Background Knee valgus angles have been identified as important predictors of ACL injury risk. To screen for athletes with poor dynamic knee stability simple field tests are needed.

Objective Purpose of this study was to find out if the subjective assessment of knee control in the single-leg squat task correlates with the two-dimensional video analysis of frontal plane knee angles.

Design Single-measure.

Setting The study took place at the UKK Institute, in Tampere, Finland from April 2011 to May 2013.

Participants 480 floorball, basketball, ice-hockey and volleyball players, out of which 283 were females (mean age 17 years, SD 4) and 197 were males (mean age 16 years, SD 2) .

Risk factor assessment The subject's ability to control the knee during the single-leg squat was assessed by an observer from the front side of the subject. The subjects squatted down to a 90 degree knee flexion angle. A scale from 0 to 2, where 0 equals “good performance” and 2 “poor performance”, was used. Frontal plane knee angles were calculated for each valid trial from the video image.

Main outcome measurements Subjective assessment of knee control, mean frontal plane knee angle.

Results The mean frontal plane knee angles were different among subjects rated “good performance” (right 2°, SD 7; left 1°, SD 7), “reduced performance” (right 10°, SD 9; left 8°, SD 7) or “poor performance” (right 20°, SD 9; left 18°, SD 9). These differences were statistically significant (P<.001) for right and left leg.

Conclusions This study suggests that the subjective assessment on knee control during a single-leg squat task is a suitable tool to screen for athletes with reduced knee control. In future, studies it is necessary to study if the athletes identified having poor or reduced control are in greater risk of knee injury.