Direct evidence of the anterior cruciate ligament-hamstring reflex arc in humans

It has been emphasized that the anterior cruciate ligament plays an importa nt role in the proprioceptive feedback system. The anterior cruciate ligame nt-hamstring reflex has been revealed in animal experiments, but it has not been established in humans. The purpose of this study was to demonstrate d irect evidence of the anterior cruciate ligament-hamstring reflex are. Nine knees in nine healthy subjects were investigated. The anterior cruciate li gament was stimulated by the use of wire electrodes inserted using an arthr oscopic technique. Electromyographic signals from the biceps femoris and th e semitendinosus muscles were recorded with surface electrodes. The change in electromyographic activity was analyzed after electrical stimulation in the normal knee condition, and again after intraarticular sensation had bee n interrupted with a local anesthetic. After electrical stimulation, subjec ts demonstrated increased electromyographic activity of the hamstring muscl es in the normal knee condition. This response indicates the existence of a n anterior cruciate ligament-hamstring reflex are. Conversely, there was no change in activity for the hamstring muscle in the anesthetized knee becau se the afferent impulse from the neural elements of the anterior cruciate l igament had been removed.