Dynamics of Respiratory Response to Sinusoidal Work Load in Humans

The present study was undertaken to investigate possible distortion in responses of respiratory variables including O₂ uptake (VO₂), CO₂ output (VCO₂), and ventilation (VE) to sinusoidal work load, and to find out whether the conventional transfer models were applicable to analyze the dynamics of these variables. Six healthy subjects performed exercise for 32 min on a bicycle ergometer with electro-magnetic braking. The work load was varied sinusoidally between 30 W and 60 % of maximum O₂ uptake (VO₂max) during periods from 1 to 16 min. The respiratory variables were measured on a breath-by-breath basis with a mass spectrometer and a computer system. The responses of VO₂, VCO₂, and VE to sinusoidal work load were not completely sinusoidal in form but were somewhat distorted, forming sawtooth waves with steeper down-slopes during periods, of 4 - 16 min, but this distortion was not observed at 1 min or 2 min periods. However, the results could be approximately described by a first-order model without or with time delay. Time constants of the first-order model without time delay were 46 sec for VO₂, 62 sec for VCO₂, and 73 sec for VE, respectively. We also found a close relationship between the time constants of VO₂ and VCO₂ and VO₂max. These results suggested that exponential functions may be applied and are expected to yield valid results in assessing physical fitness, although the control of ventilatory and gas exchange in exercise does show non-linear characteristics.

Key words

Transfer models - sinusoidal work load - time constants - O₂ uptake - CO₂ output - ventilation - exercise