Hamstrings and Quadriceps Muscles Function in Subjects with Prior ACL Reconstruction Surgery

Background: As the knee joint is a common site for injury among younger people, the purpose of this study was to measure the skeletal muscle endurance and strength on people with prior anterior cruciate ligament (ACL) knee reconstruction surgery.

Method: Young healthy female subjects who reported having knee reconstruction surgery more than one-year prior were tested. The skeletal muscle endurance index (EI) of the hamstrings and quadriceps muscles was determined as the decline in the specific muscle acceleration in response to 2 Hz, 4 Hz, and 6 Hz electrical stimulation. Maximal isometric muscle strength (MVC) was measured in the hamstrings and quadriceps muscles.

Results: The hamstrings muscles in the injured leg had less endurance than the non-injured leg at 6 Hz stimulation (55.5 ± 13.2% versus 78.0 ± 13.3%, p < 0.01). Muscle endurance was not reduced in the quadriceps muscles in the injured leg compared to the non-injured leg at 6 Hz stimulation (78.0 ± 13.3% versus 80.3 ± 10.0%, p = 0.45). There were no differences in MVC between the injured and non-injured legs for either the hamstrings (p = 0.20) or quadriceps muscles (p = 0.67).

Conclusions: Muscle endurance was reduced in the hamstrings muscles at least one-year post injury, while hamstrings strength was recovered. Reduced hamstrings muscle endurance could be a result of lack of endurance training during rehabilitation. This may contribute to re-injury in the muscle, even in people who have recovered muscle strength.