Dynamic linearity of VO2 responses during aerobic exercise

The multifrequent pseudorandom binary sequence (PRBS) technique is a useful tool for studying oxygen uptake (VO2) kinetics within the aerobic range. However, the validity of this multifrequent test may be limited by nonlinearities generated by the circulatory and pulmonary system. To check for such nonlinear effects, we compared the frequency responses computed from two PRBS protocols with the results of pure sinusoidal frequencies varying in amplitude and mean values (periods between 50 s and 450 s). According to our results the VO2 frequency response does not seem to depend on the type of testing--PRBS or sine--or the changes within each test, i.e. mean power and power amplitude of the sine tests and the switching frequency of the PRBS. In the range of higher frequencies small differences between the test conditions may have been obscured by the greater scatter of dynamic responses. It was concluded that the VO2 frequency response was quasi-linear for periods down to the least 100 s. However, even in this range nonlinearities can be provoked by rest-exercise transitions, by a varying contribution of lactate or by an insufficient noise reduction.