 After ACL reconstruction surgery, athletes must undergo a battery of tests before they're deemed ready to return to sport. Unfortunately, rates of re-injury remain high. That has motivated some researchers to take a closer look at some of the measures used to declare athletes ready to resume their previous level of activity. One group recently examined the triple hop test for distance. Their findings, reported in the American Journal of Sports Medicine, suggest that comparing hop distances between the affected and non-affected knee could mask serious differences in biomechanics. The team compared 24 male athletes cleared to return to sport after ACL reconstruction with 23 healthy male controls. They used a combination of force plate measurements and 3D motion analysis to capture the athlete's performance on the triple hop test. The triple hop test is one of several assessments clinicians used to clear athletes to return to sport after ACL surgery. The test consists of an initial propulsion phase, two rebound phases, and a final landing phase. A limb symmetry index of 90% is the recommended cutoff for safe return to sport. Limb symmetry refers to how closely an athlete's repaired limb can match the performance of their other limb. In this study, athletes who had undergone ACL reconstruction performed relatively well, achieving a high symmetry index of 97%. But their biomechanics during the triple hop test told a different story. Data on how much energy athletes generated or absorbed during each phase of the test revealed significant asymmetries between repaired and non-repaired limbs. While athletes reached up to 80% symmetry during some phases, symmetries as low as 54% and 66% were observed during the two rebound phases. These asymmetries would not be observed if only distance were measured during a triple hop test. Motion analysis showed that athletes who had undergone ACL reconstruction landed differently on their repaired limb, with the knee slightly more extended and with more flexion in the hip and trunk. An energy analysis confirmed that the repaired knees of these athletes contributed significantly less to take off during each hop compared to other knees. The results suggest that these athletes may use their hip to compensate for deficits in their affected knee. More broadly, the findings of this study reveal the limitations of the metric used during the triple hop test. Looking at distance alone, masks important asymmetries in knee function. Unfortunately, the expertise and equipment required to perform biomechanical assessments aren't available in routine clinical settings. Future studies are needed to explore viable test options for measuring athletes' readiness to return to sport, and to better understand how asymmetries in knee function can affect athlete performance on the road to recovery.