It is controversial whether muscles ever reach their mitochondrial oxidative capacity (ox cap) in vivo. We hypothesized that this is possible if an adequately high [ADP] is achieved, i.e., under conditions of very low [PCr] without a similarly low pH. PURPOSE: To determine if under conditions of low [PCr] and stable pH muscle in vivo can achieve and sustain ATP supply at its ox cap. METHODS: Five subjects aged 38.0 ± 7.3 y performed voluntary isometric contractions of the leg anterior compartment (LAC) and first dorsal interosseous (FDI) muscles in 1) a protocol designed to elicit maximum energetic steady-state (SSmax) and 2) a shorter protocol of high-frequency contractions (Max) that reduced PCr as low as possible with little change in pH. We made magnetic resonance spectroscopic measurements of PCr, Pi, and ATP along with force measurements before, during, and after exercise. We estimated ox cap by kPCr*resting[PCr] and determined oxidative phosphorylation (ox phos) with the initial rate of PCr recovery following exercise. RESULTS: The estimated ox caps for FDI and LAC were 0.47 ± 0.04 and 0.70 ± 0.07 mM ATP s−1, respectively. In the Max test both muscles had ox phos rates equal to their ox caps. In both cases [PCr] at the end of exercise was very low (FDI: 6.3 ± 0.6 mM; LAC: 6.1 ± 1.3 mM) with no significant change in pH. At SSmax the FDI sustained an ox phos rate equal to its ox cap, with [PCr] in steady-state at 5.5 ± 1.0 mM. The LAC's [PCr] at SSmax was much higher (16.9 ± 1.6 mM) and its ox phos rate was only about half of its ox cap. Neither muscle showed a change in pH at SSmax. Exercise beyond SSmax exhibited steadily declining pH. CONCLUSION: This study shows that with a very low [PCr] and stable pH, muscle in vivo can achieve an ox phos rate equal to its ox cap. The ability to reach energetic steady-state at ox cap requires that the muscle achieve proton balance as evidenced by stable pH. Supported by NIH grants AR45184 and AR41928