Net ATP production rates in exercising human muscle were calculated from 31P MRS kinetic data collected with 1-8 s time resolution. During the exercise, the muscle output was measured using a force transducer. Muscle output (force/unit volume) is then divided by the calculated net ATP used to estimate the ATP cost of force production. These measurements have been performed using an exercise protocol of isometric maximum voluntary contraction of the gastrocnemius/soleus muscle group. Results show that by the end of 30 s of exercise, total ATP production and ATP cost of force production had stabilized and remained constant (within the measurement errors) until the end of the 2-min period. The results also demonstrate that either the ATP cost of force production in the human gastrocnemius/soleus is lower in the first second than at any other time in the 2-min window which was investigated, or that the ATP production is being underestimated at the later time points. These data on ATP production rates, and the data relating to ATP cost of force production in muscle will aid in understanding the causes of muscle failure as well as the progression and treatment of muscle disease.