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Force enhancement of quadriceps femoris in vivo and its dependence on stretch-induced muscle architectural changes.

The purpose of this study was to investigate if force enhancement (FE) in vivo is influenced by stretch-induced changes of muscle architecture. Therefore, 18 subjects performed maximum voluntary isometric (100 degree knee flexion angle) and isometric-eccentric-isometric stretch contractions (80 degrees-100 degrees; ω=60 degrees.s(-1)) whereby pennation angle and fascicle length of vastus lateralis was determined using ultrasonography. We found significant (2-way repeated ANOVA; α=0.05) enhanced torque of 5-10% after stretch as well as significant passive FE but no significant differences in muscle architecture between isometric and stretch contractions at final knee angle. Furthermore, EMG recordings during a follow-up study (n=10) did not show significant differences in activation and mean frequency of contraction conditions. These results indicate that FE in vivo is not influenced by muscle architectural changes due to stretch.