Exp Brain Res (1999) 124:80±88  Springer-Verlag 1999 RESEARCH ARTICLE Anne Kavounoudias ´ Jean-Claude Gilhodes RØgine Roll ´ Jean-Pierre Roll From balance regulation to body orientation: two goals for muscle proprioceptive information processing? Received: 16 September 1997 / Accepted: 16 July 1998 A. Kavounoudias ( ) ) ´ J.-C. Gilhodes ´ R. Roll ´ J.-P. Roll Laboratoire de Neurobiologie Humaine, UMR 6562, CNRS-UniversitØ de Provence ± centre de Saint JØrôme, avenue Escadrille Normandie-Niemen, F-13397 Marseille cedex 20, France e-mail: anneka@newsup.univ-mrs.fr, Tel.: +33-4-91-28-82-95 Abstract This study was based on the assumption that the central processing of proprioceptive inputs that arise from numerous muscles contributes to both awareness and control of body posture. The muscle-spindle inputs form a ªproprioceptive chainº which functionally links the eye muscles to the foot muscles. Here, we focused on the specific contribution of two links in the control of human erect posture by investigating how propriocep- tive messages arising from ankle and neck muscles may be integrated by the central nervous system. Single or combined mechanical vibrations were applied to different muscle tendons at either one (ankle or neck) or both (an- kle plus neck) body levels. The amplitude and the specific direction of the resulting oriented body tilts were ana- lyzed by recording the center of foot pressure (CoP) through a force platform with four strain gauges. The re- sults can be summarized as follows: (1) the vibration-in- duced whole-body tilts were oriented according to the muscles stimulated; furthermore, the tilts were in opposite directions when neck or ankle muscles on the same side of the body were stimulated; (2) except for the ankle an- tagonist muscles, co-vibrating adjacent or antagonist mus- cles at the same body level (ankle or neck) resulted in body sways, whose orientation was a combination of those obtained by stimulating these muscles separately; and (3) likewise, co-vibrating ankle and neck muscles in- duced whole-body postural responses, whose direction and amplitude were a combination of those obtained by separate vibration. We conclude that the multiple proprio- ceptive inputs originating from either one or both body levels may be co-processed in terms of vector-addition laws. Moreover, we propose that proprioceptive informa- tion from ankle and neck muscles may be used for two tasks: balance control and body orientation, with central integration of both tasks. Key words Human erect posture ´ Proprioception ´ Vibratory stimulation ´ Whole-body motor responses ´ Orientation ´ Regulation Introduction Previous ideas about the organization and control of hu- man erect posture have been considerably revised dur- ing the last decade. The idea of an ªinternal modelº has been added to the traditional assumption that postur- al activities basically depend on interactions between ªsingle level reflexesº (Sherrington 1906). This model is thought to operate with highly integrated multi-senso- ry information and to deal much more adaptively and flexibly with a wide range of tasks (Gurfinkel et al. 1988; Horak and MacPherson 1996; Massion 1992; Merfeld et al. 1993). For example, the perceptual or mo- tor responses after muscle vibration can involve either the whole-body or a single segment, depending on the instructions given to the subjects or their environment (Quoniam et al. 1990; Roll et al. 1986). The ªpostural body schemeº, considered to be an unconscious repre- sentation of the body's configuration and dynamics, may form a reference frame, which the central nervous system uses to continuously restore the body's balance or its required orientation (ClØment et al. 1984; Gurfinkel et al. 1988, 1995; Lestienne and Gurfinkel 1988). On these lines, postural control is approached not only in terms of an automatic regulative process for the main- tenance of the body equilibrium, but also in terms of a high-level process, including body spatial representation. In fact, every directed activity in extrapersonal space im- plies that the body was previously oriented (Gurfinkel and Levick 1991). As pointed out by Horak and MacPherson (1996), posture seems to subserve two behavioral goals: orientation and balance. Visual, vestibular, cutaneous, and muscle propriocep- tive sensory modalities (Fetter and Dichgans 1996; Horak and Shupert 1994) are involved in postural con-