Following an anterior cruciate ligament (ACL) injury, athletes typically undergo a battery of testing to help inform their readiness to return to sport. This battery usually includes the following:
- Physical examination (e.g. range of motion, quadriceps and hamstring strength)
- Subjective questionnaires about knee function (e.g. International Knee Documentation Committee 2000, Knee Injury and Osteoarthritis Outcome Score) and psychological readiness (e.g. The ACL Return to Sport After Injury scale)
- Functional testing (e.g. single-leg squats, single-leg hops, jump/landing assessments and cutting/stepping assessments)
A key component of the functional testing consists of a variety of hop tests. Some of the most common are the (1) single hop for distance, (2) triple hop for distance, (3) triple crossover hop for distance, and (4) six-meter timed hop.
Pass/fail on these hop tests is traditionally determined by the limb symmetry index (LSI), which is simply a ratio of the performances on each leg (i.e. hop distance on the involved leg divided by hop distance on the uninvolved leg). To pass a hop test, a LSI of 90%+ is generally considered acceptable (van Melick et al., 2016).
The truth is, by the time an athlete is most of the way through rehab, it’s not too difficult to hop within 10% distance of the uninvolved leg. In fact, athletes can often pass these hop tests even before achieving symmetry in quadriceps and hamstring strength! This is because hopping involves more joints than just the knee.
Remember, athletes are master compensators. When a challenge is put in front of them, they can often find a way to reach the target while working around any physical deficiencies they might still have. During a hop test, an athlete can make up for a lack of quadriceps power by generating extra power at the hip and/or ankle.
No doubt, the LSI from hop testing is important for return to sport decision-making. But by considering only the LSI from hop tests, we’re leaving a ton of useful information on the table that these tests can provide. Chiefly, in addition to the distances hopped, we should also be assessing movement strategies and movement quality (Kotsifaki et al., 2019).
Because there’s no one “optimal” or “right” way to move, our best bet is to compare the symmetry of movement strategies and movement quality between sides. In particular, here are five aspects of hop tests to compare sides on:
- Different take-off strategies between sides (hip- or knee-dominant). An athlete with deficiencies in quad strength/power can rely more on the hip musculature to propel themselves forward.
- Different landing strategies between sides (hip- or knee-dominant). Similar to the above, if the athlete can’t brake effectively using their quads, they can shift the force absorption requirements to the hip.
- Different impact absorption between sides (passive/overly stiff landing or active landing using the musculature to absorb the impact. If an athlete doesn’t have good eccentric quadriceps strength and rate of force development, they will likely land on a stiffer (straighter) knee and thus rely on the joint to take most of the impact when landing. This is opposed to using the musculature (and going into more knee flexion) to absorb the forces from landing, which would take stress off passive structures like the ACL.
- Different knee control during landing between sides (valgus collapse or knee stacked vertically over foot). If an athlete doesn’t have sufficient strength or neuromuscular control when landing, they will often land with a valgus moment (the knee collapses in on landing). This is something to be wary of, as this is a key mechanism of ACL injury.
- Different trunk control during landing between sides (lateral flexion/rotation or upright). If an athlete doesn’t have sufficient neuromuscular control around the trunk, they will often tip to the side of the leg they land on, or rotate their trunk toward this side. These two movement patterns have also been linked to ACL injury, so they are worth keeping an eye on.
Oftentimes even an experienced eye can miss these subtleties. For this reason, we recommend filming athletes during hop testing from both the front (frontal plane) and the side (sagittal plane). Filming allows you to slow the action down, watch the hops as many times as you need to, and even compare movement quality on each leg side-by-side (if you’re savvy with video editing).
Clearly, an athlete can continue to exhibit important differences in function despite achieving a LSI of even 100%. In addition to LSI, examining the above strategies and qualities during hop tests gives us a much better idea of whether the athlete is truly at full function compared to the uninvolved side.