The present study tested the hypotheses that Achilles tendon forces during fast concentric actions do not differ between extended and flexed knee positions, and this phenomenon is attributable to the force-length characteristics and electromyograms (EMGs) of gastrocnemius muscle. Seven healthy men performed static and concentric plantarflexions at fully extended (K0) and 0.785 rad (45°) flexed (K45) knee positions with the maximal effort. In concentric actions, the angular velocities were set at 0.524 (slow) and 6.109 rad s-1 (fast). Fascicle length of medial gastrocnemius (MG) was determined with ultrasonography. Surface EMGs of the MG were recorded during each action. Achilles tendon force was calculated from the torque and moment arm of the tendon. Peak tendon forces in fast concentric actions were similar in K0 and in K45, but those in static and slow concentric actions significantly (P<0.05) differed between the two positions. When the tendon force peaked, the fascicle lengths in each action and fascicle velocities in both concentric actions were significantly (P<0.05) greater in K0 than in K45. The EMGs were significantly (P<0.05) higher in K0 than K45 during each action. The results suggest that (1) the difference in the tendon forces between the two positions can be explained by the force-length and -velocity characteristics and the EMGs of the MG, and (2) the contribution of the MG to the tendon force in flexed knee positions is greater in concentric actions than expected from the results in static actions.
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