Influence of exercise intensity on the on- and off-transient kinetics of pulmonary oxygen uptake in humans
Corresponding author B. J. Whipp: Department of Physiology, St George's Hospital Medical School, Cranmer Terrace, Tooting, London SW17 0RE, UK. Email: firstname.lastname@example.org
- 1The maximal oxygen uptake (V̇O2,peak) during dynamic muscular exercise is commonly taken as a crucial determinant of the ability to sustain high-intensity exercise. Considerably less attention, however, has been given to the rate at which V̇O2 increases to attain this maximum (or to its submaximal requirement), and even less to the kinetic features of the response following exercise.
- 2Six, healthy, male volunteers (aged 22 to 58 years), each performed 13 exercise tests: initial ramp-incremental cycle ergometry to the limit of tolerance and subsequently, on different days, three bouts of square-wave exercise each at moderate, heavy, very heavy and severe intensities. Pulmonary gas exchange variables were determined breath by breath throughout exercise and recovery from the continuous monitoring of respired volumes (turbine) and gas concentrations (mass spectrometer).
- 3For moderate exercise, the V̇O2 kinetics were well described by a simple mono-exponential function, following a short cardiodynamic phase, with the on- and off-transients having similar time constants (τ1); i.e. τ1,on averaged 33 ± 16 s (± S.D.) and τ1,off 29 ± 6 s.
- 4The on-transient V̇O2 kinetics were more complex for heavy exercise. The inclusion of a second slow and delayed exponential component provided an adequate description of the response; i.e. τ1,on= 32 ± 17 s and τ2,on= 170 ± 49 s. The off-transient V̇O2 kinetics, however, remained mono-exponential (τ1,off= 42 ± 11 s).
- 5For very heavy exercise, the on-transient V̇O2 kinetics were also well described by a double exponential function (τ1,on= 34 ± 11 s and τ2,on= 163 ± 46 s). However, a double exponential, with no delay, was required to characterise the off-transient kinetics (i.e. τ1,off= 33 ± 5 s and τ2,off= 460 ± 123 s).
- 6At the highest intensity (severe), the on-transient V̇O2 kinetics reverted to a mono-exponential profile (τ1,on= 34 ± 7 s), while the off-transient kinetics retained a two-component form (τ1,off= 35 ± 11 s and τ2,off= 539 ± 379 s).
- 7We therefore conclude that the kinetics of V̇O2 during dynamic muscular exercise are strikingly influenced by the exercise intensity, both with respect to model order and to dynamic asymmetries between the on- and off-transient responses.