Effect of pH on sensation and vastus lateralis electromyogram during cycling exercise

Six male subjects performed 30 min of cycling exercise: 15 min at 50% of maximum oxygen consumption (VO2 max) and 15 min at 80% VO2 max. On random days subjects ingested 300 mg/kg body wt of NH4Cl to induce acidosis, NaHCO3 to induce alkalosis, or CaCO3 as a placebo during a 3-h preexercise period. Blood pH at the onset of exercise was 7.238 in acidosis, 7.435 in alkalosis, and 7.394 in the placebo control. A direct ratio scaling technique was employed to measure the sensation of how hard exercise felt. Sensory intensity increased twofold after 15 min at 50% VO2max (P less than 0.01). There was no effect of pH on how hard exercise felt during 50% VO2max bouts. Sensory intensity at the end of heavy exercise increased 20% more in acidosis but was not different in alkalosis compared with placebo (P less than 0.05). There was no difference in integrated electromyogram (EMG) between conditions, but there was an average 13% integrated EMG increase by the end of exercise (P less than 0.05). Plasma lactate was lower in acidosis and higher in alkalosis than placebo at 80% VO2max. These data indicate that during heavy exercise acidotic pH accelerates the change in sensory intensity, but this phenomenon is not necessarily associated with changes in the integrated surface EMG.