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Within the fields of sports medicine and strength and conditioning the concept of symmetry has long been used to quantify injury risk, the safe return to play and the preferred way to perform exercises. Listening to other practitioners and podcasts this is common practice, and something often cited in the return to play research. But why is this? The idea that human beings should be symmetrical in every detail is accepted but not is actually observed in reality.
From an internal perspective our heart and lungs are asymmetrical in size and position. In daily life we use different hands and legs to perform tasks such as opening doors or balancing, causing a natural asymmetry to develop. Therefore, to expect athletes to present perfect symmetry in both anatomy and function is unrealistic. For more information on limb dominance and preference I recommend an excellent review by Virgile and Bishop.
Focusing specifically on injury risk and return to play, it is understandable why asymmetry took hold as a yard stick. The evidence post injury, is that one limb is weaker and less functional than the other. An asymmetry is observed which is a proverbial smoking gun. Second, when judging if a player can return to play safely, the only individualised reference point available to the therapist is the opposing limb.
In the absence of complex and expensive biomechanical testing, therapists will often compare the function of the injured limb to the non-injured (n.b. I avoid the term healthy as this is not always the case) thus establishing the mental loop that ties injury to symmetry.
However, many athletes who are asymmetrical do not get injured and many athletes who get injured are symmetrical. This side of the data is often not considered or ignored as only those that are injured are assessed.
“In the absence of complex and expensive biomechanical testing, therapists will often compare the function of the injured limb to the non-injured...thus establishing the mental loop that ties injury to symmetry".
When considering an athlete has having an asymmetry, how confident can you be in that judgement? Asymmetry is highly variable, unreliable and task specific. To illustrate this an athlete may perform two screening tests: the Y balance test and a unilateral counter-movement jump. These are often used in player readiness and risk assessments.
Measuring unilateral CMJ data with Output
It is frequently observed that an athlete may have better dynamic stability on their left leg but greater power output on their right. But re-test that athlete three days later and the size and side of asymmetry could have shifted, something not uncommon in the asymmetry literature. Effectively making the asymmetry value of most tests unreliable, with some exceptions, such as the rear foot elevated split squat 5RM (Helme et al., 2019).
The measurement of asymmetry is fraught with measurement issues, which were reported In a recent systematic review of the research into functional lower-limb asymmetry and injury in athletes (Helme et al. 2021). This paper showed that the evidence was weak, inconsistent and from moderate quality research.
Essentially, there is no sound evidence that asymmetry in any measure of lower body performance is related to suffering an injury (how long or how many injuries was not studied in these papers). The review commented that far more research of greater quality is required to substantiate this lack of evidence, but the fact remains, there is still nothing to support the measurement of asymmetry to measure the risk of injury.
This is common sense, once the rule of thumb that we should be symmetrical is discarded. If you were to try and explain the mechanism of why an imbalance between limbs causes an injury, it is difficult to give a clear and concise answer (I have tried many times). On the other hand, explaining why a lack of flexibility or strength causes an injury it is much easier.
During the rehab process, if symmetry is the goal, why not let both legs detrain to the same level of function? How does anyone know if the other limb is a valid reference point for injury prevention or not? After all, the athlete got injured in the first place, so something had to be dysfunctional for that to happen.
Symmetry is moving a goal post, forever changing as the functional ability of each limb changes in response to the environment. Chasing and maintaining symmetry, in the hope of preventing injury is a fool’s errand.
“Symmetry is moving a goal post, forever changing as the functional ability of each limb changes in response to the environment".
I propose that we stop interpreting the performance of a limb relative to the other leg, instead treating each limb independently. To do this external benchmark data is required, to determine the minimum and/or optimal functional capacity of a limb. The outcome being less reliant on the other limb for information. This can be taken from the research literature or internally created over time in an athletic group, by collating the limb performance data from those athletes who lose the least time to injuries.
“I propose that we stop interpreting the performance of a limb relative to the other leg, instead treating each limb independently".
Using the straight leg raise test, for example, over the course of time and repeated measurements in a group of athletes a ROM of above 80° may be indicative of reduced time lost to injury. Athlete A may have right and left leg mobility scores of 75° and 82° respectively. So, the action here is to ensure that both legs achieve this threshold score of 80°. It doesn’t matter if both arrive at 82° or that after a month the values are 85° and 90°.
Measuring straight leg raise ROM using Output
What actually matters is that both achieve the required level of functionality. If both limbs meet these standards, to differing degrees, so what? It doesn’t matter. This is probably perfect normally. If one limb is a great deal different from the other, it is my expectation that the weaker limb will be below the minimum threshold for performance and prevention.
The process can be just as importantly applied the rehabilitation process to make sure that one limb is progressing in rehab, but equally that the opposing limb doesn’t fall below the required standard. In a symmetry-based model, having two equal yet sub-optimal limbs is better than one being strong and the other weak. This is evidently not an effective strategy.
Through the exercise modes offered in Output, such as mobility range of motion and movement velocity an athlete can undergo a wide battery of tests, with little equipment and time costs. More importantly data can be collected and analysed on each limb for each of these limb functions independent on the other. When considered in relation to individual, squad and external research data practitioners can make highly reliable decisions about the robustness of an athlete, without having to rely on the moving goalpost that is the uninjured limb.
Dr. Mark Helme is Lecturer in Sport coaching at Leeds Beckett University, focusing specifically in the areas of Strength and Conditioning and Biomechanics. Mark completed his PhD at Leeds Beckett University examining the implications of leg strength asymmetry in sub-elite Rugby League players. His research interests include the development of strength, specifically how this interacts with sprinting, change of direction and injury costs in team sport players. Mark has been a strength and conditioning coach for nearly 20 years and has been accredited with the UKSCA since 2007, working with youth and senior athletes, specialising in Rugby League.