WHAT FUNCTIONS DO REFLEXES SERVE DURING HUMAN LOCOMOTION? E. PAUL ZEHR 1,2 % and RICHARD B. STEIN 2 1 Neurophysiology Laboratory, Faculty of Physical Education and Recreation, University of Alberta, Edmonton, Alberta Canada T6G 2H9 and 2 Division of Neuroscience, 513 Heritage Medical Research Centre, University of Alberta, Edmonton, Alberta, Canada T6G 2S2 (Received 29 July 1998) AbstractÐStudies on the re¯ex modulation of vertebrate locomotion have been conducted in many dierent laboratories and with many dierent preparations: for example, lamprey swimming, bird ¯ight, quadrupedal walking in cats and bipedal walking in humans. Emerging concepts are that re¯exes are task-, phase- and context-dependent. To function usefully in a behaviour such as locomotion wherein in- itial conditions change from step to step, re¯exes would have to show modulation. Papers are reviewed in which the study of dierent re¯exes have been conducted during dierent beha- viours, with an emphasis on experiments in humans. A framework is developed in which the modulation and ¯exibility of re¯exes are demonstrated. Alterations in cutaneous, and muscle (stretch and load recep- tor) re¯exes between sitting, standing and walking are discussed. Studies in which both electrical, mech- anical and `natural' receptor activation have been conducted during walking are reviewed. Re¯exes are shown to have important regulatory functions during human locomotion. A framework for discussion of re¯ex function throughout the step cycle is developed. The function of a given re¯ex pathway changes dynamically throughout the locomotor cycle. While all re¯exes act in concert to a cer- tain extent, generally cutaneous re¯exes act to alter swing limb trajectory to avoid stumbling and falling. Stretch re¯exes act to stabilize limb trajectory and assist force production during stance. Load recep- tor re¯exes are shown to have an eect on both stance phase body weight support and step cycle timing. After neurotrauma or in disease, re¯exes no longer function as during normal locomotion, but still have the potential to be clinically exploited in gait modi®cation regimens. # 1999 Elsevier Science Ltd. All rights reserved CONTENTS 1. Introduction 186 2. A note on methods 187 3. Re¯ex conditioning in static postures 187 3.1. Modulation by remote muscle contraction 187 3.2. Modulation of H-re¯ex and tonic motor output by muscle nerve stimulation 187 3.3. Modulation of H-re¯ex and tonic motor output by cutaneous nerve stimulation 188 4. Task-dependency of re¯exes with changes in postural orientation 188 4.1. Modulation of tonic motor output and H-re¯ex by muscle nerve stimulation 188 4.2. Modulation of tonic motor output by cutaneous nerve stimulation 189 5. Phase-dependency of re¯exes during rhythmic movement 190 5.1. H-re¯ex modulation during walking 190 5.2. H-re¯ex modulation during leg cycling 190 5.3. Cutaneous re¯ex modulation during walking 190 5.4. Cutaneous re¯ex modulation during leg cycling 191 6. Studies of re¯ex function during locomotion 191 6.1. Stretch re¯exes 192 6.1.1. Background studies in quadrupeds 192 6.1.2. Studies in humans 192 6.2. Load receptor re¯exes and step cycle timing 193 6.2.1. Background studies in quadrupeds 193 6.2.2. Studies in humans 193 6.3. Cutaneous re¯exes 194 6.3.1. Initial studies in quadrupeds 194 6.3.2. Studies in humans 194 6.3.3. Methodological issues 196 6.4. Re¯ex function during locomotion after neurotrauma and neurodegenerative disease 196 7. Studies of mechanically-induced stumbling during human locomotion 198 7.1. Support surface perturbation 198 Progress in Neurobiology Vol. 58, pp. 185 to 205, 1999 # 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0301-0082/99/$ - see front matter PII: S0301-0082(98)00081-1 % Corresponding author. Tel.: 001 403 492 2004; Fax: 001 403 492 2364; E-mail: pzehr@gpu.srv.ualberta.ca. 185