Purpose: This study was conducted to identify factors other than morphological muscle strength factors that affect injured and uninjured sides of knee flexors with anterior cruciate ligament (ACL) lesions. Methods: The study population consisted of 22 patients with ACL lesions. Their hamstring muscle volume was measured on MRI, and muscle torque per muscle volume was calculated as the peak torque of knee flexion divided by hamstring muscle volume. Results: The mean muscle torque per unit volume of hamstrings in patients with ACL rupture was 0.09 ± 0.02 Nm/cm3 at 60°/s and 0.08 ± 0.01 Nm/cm3 at 180°/s on the injured side, and 0.11 ± 0.02 Nm/cm3 at 60°/s and 0. 08 ± 0.01 Nm/cm3 at 180°/s on the uninjured side. The mean muscle torque per unit volume of hamstrings in control subjects was 0.11 ± 0.02 Nm/cm3 at 60°/s and 0.08 ± 0.03 Nm/cm3 at 180°/s. One-factor ANOVA analysis found no significant differences between the three groups at either flexion velocity. Conclusions: Neurological dysfunction does not appear to exist in knee flexor muscles after ACL injury, unlike the quadriceps. Since the mechanism of muscle weakness will differ depending on the muscle, it is important for clinicians to take this discrepancy into consideration. Level of evidence: II.
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