Background A high knee abduction moment (KAM) and an erect movement pattern during landing have been suggested as anterior cruciate ligament (ACL) injury risk factors. Previous research focused mainly on determination of parameters with a predictive value for ACL injuries. However, few studies have investigated neuromuscular activity patterns.

Objective To investigate muscular activity of quadriceps and hamstrings with respect to a flexed or erect landing pattern during drop vertical jumps (DVJ) and its association with the KAM.

Design Cross-sectional exploratory study.

Setting Controlled laboratory study.

Participants 48 uninjured female elite athletes were recruited. Based on “peak knee flexion angle” (pKFA) and “peak hip flexion angle” during landing, subjects were subdivided in a “high flexion group” (hip and knee flexion>mean; n=17) or “low flexion group” (hip and knee flexion<mean; n=15).

Risk factor assessement 3D movement analysis was used during DVJ.

Main Outcome Measurements The root mean square of normalized EMG amplitude was calculated for medial vastus (VM), lateral vastus (VL), medial hamstrings (HM) and lateral hamstrings (HL) from touchdown until pKFA during the first landing of the DVJ. Muscular activity, co-contraction ratios and KAM were compared between groups with independent t-tests.

Results Subjects that performed landing with low hip and knee flexion (low flexion group), showed a significantly higher activity of the VL (P=.007), a significantly lower VM/VL activity ratio (P=.01) and a significantly higher absolute and normalized (to bodyweight * height) peak KAM compared to the ones with a high flexion landing pattern (respectively P=.03; P=.02). No significant results were found for hamstrings activity.

Conclusions A more erect landing pattern is associated with higher VL activity, a more disproportional medial/lateral activation pattern, and a higher KAM. These associations provide new support to proposed mechanisms of increased ACL injury risk.