The force-velocity properties of skeletal muscle have an important influence on locomotor performance. All skeletal muscles produce less force the faster they shorten, and typically develop maximal power at velocities of about 30% of maximum shortening velocity (V_max). We used direct measurements of muscle mechanical function in two ankle extensor muscles of wild turkeys to test the hypothesis that during level running muscles operate at velocities that favor force, rather than power. Sonomicrometer measurements of muscle length, tendon strain gage measurements of muscle force, and bipolar electromyographs were taken as animals ran over a range of speeds and inclines. These measurements were integrated with previously measured values of muscle V_max for these muscles to calculate relative shortening velocity, V/V_max. At all speeds for level running the V/V_max values of the lateral gastrocnemius and the peroneus longus were low (<0.05), corresponding to the region of the force-velocity relationship where the muscles were capable of producing 90% of peak isometric force but only 35% of peak isotonic power. V/V_max increased in response to the demand for mechanical power with increases in running incline, and decreased to negative values to absorb energy during downhill running. Measurements of integrated EMG activity indicated that the volume of muscle required to produce a given force increased from level to uphill running. This observation is consistent with the idea that V/V_max is an important determinant of locomotor cost because it affects the volume of muscle that must be recruited to support body weight.