Forty-one percent of injuries affecting athletes occur in the knee, with one-fifth of knee injuries involving the anterior cruciate ligament (ACL). Single-leg hop tests are a common component of assessing readiness to return-to-sport (RTS) after ACL reconstruction (ACLR). However, patients can meet hop test benchmarks of overall distance and time, despite significant movement compensations in the injured limb. The purpose of this study was to determine if temporospatial hop test characteristics (i.e., stance time, flight time) differed between limbs in patients who passed clinical RTS testing after ACLR.
Thirty-five participants 10-25 years old and within 5-15 months of ACLR who were planning to return to sport were included. Patients who previously had surgery in either knee or concomitant posterior cruciate ligament reconstruction were excluded. Participants completed clinic testing when RTS was being planned until they passed RTS criteria, which required >90% symmetry between limbs for quadriceps strength (isometric at 90° knee flexion using a crane scale and 1-repetition maximum using a knee extension machine) and 4 single-leg hop tests (single, crossover, and triple hops for distance, 6-meter timed hop). Participants also had to score >90% on the IKDC 2000 and Global Rating Scale to pass RTS testing. During the session when patients passed RTS testing, 2 trials of the single, triple and 6-meter timed hop tests were conducted on a 6-meter instrumented pressure walkway (Protokinetics Zeno Walkway System, Havertown, PA) on each limb. Variables of interest included flight and stance time during hop tests. Flight-to-stance ratio during the triple and 6-meter timed hop was calculated as the ratio of the average flight time to average stance time during each trial. Paired t-tests were used to compare hop characteristics between limbs.
Participants were 17.5±3.0 years old, 51.4% female, and 10.8±2.8 months after ACLR. Thirty-four percent had a quadriceps tendon autograft, 43% had a patellar tendon autograft, 17% had a hamstring tendon autograft, and 6% had an iliotibial band autograft. Flight time in the involved limb was shorter during the single hop (Involved: 0.209±0.046 sec, Uninvolved: 0.221±0.050 sec; p=0.011), triple hop (Involved: 0.648±0.128 sec, Uninvolved: 0.671±0.135 sec; p=0.010) and 6-meter timed hop (Involved: 0.168±0.029 sec, Uninvolved: 0.176±0.031 sec; p<0.001). Average hop distance during the 6-meter timed hop was shorter in the involved limb (Involved: 135.9±21.3 cm, Uninvolved: 140.6±21.8 cm; p<0.001). Stance time did not differ between limbs during the triple hop (p=0.162) or the 6-meter timed hop (p=0.140). Flight-to-stance ratio was shorter in the involved limb for the triple hop (Involved: 0.907±0.218, Uninvolved: 0.986±0.269; p=0.001) and the 6-meter timed hop (Involved: 0.646±0.144, Uninvolved: 0.684±0.154; p<0.001).
Distinct differences in temporospatial hop test characteristics were present in patients who met clinical RTS criteria, most notably in decreased flight time in the injured limb. These findings indicate that movement patterns may not be restored despite meeting traditional hop benchmarks of total distance and time. Further testing to investigate the impact of hop characteristics on patient outcomes during ACL recovery and potential interventions to improve these hop characteristics is warranted.