Objective To investigate the effects of anterior cruciate ligament reconstruction (ACLR) on knee muscle strength of the operated and non-operated limbs and on bilateral knee biomechanical characteristics during 45° and 90° side-cutting maneuvers in football players, and to provide theoretical support for the development of precise postoperative rehabilitation programs and the optimization of football shoe design.

Methods G*Power was used to calculate the sample size, with power set at 0.8, effect size f set at 0.8, and significance level α set at 0.05. The required total sample size was calculated to be 15 participants. Seventeen football players with ACLR were randomly recruited. The Biodex isokinetic dynamometer was used to assess knee flexion and extension moments of the operated and non-operated limbs at an angular velocity of 60°/s. The Simi Motion capture system and Kistler 3D force platform were used to evaluate biomechanical characteristics including knee angles, moments, and peak ground reaction forces of the operated and non-operated limbs during 45° and 90° side-cutting maneuvers. Kinematic and kinetic data were filtered using a fourth-order Butterworth low-pass filter with cutoff frequencies of 10 Hz and 50 Hz, respectively. Kinetic variables were normalized to body weight. Initial contact was defined as the instant when the vertical ground reaction force exceeded 10 N after the test foot contacted the force platform. The Shapiro-Wilk test was used to check the normality of data distribution. Paired sample t-tests were conducted to compare knee muscle strength between the operated and non-operated limbs. Two-factor repeated measures ANOVA was used to examine the effects of side (operated vs. non-operated) and movement (45° side-cutting vs. 90° side-cutting) on knee biomechanical characteristics. The significance level was set at P＜0.05.

Results Compared with the non-operated limb, the operated limb showed significantly lower peak knee extension moment at an angular velocity of 60°/s (P=0.031). Compared with the 45° side-cutting maneuver, the 90° side-cutting maneuver exhibited significantly lower values in knee flexion angle at initial contact (operated: P=0.001; non-operated: P=0.045), peak knee flexion angle (operated: P=0.000; non-operated: P=0.009), peak knee extension moment (operated: P=0.000; non-operated: P=0.042), and peak anterior-posterior ground reaction force (operated: P=0.007; non-operated: P=0.025); while significantly higher values were observed in knee valgus angle at initial contact (operated: P=0.002; non-operated: P=0.003), peak valgus angle (operated: P=0.043; non-operated: P=0.038), peak valgus moment (operated: P=0.028; non-operated: P=0.027), peak external rotation moment (operated: P=0.041; non-operated: P=0.034), and peak medial-lateral ground reaction force (operated: P=0.008; non-operated: P=0.000). During the 90° side-cutting maneuver, the non-operated limb demonstrated significantly greater knee flexion angle at initial contact (P=0.038), peak knee flexion angle (P=0.043), peak knee extension moment (P=0.041), and peak medial-lateral ground reaction force (P=0.033) compared with the operated limb.

Conclusions One year after ACLR, knee muscle strength, particularly knee extensor strength, had not recovered to the level of the non-operated limb. The kinematic and kinetic characteristics of the knee during the 90° side-cutting maneuver indicate a significant increase in ACL load during large-angle side-cutting maneuvers, which may represent a compensatory strategy whereby the body enhances neural regulation and muscle coordination to cope with the higher injury risk. Therefore, it is recommended that post-ACLR rehabilitation emphasize strengthening knee muscle strength, especially knee extensor strength, and incorporate neuromuscular control training such as dual-task interference training and proprioceptive training to manage unavoidable large-angle side-cutting maneuvers.