Do ‘kissing knees’ increase the risk of ACL injury?

4 min read. Posted in Knee
Written by Physio Network info

A recent study aimed to explore the association between inward knee motion and risk of future non-contact ACL injury in elite female athletes.

We reviewed this study in the latest issue of our Research Reviews – where industry experts break down the most recent and clinically relevant studies, for immediate application in the clinic.

What you’ll read below is a snippet from the review.

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Back to the study!


STUDY TITLE: Kiss goodbye to the ‘kissing knees’: no association between frontal plane inward knee motion and risk of future non-contact ACL injury in elite female athletes – Nilstad et al (2021)

Study reviewed by Travis Pollen in the June 2021 issue of the Research Reviews


Key points from the study

  • This study showed no association between inward knee motion in a double-leg drop vertical jump and single leg squat, and non-contact ACL injury.
  • However, it remains to be seen whether inward knee motion should never be considered cause for concern, or whether we’re yet to find the correct test to assess it.


Background and Objective

The debate over whether pre-season screening tests are useful for identifying ACL injury risk has persisted for years, due in part to mixed findings in the research. Therefore, the purpose of this study was to determine whether there was an association between inward knee motion in the double-leg drop vertical jump and single leg squat, and future non-contact ACL injury, in a large sample of elite female athletes.



Over an 8-year period, the researchers screened 880 elite female handball and football athletes in their pre-seasons and registered non-contact injuries twice yearly. Screening included 2D video recordings of a double-leg drop vertical jump (from a 30-cm box) and single-leg squats. From video, the researchers analyzed several hip and knee kinematic variables corresponding to inward knee motion (see Figure 1).




Out of the 880 athletes, there were 74 non-contact ACL injuries over the course of the study. There were no significant differences between the injured and uninjured athletes in any of the measures of inward knee motion (p > 0.05), and small effect sizes (Cohen’s d < 0.27).



Each athlete in this study was tested once and then followed up for injury over multiple seasons. The median time between testing and injury was 434 days, with >60% of injuries occurring over a year after testing. With so much time between testing and injury, it raises the question of whether the athletes’ screening data still reflected their mechanics at the time of injury.


Clinical Implications

Previous studies showing inward knee motion increased the risk of non-contact ACL injury (e.g. Hewett et al., 2005) were limited by relatively small sample sizes and low numbers of injuries, which increase the risk of spurious findings. The large sample size and number of injuries in this study increase our confidence that the null findings are not a statistical error.

Although the authors suggest we “kiss goodbye to the ‘kissing knees’”, we must not overextend the conclusions. Given that the sample was elite female handball and football athletes, we should be cautious in generalizing the findings to males, non-elite athletes, and other sports.

Furthermore, this study only investigated two movements: a double-leg drop vertical jump, which isn’t specific to the single-leg context of ACL injuries; and single-leg squats, which aren’t specific to the speed and load of ACL injuries. Prospective research on single-leg drop vertical jumps and cutting maneuvers is crucial to furthering our understanding of the importance of inward knee motion in ACL injury.


Here’s what this review looks like in our June issue.


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If you have a question, suggestion or a link to some related research, share below!

  • Jonathan Khoo

    yep this is going to require more research. while potentially it doesnt matter i would argue if it is an uncontrolled valgus movement with lots of vacillation in the hip and knee joint this is a sign of poor balance and .: increased ACL risk. If its just part of their normal anatomy to valgus then they may have full control of it. People with anteverted hips are more likely to valgus in and also people with longer femurs and i think likely pes planus too. Ladies and more hyperflexible people also have this tendency

    Jonathan Khoo | 11 January 2023 | Likes

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