Is observational screening for increased risk of ACL injury reliable?

2018 United States forward Alex Morgan (13) scores a goal past Canada goalkeeper Stephanie Labbe (1) during the second half in the 2018 CONCACAF Women’s Championship final soccer match at Toyota Stadium. Credit: Kevin Jairaj-USA TODAY Sports

Knee injuries plague young female athletes at an alarming rate. Anterior cruciate ligament (ACL) injuries in particular take young athletes out of sports participation more than any other injury. Female athletes are up to 10 times more susceptible to this injury than their male counterparts(1). Preventing these injuries starts with identifying the risk factors that contribute to their incidence. One such associated finding in female athletes with ACL injuries is increased knee abduction moment (KAM) when performing a vertical drop jump.

The KAM forces increase when landing from a box jump with femoral adduction, knee valgus, and ankle eversion. Researchers in Ohio hypothesized that the KAM would be greater in young female athletes that went on to suffer an ACL injury. They evaluated the jumping biomechanics of 205 female athletes during their preseason training and continued to monitor them throughout their competition season. The researchers found that those athletes who demonstrated a peek landing KAM greater than 25.3 Nm showed a 6.8% increased risk of suffering an ACL injury.

Detection of high KAM forces requires a three-dimensional (3-D) movement evaluation laboratory such as used in the Ohio study. Therefore, sport scientists in Norway wondered if there was a more portable and economical way to identify female athletes at risk for ACL injury(2). They compared the results of real-time observational screening of athletes performing a box jump with a 3-D evaluation of the same athletes performing the same movements. The study trained three experienced physiotherapists to rate the landing posture of 60 elite Norwegian female football players. The therapists graded the landing posture as either:

  • 0 (good control with proper knee alignment over the middle toes without extraneous motion);
  • 1 (less control with some valgus movement at the knees or side to side instability at the knees upon landing);
  • 2 (poor knee alignment and control with significant valgus motion).

The training prior to the study was extensive, with each therapist evaluating up to 30 subjects in the training phase. The results of the observational portion of the study showed almost perfect consensus between the raters. The training program, therefore, produced excellent inter-rater reliability. Beyond showing reliability within their own observational measurements, the therapists’ ratings showed very good correlation with identifying those athletes with high knee valgus angles. However, when compared to the 3-D analysis, the ability to identify athletes with higher KAM using observational screening was moderate.  So while those athletes observed to have poor knee control did indeed exhibit higher knee valgus angles, they were not necessarily the ones with higher KAM forces. The researchers speculate that other factors besides knee valgus angles contribute to KAM.

Athletes with knee valgus angles of 10.8º or higher have shown increase risk of ACL injury. Therefore, identifying those with poor knee control using observational analysis may aid injury prevention programs. Observational analysis is consistent, reliable, portable, inexpensive, and quick when compared to laboratory 3-D motion analysis. While not able to identify high KAM with observation, physios can speedily recognize those players who may need extra preventative or neuromuscular training to maintain knee health throughout a playing season.


1. Br J Sports Med 2015;49:118-122.

2. JOSPT. 2014 May;44(5):358-65

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