Muscle metabolic status and acid-base balance during 10-s work:5-s recovery intermittent and continuous exercise

Gastrocnemius muscle phosphocreatine (PCr]) and hydrogen ion (H(+)]) were measured using (31)P-magnetic resonance spectroscopy during repeated bouts of 10-s heavy-intensity (HI) exercise and 5-s rest compared with continuous (CONT) HI exercise. Recreationally active male subjects (n = 7; 28 yr ± 9 yr) performed on separate occasions 12 min of isotonic plantar flexion (0.75 Hz) CONT and intermittent (INT; 10-s exercise, 5-s rest) exercise. The HI power output in both CONT and INT was set at 50% of the difference between the power output associated with the onset of intracellular acidosis and peak exercise determined from a prior incremental plantar flexion protocol. Intracellular concentrations of PCr] and H(+)] were calculated at 4 s and 9 s of the work period and at 4 s of the rest period in INT and during CONT exercise. PCr] and H(+)] (mean ± SE) were greater at 4 s of the rest periods vs. 9 s of exercise over the course of the INT exercise bout: PCr] (20.7 mM ± 0.6 vs. 18.7 mM ± 0.5; P < 0.01); H(+)] (370 nM ± 13.50 vs. 284 nM ± 13.6; P < 0.05). Average H(+)] was similar for CONT vs. INT. We therefore suggest that there is a glycolytic contribution to ATP recovery during the very short rest period (<5 s) of INT and that the greater average power output of CONT did not manifest in greater H(+)] and greater glycolytic contribution compared with INT exercise.