This study assessed differences in reaction speed of hamstring muscles between knees with chronic anterior cruciate ligament injuries (an ACL group; n=10) and healthy subjects (a control group; n=10). All subjects were instructed to react to a sudden forward movement caused by the KIN/COM Isokinetic Dynamometer with hamstring contraction 10 times. The input arm of the Dynamometer located at the posterior side in the middle of the calf was unexpectedly moved 15° forward at a speed of 30°/s. Response time was divided into premotor reaction time (PMRT), electromechanical delay (EMD) and reaction time (RT). Results showed that the PMRT of the medial hamstrings was significantly shortened (p=0.046) and EMD lengthened (p=0.009) in the ACL group compared to the control. There were no differences in PMRT of the lateral hamstrings nor in RT between the two groups. Whether compensatory mechanisms for ACL-deficient knees were obtained was determined by this study.
