The Effect of Quadriceps Fatigue of Repeated Isometric Contractions on Knee Passive Stiffness

Purpose: Fatigue has a potential effect on knee stiffness reduction. Quadriceps fatigue intervention of repeated contractions is one traditional type of inducing knee fatigue. Therefore, defining the connection between quadriceps fatigue and knee stiffness may be beneficial to early knee rehabilitation. Knee passive stiffness was passive component of overall knee stiffness, this study aimed to investigate the effect of quadriceps fatigue on knee passive stiffness. Methods: The preliminary experiment was first performed with a separate cohort of 6 participants to verify the passive stiffness measurements. Then 30 participants were recruited in fatigue experiment of lower hip angle conditions. The fatigue protocol was induced by repetitive quadriceps isometric contractions in three fatigue cycles. Knee passive stiffness between pre and post 50% range of motion (ROM) groups were calculated by the regression slope of angular displacement-torque curve. Results: Knee passive stiffness measurements were lower under conditions of lower hip angle. There was a significant difference in passive stiffness between pre and post 50% ROM groups. Only passive stiffness of pre 50% ROM showed a remarkable decrease after fatigue (approximately 3.8%, p < 0.05). Conclusion: When quadriceps reached fatigue with the repeated isometric contractions, the influence of fatigue on knee passive stiffness of pre and post 50% ROM was different. Passive stiffness of pre 50% ROM decreased significantly when the fatigue threshold reached 90-fold isometric contractions. The finding may help understand how passive knee stiffness changes under fatigue conditions. It may also provide references of fatigue dosage in rehabilitation training to improve knee passive stiffness and ROM in early rehabilitation.