NASA: Several indicators of muscle metabolism were measured in dogs during exercise following 8 weeks of confinement in cages. Muscle tissue samples were studied at rest and following exercise for adenine nucleotides, creatine phosphate, creatine, glycogen, pyruvate, and lactate. Results indicate that confinement results in less efficient metabolic responses to exercise, decreased muscle glycogen at rest, and changes in the equilibrium between ATP breakdown and resynthesis during exercise.