Faster O2 uptake (VO2) kinetics has been observed during moderate-intensity (Mod) exercise following a prior bout of heavy-intensity (HVY) exercise (J. Appl. Physiol. 98:1371, 2005). We examined the activation of pyruvate dehydrogenase activity (PDHa) during Mod both with and without prior HVY. Subjects (n = 9) performed a Mod1-HVY-Mod2 protocol preceded by 20W baseline (BSL). Breath-by-breath VO2 kinetics and NIRS-derived muscle oxygenation were measured continuously and muscle biopsies were taken at specific times during the transition to Mod. In Mod1, PDHa increased from BSL (1.08 ± 0.2 mmol/min/kg ww) to 30 s (2.05 ± 0.2 mmol/min/kg ww), with no additional change at 6 min exercise (2.07 ± 0.3 mmol/min/kg ww). In Mod2, PDHa was already elevated at BSL (1.88 ± 0.3 mmol/min/kg ww) and was greater than in Mod1, and did not change at 30 s (1.96 ± 0.2 mmol/min/kg ww) but increased at 6 min exercise (2.70 0.3 mmol/min/kg ww). VO2 kinetics were faster in Mod2 (19 ± 2 s) than Mod1 (24 ± 3 s). PCr breakdown from BSL to 30 s was greater (P<0.05) in Mod1 (13.6 ± 6.7 mmol kg dw-1) than Mod2 (6.5 ± 6.2 mmol kg dw-1) but total PCr breakdown was similar between conditions (Mod1, 14.8 ± 7.4 mmol kg dw-1; Mod2, 20.1 ± 8.0 mmol kg dw-1). Both oxy- and total hemoglobin were elevated prior to and throughout Mod2 compared to Mod1. In conclusion, the greater PDH activity at BSL prior to Mod2 than Mod1 may have contributed in part to the faster VO2 kinetics in Mod2. That oxy- and total- hemoglobin were elevated prior to Mod2 suggest that greater muscle perfusion also may have contributed to the observed faster VO2 kinetics. These findings are consistent with metabolic inertia, via delayed activation of PDH, in part limiting the adaptation of pulmonary VO2 and muscle O2 consumption during the normal transition to exercise.