The main factor causing prolonged reaction time on force producing process following anterior cruciate ligament...

This study investigated the electromechanical properties of atrophied human quadriceps femoris muscle during a voluntarily elicited maximal isometric contraction (MVC) and a peripherally stimulated twitch contraction. Nineteen patients were recruited 2-3 months following a unilateral anterior cruciate ligament (ACL) reconstruction. Both the involved leg as well as the uninvolved leg were studied. Maximal twitch response was elicited and surface electromyograms (EMG) were recorded from the vastus lateralis. Total reaction time (TRT) for both MVC and twitch on involved leg was prolonged (251.47 msec, 26.01 msec). This prolongation suggests an extended lag in avoiding injury such as during sports. Pre-motor time during both MVC and twitch (PMTmvc, PMTtwitch) did not differ between both groups. Electromechanical delay during MVC (EMDmvc) was prolonged on involved leg (53.42 msec), and also evoked twitch EMD (EMDtwitch) (20.04 msec) as compared to the opposite side. Prolonged EMDtwitch may be due to a decrease in stiffness of the series elastic component, changes of peripheral muscle composition to containing more slow type muscle fibers, or a decrease in function of the excitation-contraction (E-C) coupling process. A prolonged EMDtwitch can also explain the prolonged EMDmvc. These findings also suggested that prolonged TRTmvc to visual stimulus during MVC in atrophied human quadriceps femoris muscle after disuse was principally due to prolongation of EMDmvc. Prolonged EMDmvc may have resulted from decreased muscle stiffness, which was evident in the prolongation of the EMDtwitch.