Objective To determine whether voluntary activation of quadriceps femoris persons with chronic knee injuries is impaired. Methods Thirty participants with knee injuries and thirty healthy participants voluntarily participated in the study. Muscle strength in maximum voluntary contraction （MVC），muscle activation （MA），and the rate of force development （RFD） in isometric knee extension were assessed for both legs using a custom-built dynamometer. Independent-Samples T test was used to compare the mean values between the two groups or/and both legs. Results No significant differences were found in the measured variables between the two legs in the healthy control group. There was no significant difference in the MVC between legs in the injury group（P > 0.05）. The MVC and RFD of the right leg in the control group were lower than the uninjured left side in the injury group （P < 0.05，P < 0.01）. The MA and RFD of the injured side in the injury group were lower than uninjured side （P < 0.05，P < 0.01）. Conclusion Although the muscle strength was recovered back to the level of the uninjured side，the voluntary activation and the rate of force development of quadriceps femoris were still impaired at the injured knee，indicating that the neuromuscular control of the injured knee was not fully recovered and thus affecting the functional status of the patients. The twitch interpolation method for determining MA was sensitive to the functional changes in knee extensor.
Xu, G, Huang, L, Zhou, S, Wang, Z, Li, M, Ao, M, Wang, C, Zhao, M & Liang, J 2012, 'Chronic knee injuries impair voluntary activation of quadriceps femoris', Chinese Journal of Sports Medicine, vol. 31, no. 2, pp. 109-112.