During exercise, defects in calcium (Ca²⁺) release have been proposed to impair muscle function. Here, we show that during exercise in mice and humans, the major Ca²⁺ release channel required for excitation–contraction coupling (ECC) in skeletal muscle, the ryanodine receptor (RyR1), is progressively PKA-hyperphosphorylated, S-nitrosylated, and depleted of the phosphodiesterase PDE4D3 and the RyR1 stabilizing subunit calstabin1 (FKBP12), resulting in “leaky” channels that cause decreased exercise tolerance in mice. Mice with skeletal muscle-specific calstabin1 deletion or PDE4D deficiency exhibited significantly impaired exercise capacity. A small molecule (S107) that prevents depletion of calstabin1 from the RyR1 complex improved force generation and exercise capacity, reduced Ca²⁺-dependent neutral protease calpain activity and plasma creatine kinase levels. Taken together, these data suggest a possible mechanism by which Ca²⁺ leak via calstabin1-depleted RyR1 channels leads to defective Ca²⁺ signaling, muscle damage, and impaired exercise capacity.