Fatigue Induced Changes in Knee Joint Force, Angular Velocity and Joint Moments During Saggital Perturbations of Single-Leg Stance

2 Abstract: The purpose of the current study was to investigate the effect of knee muscle fatigue on knee joint moment, knee joint force and knee angular velocity during saggital perturbations of single-leg stance. 15 healthy men (age = 26.2±2.1 yr) with no history of knee injury or trauma participated in this study. Fatigue induced to knee muscle using a standard Monark bicycle ergometer. Isokinetic Dynamometer (Chattanooga, TN) was used to measure maximal knee force and time to task failure before and after fatigue protocol. Reflex muscle activity measured during single- leg stance perturbation before and after fatigue protocol. Using a motion capture system and force plate, knee kinematics data and ground reaction force were also recorded before and after fatiguing. Maximal knee force, time to task failure and associated EMG activities were significantly reduced (for both flexion and extension directions) after fatigue (P< 0.05). Moreover reflex activity of the knee muscles was significantly lower during post exercise single leg stance perturbation with respect to pre exercise phase (P < 0.05). However knee angular velocity increased (P < 0.05), knee joint force decreased (P< 0.05) and knee joint moment increased in the frontal (P< 0.05) and horizontal plan (P< 0.05) after dynamic fatigue protocol. The result of the current study demonstrated, that fatigue induced changes in muscle activity patterns around knee joint. The altered muscle activity pattern around knee joint can contribute to an increased knee varus moment and an excessive internal rotation of the tibia, during destabilizing perturbation of single-leg stance. This may further increase the risk ACL injuries during single-leg stance cutting action.