Abstract. It is known that the springlike properties of muscles provide automatic load compensation during weight bearing. How crucial is sensory control of the motor output given these basic properties of the locomotor system? To address this question, a neuro- muscular model was used to test two hypotheses. (1) Stretch reflexes are too weak and too delayed to contribute significantly to weight-bearing. (2) The important contributions of sensory input involve state-dependent processing. We constructed a two- legged planar locomotor model with 9 segments, driven by 12 musculotendon actuators with Hill-type force-velocity and monotonic force-length properties. Electromyographic (EMG) profiles of the simulated muscle groups during slow level walking served as actuator activation functions. Spindle Ia and tendon organ Ib sensory inputs were represented by transfer functions with a latency of 35 ms, contributing 30% to the net EMG profile and gated to be active only when the receptor-bearing muscles were contracting. Locomotor stability was assessed by parametric vari- ations of actuator maximum forces during locomotion in open-loop (‘‘deafferented’’) trials and in trials with feedback control based on either sensory-evoked stretch reflexes or finite-state rules. We arrived at the following conclusions. (1) In the absence of sensory control, the intrinsic stiffness of limb muscles driven by a stereotyped rhythmical pattern can produce surprisingly stable gait. (2) When the level of central activity is low, the contribution of stretch reflexes to load compensation can be crucial. However, when central activity provides adequate load compensation, the contribution of stretch reflexes is less significant. (3) Finite-state control can greatly extend the adaptive capability of the locomotor system. 1 Introduction At an international symposium on movement and sensation held in Cairns, Australia in 2001 we made the provocative suggestion that stretch reflexes do not contribute substantially to load compensation in mam- malian locomotion (Prochazka et al. 2002). We made this suggestion on the basis of the relatively small size and long latency of electromyogram (EMG) responses that occur after ground contact in the stance phase of the locomotor step cycle. Some initial biomechanical modeling we had done indicated that these responses may only have a modest effect on the kinematics of quadrupedal gait. Classical studies demonstrated many years ago the ability of the spinal cord to produce the basic loco- motor rhythm in the absence of sensory feedback (Brown 1911). Brown coined the term ‘‘intrinsic fac- tor’’ to describe the underlying neural mechanism, and more recently Grillner renamed this the ‘‘central pat- tern generator’’ (CPG) (Grillner and Zangger 1974). The fact that under some circumstances rudimentary weight-bearing locomotion can occur in the absence of sensory input indicates that the biomechanical prop- erties of the limbs provide some flexibility in load compensation (Pearson et al. 2003). However, studies on animal and human subjects have also shown that after sensory loss gait is far less coordinated and less able to adapt to changes in terrain and body posture (Bickel 1897; Lajoie et al. 1996; Bloem et al. 2002). Thus two main roles are usually attributed to sensory feedback: it provides control of the stiffness of indi- vidual muscles and it allows higher-level control of balance, stability, and coordination. Stretch reflexes associated with locomotion, on which we will concentrate in the following discussion, have been extensively studied with a variety of physiological techniques for over a century. Most of the studies have dwelt on the electrical responses of muscles to electri- cally evoked sensory inputs or to imposed muscle stretching and shortening. The amplitude of sensory- evoked EMG responses is modulated throughout the Correspondence to: A. Prochazka (e-mail: arthur.prochazka@ualberta.ca) Biol. Cybern. (2004) DOI 10.1007/s00422-003-0449-z Ó Springer-Verlag 2004 Contribution of stretch reflexes to locomotor control: a modeling study S. Yakovenko, V. Gritsenko, A. Prochazka Centre for Neuroscience, 513 HMRC, University of Alberta, Edmonton, Alberta T6G 2S2, Canada Received: 19 December 2002 / Accepted: 9 October 2003 / Published online: 20 January 2004