Skeletal muscle maximal oxidative capacity (ATP _max ) is a key component of age-related sarcopenia and muscle health. The contribution of mitochondrial morphology and electron transport chain supercomplex (SC) assemblies to ATP _max has yet to be determined in human muscle. ATP _max measured in vivo by ³¹ phosphorus magnetic resonance spectroscopy in the quadriceps femoris of nine volunteers (65.5 ± 3.3 years old) was correlated with muscle biopsy outcomes before and after 4 months of supervised exercise. Mitochondrial morphology was assessed in electron micrographs, and SCs were measured by blue native gel electrophoresis. In the sedentary conditions, ATP _max was positively associated with complex (C) I and CIII in SC I+III ₂ +IV _n and negatively associated with CI and CIII in SC I+III ₂ . Regarding mitochondrial morphology, ATP _max was positively associated with markers of mitochondrial elongation. Exercise training-induced increases in ATP _max were accompanied by mitochondrial elongation and by the redistribution of free complex III. Indicators of mitochondrial elongation were associated with the redistribution of specific complexes to SC I+III ₂ +IV _n . Higher skeletal muscle oxidative capacity in older adults is associated with mitochondrial elongation and the redistribution of electron transport chain complexes into higher rank SCs in the same muscle. Further, we provide evidence that mitochondrial elongation favors mitochondrial SC assembly.