Clearing athletes to return to sport after injury can be challenging. What tests should be used? How much testing is enough? What is the passing criteria?
Hop tests are often considered an important part of return-to-sport testing after knee injury. The tests can be performed nearly anywhere using minimal equipment, and they allow comparison between the injured and uninjured limb. But, should we be using hop tests?
Here are the pros and cons of hop testing.
Pros of Hop Testing
1. Excellent Reliability
For a test to be considered reliable, it must consistently produce the same values when tested by the same examiner during separate sessions (inter-rater) as well as when tested by different examiners (intra-rater). Described by Noyes et al. (1991), hop tests have been studied extensively and have been shown to have excellent reliability (Ross 2002, Munro 2011, Haitz 2014, Dingenen 2019).
2. Lower Extremity Power
Muscular power is a critical aspect of dynamic functional performance during sport. Maximal distance (vertical and horizontal) hop tests appear to measure this construct, as they require the athlete to accelerate and decelerate their bodies in a manner that is representative of typical sporting movements.
3. Neuromuscular Control/Balance
Typical hop testing protocols require athletes to “stick” their landings for two seconds in order for their trials to be recorded. This means that if the athlete has to touch down with their opposite leg, loses their balance, and/or has to perform an additional hop after landing, the hop has to be repeated (Logerstedt 2012). This criteria requires that a baseline level of neuromuscular control/balance must be achieved in order for the test to be completed.
4. Multi-directional
While the most commonly used hop tests (single hop for distance, triple hop for distance, triple crossover hop for distance, and 6m timed hop) predominantly consist of movements in the forward, horizontal direction, there are other hop tests that utilize multi-directional movements common in sports. The side hop, medial side triple hop for distance, and medial rotation hop for distance are examples (Gustavsson 2006, Dingenen 2019).
5. Endurance
While the role of fatigue as a risk factor in the incidence of ACL injury is up for debate, endurance is still an important capacity to assess when determining readiness to return to sport (Bourne 2019). Abrams et al. (2014) found that limb asymmetries became more apparent when more demanding tests requiring increased stamina were used. The side hop test is an endurance hop test that requires the athlete to hop laterally over a 40-cm distance as many times as possible for 30 seconds (Gustavsson 2006).
Cons of Hop Testing
1. Overestimates Knee Function
The involved limb is not the only side affected by injury. The uninvolved limb also becomes deconditioned due to decreased activity levels.
Gokeler et al. (2017) compared hop testing in patients after ACL reconstruction to normative values for healthy controls published by Myers et al. (2014). The mean limb symmetry index of their patients (95.4%) was well over the clinical cut-off of 90%, but they still demonstrated significant deficits when compared to normative data. Comparing both limbs to normative values showed that shorter hop distances on the uninvolved limb are a contributing factor to the high level of symmetry.
2. Does Not Isolate Quadriceps Function
Testing quadriceps strength in isolation requires more technical equipment than hop testing. Quadriceps strength can be objectively measured using an isokinetic dynamometer (gold standard) or handheld dynamometer with external fixation. Clinicians without this equipment may believe that hop testing is sufficient to test quadriceps function, but this is not the case.
For any test to be useful, it needs to be reliable and valid. One type of validity is called criterion validity, and refers to the degree to which a test/measure is correlated with another variable. In this case, we’re talking about how hop test performance correlates with quadriceps strength.
Barfod et al. (2019) investigated whether hop distance was correlated to quadriceps strength. They found that a “single leg hop test cannot be used as a surrogate measure for knee extensor strength as satisfactory hop test performance can be achieved in the absence of satisfactory knee extensor strength” (Barfod 2019).
This aligns with findings from Nagai et al. (2020). While there is a correlation between quadriceps strength and hop distance, hop testing consistently demonstrates higher limb symmetry values than isokinetic testing. Hop testing likely overestimates knee function, potentially leading to premature clearance for return to sport.
3. Neuromuscular Control/Balance
Wait, didn’t you just say that this was one of the benefits of hop testing above? Yeah, about that… While the output of hop testing (i.e. distance traveled/time) does depend on having a baseline level of control (i.e., you have to “stick” the landing). There is a large amount of variability in the amount of control a person can display before their trial is discarded.
A person could jump the same distance on each leg, but one leg could be “sloppier” than the other without nullifying the trial. Unfortunately, quality is not factored in when determining limb symmetry with hop tests. Thus, it’s important to not only assess the quantity of the hop test, but the quality as well.
4. Compensations
Because hop testing does not isolate a specific joint or muscle, the athlete may compensate with uninjured joints (hip and ankle). Garrison et al. (2018) found that during simple tasks such as a double-leg squat, patients after ACL reconstruction shift load away from the knee to the hip.
Kotsifaki et al. (2020) found this to be the case when analyzing hop testing as well. Patients after ACL reconstruction had lower knee flexion angles and lower knee power absorption during landing. Check out the tweet below.
Here’s an example from @RoulaKotsifaki where the performance is identical, kinematics are similar but joint power quite different.
Knee joint angle at Peak knee power absorption <10 degrees
Almost double the power difference pic.twitter.com/eH8WuNTbbz— Rod Whiteley (@RodWhiteley) December 20, 2019
As you can see, the darker skeleton takes off and lands with less knee flexion compared to the lighter skeleton. This decrease in knee flexion corresponded to less power absorption at the knee, despite hopping the same distance.
Wren et al. (2018) also found differences in lower extremity kinematics and kinetics during hop testing. Compared to healthy controls, patients after ACL reconstruction had lower knee flexion moments when landing.
Also, patients after ACL reconstruction who did not have symmetric hop distance between limbs tended to offload more towards the ankle when landing on the involved limb. Patients who were symmetric tended to offload more towards the hip of the involved limb.
Patients with symmetric hop distances may just develop more efficient compensations by offloading towards the larger hip musculature.
While differences in joint angles are often seen in research, these differences can be difficult to detect with the naked eye. Fortunately, commercially available video cameras and freely available software can provide a practical and viable alternative (Welling 2018).
5. Minimal Neurocognitive Demands
While hop testing may mimic some of the demands of a sport, sporting activities occur in an open environment where the athletes have to react to the situation and their surroundings. Hop testing assesses the physical demands, but not the neurocognitive demands critical to performance and potentially re-injury prevention.
Reproducing the demands of a sporting environment typically requires the use of expensive laboratory equipment to add visual and/or cognitive load to functional testing. However, emerging research has investigated the use of light system to introduce a reactive cognitive load to traditional functional testing in order to better simulate the demands of sport (Simon 2020).
These neurocognitive hop tests were found to be more challenging, resulting in decreased performance. While this study was performed on healthy individuals, it could potentially be more sensitive to between limb differences that may occur after injury.
Summary
Hop tests have clinical utility, but should NOT be used in isolation when determining readiness to return to sport. When utilizing hop testing in your return-to-sport decision-making process, be sure to consider the pros and cons.
Pros of hop testing
- Excellent reliability
- Simple measure of lower extremity power
- Requires neuromuscular control and dynamic balance
- Can test movement in multiple planes
- Simple measure of lower extremity endurance
Cons of hop testing
- Overestimates knee function
- Cannot be used as surrogate measure for quadriceps strength
- Difficult to measure quality of hop
- Ankle and hip can compensate for deficits in knee function
- Minimal neurocognitive demands for traditional tests