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

1. The maximal oxygen uptake ((V) over dot (O2,peak)) during dynamic muscul ar exercise is commonly taken as a crucial determinant of the ability to su stain high-intensity exercise. Considerably less attention, however, has be en given to the rate at which (V) over dot (O2), increases to attain this m aximum (or to its submaximal requirement), and even less to the kinetic fea tures of the response following exercise. 2. Six, healthy, male volunteers (aged 22 to 58 years), each performed 13 e xercise tests: initial ramp-incremental cycle ergometry to the limit of tol erance and subsequently, on different days, three bouts of square-wave exer cise 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) over dot O-2, kinetics were well describe d by a simple mono-exponential function, following a short cardiodynamic ph ase, 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) over dot (O2) 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) over dot (O2) kin etics, however, remained mono-exponential (tau (1,off) = 42 +/- 11 s). 5. For very heavy exercise, the on-transient (V) over dot (O2), kinetics we re 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 Tie, kinetics revert ed to a mono-exponential profile (tau (1,on) = 34 +/- 7 s), while the off-t ransient 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) over dot (O2) during dyna mic 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.