Mitochondrial targeted SS-31 improves skeletal muscle function. In vivo mitochondrial coupling ratio (P/O) (A) and maximum mitochondrial ATP production (B) in the hindlimb muscles of aged mice were both increased 1 h after treatment with SS-31. In situ fatigue resistance in the aged mice was also increased 1 h after SS-31 treatment (C). Eight days of daily treatment with SS-31 led to increased endurance capacity in the aged mice (D) as well. Means ± SEM. n = 5-7 per group. **P <0.01 relative to age-matched control. ##P <0.01 relative to young control. Young - 5 months old; Old - 27 months old. Modified from Siegel et al. .