Muscle fibre types differ in both contractile and metabolic properties. Here we show that muscle fibre types also vary in inherent mitochondrial properties, such as coupling status (ATP/O2 or P/O), and have impact on the pace of mitochondrial and cell aging. Innovative optical and magnetic resonance spectroscopic methods applied to non-invasively measure ATP synthesis and O2 uptake revealed well-coupled mitochondria (P/O=2.3-2.5) in vivo in resting hand (first dorsal interosseus, FDI: P/O=2.7±0.1 SEM, n=10) vs. leg (tibialis anterior, TA: P/O=2.0±0.2 SEM, n=10) in adult subjects (34±4 years). These methods also revealed a significant decrease in coupling correlated with depletion of [ATP] in the FDI but not in the TA in the elderly (74±3 years). The degree of age-related uncoupling increased with type II muscle fibre content in agreement with the higher reactive oxygen species production in this cell type. This variation in mitochondrial coupling with age and muscle fibre type points to intrinsic cellular factors as critical to mitochondrial function and to the tempo of mitochondrial aging in human muscle.