Background Single-leg squat (SLS) is a clinical tool used to assess knee control. Previous studies have proposed that knee valgus motion in the SLS could be related to injury risk, but this association has not been previously examined.

Objective The objective of this study was to investigate the association between acute lower extremity injuries and frontal plane knee control in young athletes.

Design This study is part of a prospective cohort study (PROFITS-study). As part of the baseline test battery, we measured frontal plane projection angles during SLS. We also measured the athlete's height and weight, and the athletes filled out a questionnaire describing their time-loss injuries during the past 12 months. We registered new time-loss injuries in the following 12 months on a weekly basis. Only athletes free from lower extremity injuries during the 12 months prior to baseline were included in the analyses.

Setting Research institute, youth sports.

Participants Ten local basketball and ten local floorball teams were invited to the study. Out of these, 9 basketball and 9 floorball teams agreed to participate. Only athletes who were 21 years or younger and were free from injury during baseline were eligible. Complete data were obtained from 306 athletes.

Assessment of Risk Factors A generalized linear mixed model was used to analyse the potential risk factors.

Main Outcome Measurements Acute time loss lower extremity injuries sustained during the 12 month period after baseline.

Results Displaying a frontal plane projection angle greater than 1 standard deviation above the mean (>23.8°) in the SLS was associated with a higher risk of lower extremity injuries (OR 5.57, 95% CI 1.61–19.32) and ankle injuries (OR 2.37, 95% CI 1.13–4.98).

Conclusions Based on this study, athletes who display large frontal plane projection angles during the SLS have an elevated risk of lower extremity injuries.