Maximal fascicle shortening velocity measurements in human medial gastrocnemius muscle in vivo

This study evaluated the maximal fascicle shortening velocity under near‐no‐load conditions. In addition, we determined whether the rate of torque development during ballistic contraction was related to maximal fascicle shortening velocity. Under passive and active conditions, the medial gastrocnemius muscle fascicle shortening velocity was measured using ultrasonography at 300, 400, 500, 600, 700, 800, 900, and 1000 ° s(−1). The maximal fascicle shortening velocity was defined as the fascicle shortening velocity under the lowest angular velocity that satisfied the following two conditions; (1) the difference in torque values between passive and active conditions was below 2.4 Nm and (2) the difference in fascicle shortening velocities between passive and active conditions was below 10 mm s(−1). The rate of torque development was analyzed during the periods of 32, 48, 96, 152, and 200 ms after the onset of contraction during ballistic contraction. At the angular velocity (678.6 ± 147.7 ° s(−1)) that satisfied the two previously mentioned conditions, the exerted torque and the maximal fascicle shortening velocity were 1.4 ± 1.3 Nm and 251.0 ± 40.5 mm s(−1). No significant correlations were found between the maximal fascicle shortening velocity and the rate of torque development at each time point. In conclusion, the maximal fascicle shortening velocity was quantified when the angular velocity satisfied the two conditions. Furthermore, the rate of torque development, often used as an indicator of muscle velocity, did not represent the maximal fascicle shortening velocity.