Objective To assess whether anterior cruciate ligament (ACL) reconstruction with a hamstring graft leads to reduced hamstring endurance 9–12 months post-surgery, to investigate the relationship between hamstring endurance and knee function, and to assess if individuals have reduced hop performance when fatigued.

Study Design A cross-sectional inter-limb comparison study was undertaken with participants 9–12 months after an ACL reconstruction with a hamstring graft, and a group of age-, gender- and activity-matched controls.

Background In New Zealand, 80% of individuals with an ACL rupture have surgical reconstruction to reduce knee instability. Endurance capability in the muscles controlling the knee is poorly understood in this population despite many sporting activities requiring notable muscle endurance. The hamstring muscles, when active, provide important anatomical support to protect the reconstructed graft. In the absence of good hamstring endurance, fatigue may predispose individuals to re-injury.

Method Hamstring endurance was measured using a progressive fatigue test on an isokinetic dynamometer at a joint angular velocity of 120°/second. The dependant variable was the maximum number of repetitions performed. Statistical comparisons were made across injured, uninjured and control group limbs. Hop performance was measured using a series of hop tests. The single and triple hop for distance were repeated within two minutes of completing the hamstring fatigue protocol. Data analyses involved ANOVA and correlation coefficients.

Results There was a significant (p < 0.05) deficit in hamstring endurance observed between the injured leg (mean: 111 repetitions, SD 49) and uninjured leg (mean: 136 repetitions, SD 67) of the ACL group, but not between the uninjured and control group legs (mean: 124 repetitions, SD 50). There were no significant (p<0.05) correlations found between the perceived or performance-based knee function and hamstring endurance of the ACL-injured leg (R = -0.23 to 0.21). The single and triple hop distance of the injured leg was significantly less (p < 0.05) than the uninjured leg, pre and post hamstring fatigue (effect size 0.49 and 0.35 respectively). For both hops, there was a significant difference across pre- and post-fatigue measures irrespective of legs (effect size 0.53 and 0.76 respectively). There was no significant difference between the uninjured and control legs for the single hop for distance (effect size 0.01). A significant interaction effect (p = 0.049) indicated a different response to fatigue across the uninjured and control legs for the triple hop for distance; however, there was a low effect size (0.08).

Conclusion The observed 18% deficit in hamstring endurance across the ACL-reconstructed individual’s limbs is indicative of a notable loss in performance of the hamstring muscles at 9–12 months post-surgery, and provides initial support for the inclusion of hamstring endurance training in rehabilitation programmes post-surgery.
When the hamstring muscles are fatigued, single and triple hop distances are significantly less on the ACL-injured leg compared to the uninjured leg. Considering a large proportion of injuries occur in the final stages of a sporting event, hop testing under fatigued conditions is worthy of attention for return-to-sport testing.