AbstractThe calf muscle metabolism of 7 patients with stable chronic respiratory failure (PaO2below 65 Torr) was studied using31P NMR spectroscopy. NMR spectra were acquired at rest, during the course of 360 pedal movements at 20, 35 and 50% of the maximal voluntary contraction (MVC) and during recovery. Eight normal aged‐matched subjects served as a control group. In resting muscle, no significant differences were observed between both groups as regards intracellular pH, inorganic phosphate/phosphocreatine (Pi/PCr) and β‐ATP/PCr + Pi + phosphomonoester (PME) ratios. Although effective power outputs were similar for both groups at each work level, patients exhibited a higher Pi/PCr ratio than healthy controls (3.19 ± 1.01 vs 0.49 ± 0.05 at 50% MVC;p<0.01) and a lower pHi(6.65 ± 0.11 vs 7.06 ± 0.02 at 50% MVC;p<0.01). Moreover, PCr resynthesis during recovery was slower in patients than in control subjects (t1/2PCr = 1.26 ± 0.30 vs 0.47 ± 0.05 min;p= 0.01). These results suggest impairment of aerobic capacity in a non‐ventilatory working muscle, probably due to hypoxemia in patients with chronic respira