Differentiation of hamstring short latency versus medium latency responses after tibia translation.

After injuries to the anterior cruciate ligament (ACL) a functional instability is frequently observed which has been attributed to a disturbed sensorimotor function. In light of the clinical importance of ACL injuries and the resulting functional instability, it is of enormous clinical interest to elucidate the role of sensorimotor pathways that involve the ACL. In animals and humans a direct reflex pathway between the ACL and the hamstrings has been shown. The onset latencies of responses reported after ventral tibia translation were around 40-50 ms (range 17.9-65) and were regarded as medium latency responses (MLR). However, ventral tibia translation should also induce a stretch of the hamstring muscles and evoke a short latency response (SLR). Before any muscle response after ventral tibia translation can be ascribed to anatomical structures, it is crucial to analyze the obtained muscle responses carefully. The aim of the present study was the development of an algorithm to differentiate SLR and MLR responses after ventral tibia translation. In ten healthy subjects reflex responses of the hamstrings after anterior tibia translation and after tendon taps on the biceps femoris tendon were evaluated. To investigate the influence of skin afferents, control experiments were performed after lidocain injection of the dorsal calf. The mean onset latency of the tendon jerk reflex was 21.9 +/- 3.1 ms (range 17.3 - 28.7 ms). Both SLR responses (mean onset latency: 20.3 +/- 3.5 ms; range 15.4 - 25.8) and MLR responses (mean onset latency: 38.9 +/- 4.2 ms; range 32.9 - 46.7) were obtained in all subjects. Skin afferents from the calf do not play a major role. The development of an evaluation algorithm is presented that allows a safe differentiation between these partly superimposed SLR and MLR components. It is demonstrated that by measuring the first part of the SLR from the onset to the first peak the end of the SLR can be predicted and that the onset latency of the MLR component can be assessed reliably. Possible reasons are discussed why previous studies only reported responses at MLR latencies. The fact that both SLR and MLR components can be observed after anterior tibia translation underlines the necessity to differentiate the responses before they can be ascribed to any anatomical structures. As a basis for future work the algorithm presented may become a useful tool to differentiate which afferent pathways play a role in initiating hamstring activity.