Proton magnetic resonance of exercise-induced water changes in gastrocnemius muscle. Exercise-induced water concentration changes have been determined in the medial gastrocnemius muscle noninvasively with image-guided in vivo proton magnetic resonance spectroscopy (MRS). These measurements were performed on seven normal male volunteers during and after isometric and ischemic-isometric exercise at 5, 10, and 20% maximum voluntary contraction (MVC). The observed water flux changes during different phases of exercise have been interpreted in terms of fluid transfer between the vasculature and exercising muscle. Good correlation between individual MVC and changes in water fluxes and the decay of water content was observed after 20% MVC exercise level. MRS results have been found to be consistent with those reported in the literature based on invasive biopsy techniques. 