PII S0361-9230(01)00504-4 Continuous, bilateral Achilles’ tendon vibration is not detrimental to human walk Gre ´ goire Courtine, 1 Thierry Pozzo, 1 Brigitte Lucas 2 and Marco Schieppati 1,3 * 1 Groupe d’Analyse du Mouvement, Universite ´ de Bourgogne, Campus Universitaire, Dijon, France; 2 Groupe d’E ´ tudes et de Recherche sur le Handicap (GERSH), Centre de Convalescence et de Re ´e ´ ducation, Universite ´ de Bourgogne, Dijon, France; and 3 Istituto di Fisiologia Umana, Universita ` degli Studi di Pavia and Fondazione Salvatore Maugeri (IRCCS), Scientific Institute of Pavia, Pavia, Italy [Received 25 November 2000; Revised 27 March 2001; Accepted 27 March 2001] ABSTRACT: Sensory feedback from the moving limbs contrib- utes to the regulation of animal and human locomotion. How- ever, the question of the specific role of the various modalities is still open. Further, functional loss of leg afferent fibres due to peripheral neuropathy does not always lead to major alteration in the gait pattern. In order to gain further insight on proprio- ceptive control of human gait, we applied vibratory tendon stimulation, known to recruit spindle primary afferent fibres, to both triceps surae muscles during normal floor walk. This pro- cedure would disturb organisation and execution of walking, especially if spindles fire continuously and subjects are blind- folded. Vibration induced significant, though minor, changes in duration and length of stance and swing phase, and on speed of walking and kinematics of lower limb segments. No effect was induced on angular displacement of the ankle joint or trunk and head kinematics. This paucity of effects was at variance with the perception of the subjects, who reported illusion of leg stiffness and gait imbalance. These findings would speak for a selective gating of Ia input during locomotion and emphasise the notion that the central nervous system can cope with an unusual continuous input along the Ia fibres from a key muscle like the soleus. © 2001 Elsevier Science Inc. KEY WORDS: Modulation, Proprioception, Spindle Ia input, Calf muscles, Achilles’ tendon vibration, Gait. INTRODUCTION Locomotion consists of cyclic events controlled by central pattern generating networks (CPGs) located within the spinal cord. They are under the continuous influence of descending signals and peripheral input [1,23]. Proprioceptive sensory feedback from the moving limbs has repeatedly been shown to play a significant role in the regulation of the CPG activity and in the gait adaptations to the ground. For instance, in the walking system of the cat, near the end of stance, a sensory signal switches the motor command from stance to swing [22]. In general, in many vertebrate and inverte- brate motor systems, the transition from one to another phase of movement is triggered by a phasic afferent signal [32]. A possible role of muscle afferent feedback is to control the level of activity in antigravity muscles during the stance phase [33]. Positive and negative feedback pathways from proprioceptors contribute sub- stantially to the generation of activity in extensor muscles during stance in the cat [25]. This indicates that the sensory feedback mediated by muscle afferent fibres is closely integrated into the activity of the spinal networks generating the locomotion. How- ever, one may note that investigations made on de-afferented cats have underlined no major alteration of the basic gait pattern [23,39]. There is evidence that a spinal stepping generator exists in humans [7,16] and there is growing support for a role of sensory feedback in normal human locomotion [14,15]. However, the question of the specific implication of the various modalities in the control of locomotion is still open. For example, damage of leg afferent fibres due to peripheral neuropathy as in diabetes does not led to major alteration in the gait pattern [11], whilst complete loss of large afferent fibres as in ganglionopathy creates major walking problems [10]. In order to get further insight into the role of proprioceptive information on human gait control, we applied vibratory stimulation to both triceps surae muscles during normal walk. Vibration is known to adequately recruit muscle spindle group Ia afferents [6,37]. The tendon vibration reflex can be induced in the soleus muscle by the continuous vibratory stimu- lation [40], as evidence that muscle receptors are indeed ade- quately stimulated by the mechanical effects of vibration. Further, the H reflex depression, occurring both during and after vibration, indicates ongoing activation of the presynaptic inhibition onto large spindle afferent fibres. In addition, when applied at around 70 Hz, vibration can lock the firing pattern of most Ia fibres to the stimulation pattern, thereby partly occluding the spindle input during the gait cycle. Irrespective of the induced muscle activation and reflex mod- ulation, several investigations have established that vibration pro- duces illusory changes in joint position [17–19] or even complex drawing illusions [38]. Oriented whole-body tilt can be induced in standing human subjects by applying vibratory stimulation [17, 30]; the vibration-induced sway is then oriented according to the muscles vibrated [29]. At the same time, subjects report a subjec- tive feeling of whole-body displacement [36]. Vibration applied to lateral neck muscles prior to walking disrupts steering of locomo- * Address for correspondence: Marco Schieppati, Istituto di Fisiologia Umana, Universita ` degli Studi di Pavia, Via Forlanini 6, I-27100 Pavia, Italy. Fax: +00-39-382-507664; E-mail: marco.schieppati@unipv.it Brain Research Bulletin, Vol. 55, No. 1, pp. 107–115, 2001 Copyright © 2001 Elsevier Science Inc. Printed in the USA. All rights reserved 0361-9230/01/$–see front matter 107