The relationship between intramuscular pH and the frequency components of the surface electromyographic (EMG) power spectrum from the vastus lateralis muscle was studied in eight healthy male subjects during brief dynamic exercise. The studies were carried out in placebo control and metabolic
alkalosis induced by
oral administration of NaHCO3. At the onset of exercise, blood pH was 0.08 units higher in
alkalosis compared with placebo. Muscle
lactate accumulation during exercise was higher in
alkalosis (32 +/- 5 mmol/kg wet wt) than in placebo (17 +/- 4 mmol/kg wet wt), but no difference in intramuscular pH was found between the two conditions. The EMG power spectrum was shifted toward lower frequencies during
fatigue in the control condition (10.1 +/- 0.9%), and these spectral shifts, evaluated from changes in the mean power frequency (MPF) of the EMG power spectrum, were further accentuated in
alkalosis (19 +/- 2%). Although the changes in frequency components of EMG correlated with muscle
lactate accumulation (r = 0.68, P less than 0.01), no direct relationship with muscle pH was observed. We conclude that
alkalosis results in a greater reduction in MPF associated with a higher muscle
lactate accumulation. However, the good correlation observed between the two variables is not likely causative, and a dissociation between intramuscular pH and the increase in the low-frequency content of EMG power spectrum appears during muscle fatigue.