The effects of prior moderate and prior heavy intensity exercise on the subsequent metabolic response to incremental exercise were examined. Healthy, young adult subjects (n = 8) performed three randomized plantar-flexion exercise tests: 1) an incremental exercise test (~0.6 W/min) to volitional fatigue (RAMP); 2) RAMP preceded by 6 min moderate-intensity, constant-load exercise below the intracellular pH threshold (pHT; MOD-RAMP); and 3) RAMP preceded by 6 min heavy-intensity, constant-load exercise above pHT (HVY-RAMP); the constant-load and incremental exercise periods were separated by 6 min rest. ³¹Phosphorus magnetic resonance spectroscopy was used to continuously monitor intracellular pH, phosphocreatine ([PCr]), and inorganic phosphate ([P_i]). No differences in exercise performance or the metabolic response to exercise were observed between RAMP and MOD-RAMP. However, compared to RAMP, a 14 % (SD 10) increase (P<0.01) in peak power output (PPO) was observed in HVY-RAMP. The improved exercise performance in HVY-RAMP was accompanied by a delayed (P=0.01) onset of intracellular acidosis [HVY-RAMP: 60.4 % PPO (SD 11.7) vs. RAMP: 45.8 % PPO (SD 9.4)], and a delayed (P<0.01) onset of rapid increases in [P_i]/[PCr] [HVY-RAMP: 61.5 % PPO (SD 12.0) vs. RAMP: 45.1 % PPO (SD 9.1)]. In conclusion, prior heavy-intensity delayed the onset of intracellular acidosis and enhanced exercise performance during a subsequent incremental exercise test.