Measurement of human skeletal muscle oxidative capacity by ³¹P-MR spectroscopy: A cross-validation with in vitro measurements

Purpose: To cross-validate skeletal muscle oxidative capacity measured by ³¹P-MR spectroscopy with in vitro measurements of oxidative capacity in mitochondria isolated from muscle biopsies of the same muscle group in 18 healthy adults. Materials and Methods: Oxidative capacity in vivo was determined from PCr recovery kinetics following a 30-s maximal isometric knee extension. State 3 respiration was measured in isolated mitochondria using high-resolution respirometry. A second cohort of 10 individuals underwent two ³¹P-MRS testing sessions to assess the test-retest reproducibility of the method. Results: Overall, the in vivo and in vitro methods were well-correlated (r = 0.66-0.72) and showed good agreement by Bland Altman plots. Excellent reproducibility was observed for the PCr recovery rate constant (CV = 4.6%; ICC = 0.85) and calculated oxidative capacity (CV = 3.4%; ICC = 0.83). Conclusion: These results indicate that ³¹P-MRS corresponds well with gold-standard in vitro measurements and is highly reproducible.