Effects of fatigue on frontal plane knee motion, muscle activity, and ground reaction forces in men and women during landing.

Women tear their Anterior Cruciate Ligament (ACL) 2-8 times more frequently than men. Frontal plane knee motion can produce a pathological load in the ACL. During a state of fatigue the muscles surrounding the knee joint may lose the ability to protect the joint during sudden deceleration while landing. The purpose of this study was to investigate the effects of fatigue and gender on frontal plane knee motion, EMG amplitudes, and GRF magnitudes during drop- jump landing. Pretest-posttest comparison group design was used. Twenty-six volunteers (14 women; 12 Men; Mean ± standard deviation age = 24.5 ± 2.7 yrs; height = 1.73 ± 0.09 m; mass = 74.3 ± 11.8 kg) participated in the study. Knee frontal plane ranges of motion and positions, ground reaction force peak magnitudes, and surface EMG RMS amplitudes from five lower extremity muscles (vastus medialis, vastus lateralis, medial hamstring, lateral hamstring, and lateral gastrocnemius) were obtained during the landing phase of a drop-jump. MANOVA and ANOVA indicated that peak GRF significantly (p < 0.05; 2.50 ± 0.75 BW vs. 2.06 ± 0.93 BW) decreased during fatigued landings. No other variables exhibited a fatigue main effect, although there was a significant (p < 0.05) fatigue by gender interaction for the frontal plane range of motion from initial contact to max knee flexion variable. Follow-up analyses failed to reveal significant gender differences at the different levels of fatigue for this variable. Additionally, no variables exhibited a significant gender main effect. Single subject analysis indicated that fatigue significantly altered frontal plane knee motion, peak GRF, and EMG in some subjects and the direction of differences varied by individual. Fatigue altered some aspects of landing performance in both men and women, but there were no gender differences. Additionally, both group and single subject analyses provided valuable but different information about factors representing neuromuscular control during drop-jump landing. Key pointsFatigue reduces ground reaction forces in both men and women during drop-jump landings.There was no significant difference in frontal plane knee kinematics between men and women when they were fatigued.Men and women did show differences in frontal plane knee kinematics in the fatigued state compared to the pre-fatigued state and the increased motion was in the valgus direction and varus direction.Single subject analysis shows that both women and men exhibit characteristics that may predispose them to ACL injury.