[Studies on the effects of constant exercise around the ventilatory threshold using spectral analysis of EMG and...

The effects constant exercise at three levels around the ventilatory threshold (VT) were investigated by spectral analysis of EMG and RR interval variability of ECG. The subjects were 11 healthy male students (ranging from 20 to 25 years old). Surface EMG from the knee extensor muscular (m. vastus medialis) and the RR interval of the ECG were recorded by bicycle ergometer during the three levels of exercise which were 20% less than the VT level (-20% VT), the VT level and 20% more than the VT level (+20 VT). Power spectral analysis of EMG was performed to obtain the power spectral density (PSD) of 4 bands (20-50Hz, 50-100Hz, 100-150Hz, 150-200Hz) every 2 seconds by the maximum entropy method (MEM). Power spectral analysis of RR interval variability of ECG was performed every 1 minute by MEM to obtain the low-frequency band (LFB, 0.04 to 0.15 Hz), the high-frequency band (HFB, 0.15 to 0.4 Hz) and the ratio of LFB to HFB (L/H ratio). Changes in Total PSD from 20 Hz to 200 Hz on EMG showed a higher value at the +20 VT level than at the VT level or -20 VT level. The %PSD of EMG at the band of 100-150 Hz and 150-200 Hz increased in the work load at the +20 VT level more than at the VT level or -20 VT level. Comparisons of the EMG parameters suggested a progressive recruitment of fast twitch fibers (type II b) with time. HF component of the RR interval variability showed a rapid decrease in proportion to load, whereas there was no such trend in the L/H ratio during the 3 levels of constant exercise levels. In each case, HF components of RR interval variability could be used as an indicator of parasympathetic nervous activities, but the L/H ratio did not clearly show sympathetic nervous activities during the constant exercise.