Article discusses the Mexican cavefish Astyanax mexicanus, has lost moderate-to-vigorous activity following cave colonization, reaching basal swim speeds ~3.7-fold slower than their river-dwelling counterpart. Collectively, the authors reveal broad skeletal muscle changes following cave colonization, displaying an adaptive skeletal muscle phenotype reminiscent to mammalian disuse and high-fat models while simultaneously maintaining a unique capacity for sustained muscle contraction via enhanced glycogen metabolism.