Fast Responses to Stepping on an Unexpected Surface Height Depend on Intact Large-Diameter Nerve Fibers: A Study on Charcot-Marie-Tooth Type 1A Disease

1 Department of Rehabilitation and 2 Department of Neurology, Radboud University Nijmegen Medical Centre; 3 Sint Maartenskliniek, Research Development and Education, Nijmegen, The Netherlands; and 4 Motor Control Laboratory, Department of Biomedical Kinesiology, Katholieke Universiteit Leuven, Leuven, Belgium Submitted 16 October 2008; accepted in final form 13 July 2009 Abstract The contribution of reflexes from the large myelinated afferents in the control of normal and perturbed gait in humans is a highly debated issue. One way to investigate this topic is by studying normal and perturbed gait in patients lacking large myelinated fibers in the distal limb (Charcot–Marie–Tooth [CMT] type 1A disease). Such patients should have delayed and decreased reflexes if the latter depend on these large myelinated fibers. To elicit the reflexes, both patients and controls had to step on a platform that was either at the same level or lowered by 5 cm. In control subjects, landing on a level surface induced short-latency responses in the biceps femoris and tibialis anterior muscles, whereas such responses were largely absent in the patients. Similarly, stepping down unexpectedly induced a very fast muscle synergy, leading to a brake of the forward propulsion in the controls, which was significantly reduced and delayed (on average 32 ms) in the patients. The observed changes correlated with both sensory and motor deficits. Nevertheless, it is concluded that the results are primarily related to the sensory deficits, since the delayed or absent responses appeared in both upper and lower leg muscles, whereas only the latter showed motor deficits. The data are taken as evidence that large-diameter afferents from the distal leg are essential for fast reflex activations induced by stepping on a level or lowered surface unexpectedly. Address for reprint requests and other correspondence: M. H. van der Linden, Department of Rehabilitation, 898, Radboud University Nijmegen Medical Centre, P.O. Box 9101, 6500 HB, Nijmegen, The Netherlands (E-mail: mh.vanderlinden{at}hotmail.com )