A decrease of deep knee flexion torque after anterior cruciate ligament (ACL) reconstruction using semitendinosus (ST) tendon has been reported. We hypothesized that architectural and functional differences of the hamstrings influence this weakness. Fiber length of the ST, gracilis (G), semimembranosus (SM), and biceps femoris (BF) were measured in six human cadavers. Electromyography (EMG) of the hamstrings were measured in both limbs of 16 patients after ACL reconstruction. Magnetic resonance imaging (MRI) was taken over both thighs of those patients to examine muscle volume and ST tendon regeneration. The fiber length of the ST and G were longer than that of the SM and BF. EMG values for the SM and BF of both limbs, and the ST of the ACL reconstructed limb became significantly reduced as the knee flexion angle increased. The volume of the ST in the reconstructed limb was significantly smaller than in the normal limb. Regeneration of the ST tendon was confirmed in all subjects, and the musculotendinous junction of the reconstructed limb shifted proximally when compared to that of the normal limb. The atrophy and shortening of the ST after its tendon is harvested and the lack of compensation from the SM and BF due to architectural differences among the hamstrings could cause the weakness of deep knee flexion torque after ACL reconstruction.
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