At the onset of constant-load exercise in healthy humans, the rate of muscle metabolism increases to a steady state. However, when exercise is preceded by a bout of high-intensity or 'heavy exercise' several minutes prior, steady state is typically achieved more quickly. The aim of the present study was to study the metabolic effects of prior heavy exercise on a progressive exercise protocol, in which steady state is not achieved.
Scientific Abstract:
At the onset of constant-load exercise, the rate of muscle metabolism increases to a steady state. When exercise is preceded by a bout of heavy intensity (HVY) exercise, steady state is typically achieved more quickly. The effects of a prior HVY bout on a progressive exercise protocol, in which a steady state is not achieved, are unknown. The present study, therefore, was designed to study the effects of a prior HVY bout on a progressive plantar flexion exercise protocol in healthy humans. Subjects (n=5 males, age 25 ± 4 years) completed two randomized exercise protocols that included a progressive exercise protocol (RAMP) and a progressive exercise protocol preceded 6 min prior by 6 min of HVY (HVY-RAMP). Exercise involved repeated isotonic plantar flexion (0.5 Hz, ~ 40° ROM) against an incremental resistance (~1.0 W/min) and was performed inside a 3.0 Tesla magnet for data collection using 31P-MRS (15s time acquisition per spectrum). All subjects had previously completed at least one familiarization RAMP trial, from which the steady state workload in HVY was calculated as the power output halfway between the onset of intracellular acidification and volitional fatigue. Our findings were, compared to the RAMP protocol, HVY-RAMP resulted in an increased (p < 0.05) time-to-fatigue during progressive exercise (13.0 ± 2.2 vs. 14.6 ± 1.7 min, RAMP vs. HVY-RAMP), but no differences in end-exercise concentrations of phosphocreatine ([PCr], 12.6 ± 7.5 vs. 14.0 ± 8.6 mM), inorganic phosphate ([Pi], 23.5 ± 11.2 vs. 27.7 ± 7.6 mM), or of intracellular pH (pHi, 6.6 ± 0.2 vs. 6.6 ± 0.1). However, the onset of intracellular acidosis was right-shifted in HVY-RAMP, such that it occurred at a greater (p < 0.05) power output (5.74 ± 1.52 vs. 8.22 ± 2.69 W). A biphasic pattern was also detected for the ratio [Pi]/[PCr], and similar to pHi, the onset of rapid changes in this ratio occurred at a greater (p < 0.05) power output in HVY-RAMP compared to RAMP (8.90 ± 1.71 vs. 10.63 ± 1.50 W). The rate (or slope) of changes in pHi or [Pi]/[PCr] before or after these inflection points were not different between conditions. In conclusion, as a consequence of prior
HVY exercise, the onset of the rapid fall in pHi is delayed, but once initiated, occurs at the same rate as in the RAMP protocol. This delay may be the result of an increased contribution of xidative metabolism suggested by the delayed onset of rapid increases in [Pi]/[PCr].