The passive elastic and viscous properties of isolated rat diaphragm muscle were studied, under various strains and strain rates similar to those in the animal, to measure their effects on the storage and release of mechanical potential energy. 2. Increasing the muscle length or amplitude of the displacement increased energy loss per stretch/shorten cycle, and increased the relative recovery of potential energy from the stretch during subsequent shortening. 3. Increasing strain rates increased energy loss per cycle and decreased the relative recovery of energy put into the stretch. 4. These effects were due to increased viscous resistance and increased elastic tension with increased length, and increased viscous resistance with increased strain rates. 5. The effects of increasing strain rate alone (1-4 Hz) were small relative to the effects of a 10% increase in muscle length or the amplitude of the length cycle. 6. Diaphragm muscle movements involving low velocity, small amplitude displacements at long muscle lengths are most effective at conserving net passive mechanical energy, while short muscle lengths minimize gross energy loss.
