Influence of exercise intensity on the on- and off-transient kinetics of pulmonary oxygen uptake in humans.

1. The maximal oxygen uptake (V(O(2),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 V(O(2)) increases to attain this maximum (or to its submaximal requirement), and even less to the kinetic features of the response following exercise. 2. Six, 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). 3. For moderate exercise, the V(O(2)) 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 (tau(1)); i.e. tau(1,on) averaged 33 +/- 16 s (+/- S.D.) and tau(1,off) 29 +/- 6 s. 4. The on-transient V(O(2)) 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. tau(1,on) = 32 +/- 17 s and tau(2,on) = 170 +/- 49 s. The off-transient V(O(2)) kinetics, however, remained mono-exponential (tau(1,off) = 42 +/- 11 s). 5. For very heavy exercise, the on-transient V(O(2)) kinetics were also well described by a double exponential function (tau(1,on) = 34 +/- 11 s and tau(2,on) = 163 +/- 46 s). However, a double exponential, with no delay, was required to characterise the off-transient kinetics (i.e. tau(1,off) = 33 +/- 5 s and tau(2,off) = 460 +/- 123 s). 6. At the highest intensity (severe), the on-transient V(O(2)) kinetics reverted to a mono-exponential profile (tau(1,on) = 34 +/- 7 s), while the off-transient kinetics retained a two-component form (tau(1,off) = 35 +/- 11 s and tau(2,off) = 539 +/- 379 s). 7. We therefore conclude that the kinetics of V(O(2)) 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.