (SD) = 1.00 (0.27) 1.min-1 and 1.77 (0.53) 1.min-1, respectively]. Ten subjects exercised on a motor-driven treadmill while ventilation and end-expiratory partial pressures of carbon dioxide and oxygen were recorded on a breath-by-breath basis. The magnitude of the fast component of exercise hyperpnoea was determined by quantifying the abrupt changes in ventilation at the beginning and end of exercise. Five exercise tests with durations ranging from 1 min to 8 min were completed at each of the two periods of exercise at different work rates. Subsequent statistical analysis revealed that the rapid changes in ventilation at the end of exercise were significantly smaller than those at the start [pooled means (SE) = 6.27 (0.48) and 13.05 (1.06) 1.min-1 for light and moderate exercises respectively] regardless of exercise duration. Further statistical analysis failed to find a relationship between the fast ventilatory changes present at the end of exercise, expressed as a proportion of those at the start of exercise, and either exercise duration or work rate (73% and 62% for light and moderate exercises respectively). We conclude that the fast component of exercise hyperpnoea declines rapidly in the first minute of exercise, and interpret this decline as an indication that the fast neural drive to ventilation, proportional to limb movement frequency, adapts quickly at the start of exercise.