The Responses of Continuous Knee Passive Stiffness Following Fatigue

Purpose: The purpose of this study was to investigate continuous knee passive stiffness and its fatigue responses. Knee osteoarthritis is a clinically degenerative disease that mostly cause sustained stiff knee, hence, there is an urgent need to evaluate continuous knee passive stiffness, which corresponded to the stiffness of various angles during actual continuous knee rotation. However, how to measure this kind of stiffness is yet to reach an agreement. Methods: This is a study incorporating 36 individuals, in which the fatigue protocol consisted of three bouts of multiple quadriceps voluntary isometric contractions of their dominant legs. Angle and passive torque were measured before and after each fatigue intervention cycle. Continuous knee passive stiffness of various angles and rate of stiffness under fatigue were both calculated. Results: We derived the continuous knee passive stiffness data appeared as an asymmetric U-shaped curve. The passive stiffness (Nm/°) of the initial angle (0°) is 0.249 (0.159–0.285), while that of the other endpoint angle (91°) is 0.156 (0.130–0.354). The responses of continuous knee passive stiffness following fatigue interventions revealed a significant decrease in the knee angle from 10 to 36° (p = 0.02–0.05). Conclusion: This renew study experimentally develop a novel approach to expand upon the existing methods of knee passive stiffness measurements. The intrinsic continuous knee passive stiffness and its fatigue responses both vary at specific knee angles. Our findings may provide stiffness assessment for knee osteoarthritis patients, complete fatigue intervention program, and optimize the artificial prosthesis design.