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We previously reported that active muscle stiffness could be evaluated in vivo. However, we were not able to investigate active muscle stiffness as more than 250 deg·s -1 due to the limitation of the torque motor of dynamometer. The aim of the present study was to investigate the effect of angular velocities (including higher angular velocities of more than 250 deg·s -1 ) on active muscle stiffness. Eighteen males volunteered for this study. Active muscle stiffness of the medial gastrocnemius muscle was calculated according to changes in the estimated muscle force and fascicle length during fast lengthening at five different angular velocities (100, 200, 300, 500, and 600 deg·s -1 ). Electromyographic activities of the lateral gastrocnemius muscle (LG) and soleus muscle (SOL) were evaluated over two different phases: before the stretch (mEMGa) and after the stretch (mMEGb). Active muscle stiffness was higher at 300 than at 100 deg·s -1 , but decreased as the angular velocity increased from 300 to 600 deg·s -1 . There were no differences in mEMGa and mEMGb values among the five angular velocities, whereas mEMGb values were higher than mEMGa for all angular velocities. In conclusion, active muscle stiffness was highest at 300 deg·s -1 and decreased at both slower and faster angular velocities., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

The aim of the present study was to compare passive and active muscle stiffness and tendon stiffness between long distance runners and untrained men. Twenty long distance runners and 24 untrained men participated in this study. Active muscle stiffness in the medial gastrocnemius muscle was calculated according to changes in estimated muscle force and fascicle length during fast stretching after submaximal isometric contractions. Passive muscle stiffness was also calculated from estimated passive muscle force and fascicle length during slow passive stretching. Tendon stiffness was determined during isometric plantar flexion by ultrasonography. Passive muscle stiffness of long distance runners was significantly higher than that of untrained men (p<0.001). Active muscle stiffness at all torque levels of long distance runners was also significantly higher than that of untrained men (p<0.001). No significant difference was observed in tendon stiffness between long distance runners and untrained men (p=0.869). These results suggested that passive and active muscle stiffness were higher in long distance runners than in untrained men, whereas no significant difference was observed in tendon stiffness between the two groups., (Copyright © 2015 Elsevier Ltd. All rights reserved.)