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  • Writer's pictureLuke Nelson

Is the weak getting STRONGER or is the strong getting WEAKER?

Updated: Jun 1, 2020

Comparing the function of the injured leg to the uninjured leg is commonly used to assess progress & return to play readiness. But is the uninjured leg always a good comparison??

In an ideal world, the injured/weaker side gets stronger with rehabilitation to equal the uninjured side and achieve symmetry. But does this always happen?

This question was examined in a recent study by Patterson et al. They followed 59 participants with a history of ACL repair over 5 years, and compared the performance of a battery of tests (single-hop for distance, triple hop for distance, side hop and the single-leg rise test) at one year and 5 years. Healthy controls were also used as reference data.

What did they find?

Comparing the test results at 1 and 5 years, they found the injured and uninjured sides became closer to symmetry BUT this came at the expense of the uninjured (contralateral) limb, which decreased in performance in three of the hop tests!

This led the authors to conclude that: “The leg symmetry index (comparing injured & uninjured sides) should not be used in isolation as it may incorrectly indicate or overestimate improvement in functional ability following ACL reconstruction, due to worsening contralateral limb function”.

What are the key takeaways here?

  1. Firstly don’t neglect your UNINJURED leg throughout your rehab: insufficient rehab can result in the uninjured side getting WEAKER! A lot of people and therapists will only focus on the injured leg, or use the same resistance on the uninjured and injured leg, and therefore essentially underload the stronger leg.

  2. Secondly, it’s important to assess and measure the function of the uninjured leg early in the rehab process, and ensure that as you near the end of rehabilitation, the uninjured leg hasn’t decreased function!

  3. Specific to ACL injuries, ensure that you have your uninjured leg tested BEFORE surgery

  4. It is also important to consider for those with a previous significant injury (such as an ACL rupture), the uninjured leg may not be an ideal comparison, as it could also be impaired in function. This is why using matched normals are important to compare to. In fact in O'Neill's 2019 study on those with Achilles tendinopathy, he found that those with Achilles issues often had weakness on their OPPOSITE leg, and cautions using this leg as a comparison.

At Health & High Performance we utilize a variety of functional tests, including our state of the art AxIT system, to objectively assess the function of your injured AND uninjured limbs, to improve your chances of making a successful return to play.

For further assistance please don’t hesitate to contact us at


  • O'Neill, S., et al. (2019). "Plantarflexor strength and endurance deficits associated with mid-portion Achilles tendinopathy: The role of soleus." Physical Therapy in Sport 37: 69-76.

  • Patterson, B. E., et al. (2020). "Limb symmetry index on a functional test battery improves between one and five years after anterior cruciate ligament reconstruction, primarily due to worsening contralateral limb function." Physical Therapy in Sport 44: 67-74.


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