The PRBS-technique is a useful tool to investigate muscle VO2 kinetics within the aerobic range. However, the validity of this kind of multi-frequent testing may be limited by non-linearities generated by the circulatory and pulmonary system. To check for such non-linear effects we compared the frequency responses computed from the multi-frequent PRBS with the results of pure sinusoidal testings (periods between 75 s and 450 s). Both methods show a good agreement for periods in the range between 113 s and 450 s. For shorter periods the results slightly diverged. The harmonic analyses of the sinusoidal data indicate increased stimulation of higher harmonics with increase in frequency. It is concluded that the VO2 in the lower frequency range shows only little, if any, influence of the circulatory and pulmonary system. Differences between the methods in the higher frequency range may be caused by non-linearities at the same or even lower frequencies. Therefore, periods shorter than 100 s are not suitable for assessing the muscle VO2 kinetics.