Exercise Training Effects on Muscle Function in Adults With Mitochondrial Myopathy

The goal of this observational study is to learn how exercise training affects molecular processes in skeletal muscle in adults with mitochondrial myopathy, compared with healthy adults. The main questions it aims to answer are: * How does exercise training affect mitochondrial activity and energy production pathways in skeletal muscle in people with mitochondrial myopathy? * How does exercise training affect molecular signals related to muscle growth, stress responses, and muscle-nerve communication in people with mitochondrial myopathy? Researchers will compare the trained leg to the untrained leg within the same participant, and also compare responses between participants with mitochondrial myopathy and healthy control participants, to see how molecular responses to exercise differ between groups. The participants will: * Complete a 3-4-week supervised exercise training program using one leg. * Undergo muscle biopsies from both the trained and untrained leg. * Complete basic muscle strength and physical function tests.