Background Anterior cruciate ligament (ACL) injury in knees opposite to the racket-hand side frequent occurs during single-leg landing after overhead stroke in badminton. Although large lateral inclination of trunk during single-leg landing seems to generate knee valgus moment which causes ACL injury, it has not been examined.

Objective To evaluate if there was a correlation between trunk kinematics and knee valgus moment during single-leg landing after overhead stroke, in addition, to evaluate an association with muscular strength of trunk.

Design Controlled laboratory study.

Setting Female college badminton players with no history of knee injury.

Participants 8 right-handed players.

Risk factor assessment Subjects performed single-left-legged landing after an overhead stroke following back-stepping. The kinematic and kinetic data of the trunk and left lower extremity during landing were measured using 3-dimensional motion analysis system. Isometric trunk flexion and extension strength was measured using a dynamometer.

Main outcome measurement Change in lateral trunk bending angle and maximum knee valgus moment during landing were measured. The correlation between these and trunk strength were analyzed.

Results The average of maximum knee valgus moment was 0.34±0.24 Nm/kg/m, and the change in lateral trunk bending angle was 22.9±13.3 degrees, respectively. The mean trunk flexion strength was 2.2±0.9 Nm/kg. The knee valgus moment was significantly correlated with the change in lateral trunk bending angle (r=0.87, P<.05) and the trunk flexion strength (r=−0.94, P<.01). The change in lateral trunk bending angle was also significantly correlated with the trunk flexion strength (r=−0.94, P<.01).

Conclusions Larger lateral trunk bending associated with weak flexion strength increased knee valgus moment during single-leg landing. The core muscle training focused on trunk flexion strength may improve neuromuscular control of trunk and avoid excessive knee valgus moment in playing badminton, and this may play an important role of ACL injury prevention.