Eur J Appl Physiol (2011) 111:611–620 DOI 10.1007/s00421-010-1680-7 123 ORIGINAL ARTICLE InXuence of fear of falling on anticipatory postural control of medio-lateral stability during rapid leg Xexion E. Yiou · T. Deroche · M. C. Do · T. Woodman Accepted: 28 September 2010 / Published online: 12 October 2010  Springer-Verlag 2010 Abstract During leg Xexion from erect posture, postural stability is organized in advance during “anticipatory pos- tural adjustments” (APA). During these APA, inertial forces are generated that propel the centre of gravity (CoG) laterally towards stance leg side. This study examined how fear of falling (FoF) may inXuence this anticipatory pos- tural control of medio-lateral (ML) stability. Ten young healthy participants performed a series of leg Xexions at maximal velocity from low and high surface heights (6 and 66 cm above ground, respectively). In this latter condition with increased FoF, stance foot was placed at the lateral edge of the support surface to induce maximal postural threat. Results showed that the amplitude of ML inertial forces generated during APA decreased with FoF; this decrease was compensated by an increase in APA duration so that the CoG position at time of swing foot-oV was located further towards stance leg side. With these changes in ML APA, the CoG was propelled in the same Wnal (uni- podal) position above stance foot as in condition with low FoF. These results contrast with those obtained in the litera- ture during quiet standing which showed that FoF did not have any inXuence on the ML component of postural con- trol. It is proposed that ML APA are modiWed with increased FoF, in such a way that the risk of a sideway fall induced by the large CoG motion is attenuated. Keywords Anticipatory postural adjustments · Medio-lateral stability · Fear of falling · Leg Xexion · Motor control Introduction It is well known that voluntary leg movements performed from the erect posture are preceded by dynamical phenom- ena in the postural body segments referred to as “anticipa- tory postural adjustments” (APA; see Bouisset and Do 2008; Massion 1992, for reviews). One of the main func- tions of these APA is to allow the maintenance of postural equilibrium during leg movement (see Bouisset and Do 2008; Massion 1992, for reviews). Indeed, the act of lifting the swing foot (e.g. during leg Xexion or stepping initiation) induces a reduction in the size of the base of support (BoS), which is then limited to the single stance foot’s contact with the ground. It follows that if the centre of gravity (CoG) remains at the same position, its vertical projection onto the ground will then lie outside the BoS, thus creating a dis- equilibrium torque towards the swing leg side. Based on the conditions of dynamic stability raised by Pai and Patton (1997) and Hof et al. (2005), a stable posture on the single stance leg could then be reached only when suYcient iner- tial forces are generated in the opposite direction (i.e. towards the stance leg side). During voluntary leg Xexion or stepping initiation, these inertial forces are invariably gen- erated before the time of foot-oV (FO), i.e. during APA (e.g. Jian et al. 1993; Lyon and Day 1997; Mouchnino et al. 1992; Nissan and Whittle 1990; Nouillot et al. 1992; Yiou 2005). These anticipatory inertial forces thus serve to set the initial (FO) medio-lateral (ML) CoG parameters (posi- tion and velocity) that will determine the ML CoG trajec- tory during the forthcoming voluntary leg movement (Lyon Communicated by Dick Stegeman. E. Yiou (&) · T. Deroche · M. C. Do Laboratory CIAMS, Team RIME, UFR STAPS, University of Paris-Sud 11, 91405 Orsay Cedex, France e-mail: eric.yiou@u-psud.fr T. Woodman Institute for the Psychology of Elite Performance, Bangor University, Bangor LL57 2DG, Gwynedd, UK