## 1 Introduction  
Numerous studies on the control of quiet stance have focused on balance and balance control in the anterior-posterior (AP) plane [1; 2], likely because of the analogy of the body profile in the sagittal plane with an inverted pendulum that facilitates a formal treatment of AP sway [3]. Besides, a plethora of studies examining dynamic balancing behaviour in standing have mostly considered the responsesto AP perturbations, also because of the elegant theoretical framework of the control pattern proposed at earlier times [4; 5]. The postural control in the medio-lateral (ML) direction has received less attention. Under normal standing condition, the weight of the body is symmetrically distributed, so that the centre of feet pressure (CoP) remains halfway the distance between the feet [6]. This is at variance with the position of the CoP in the sagittal plane, where it lies ahead of the ankle joint, requiring the tonic activity of the foot, leg and axial postural muscles to avoid falling forwards [7; 8]. Distinctive control modes and muscle activities appear to regulate body sway in the frontal compared to the sagittal plane [6; 9; 10; 11; 12]. The modulation of both amplitude and frequency in the EEG trace is more pronounced during ML compared to AP sway, implying that AP and ML sway are supervised by independent strategies and that ML control may require greater neural resources [13; 14]. The clinical relevance of the ML control of the CoP is emphasized by the need of good medio-lateral stability during turning [15] and by the strong dependence of gait initiation on the capacity of transferring the body along the frontal plane for launching the first step [16]. While healthy ageing hardly challenges the postural control during quiet stance [17], there is now accumulating evidence that aging effects on balance may be accentuated in the medio-lateral direction. Measures of lateral stability are related to fall risk [18] and excessive movements of CoP in the ML direction predict falls in older adults [19]. The abnormal control of the CoP may depend on asymmetries in muscle force [20], sensation [21] or altered central control. Patients with stroke typically show an asymmetric distribution of the body weight between the feet [22; 23] and problems in voluntary shifting the body toward the parcite extremity [24]. Several old and recent investigations have brought to light the effects of diverse sensory manipulations on the body's postural orientation. For example, muscle spindle activation by tendon vibration produces body forward or backward leaning depending on the site of the application [24], and the alternate vibration of trunk muscles produces the displacement of the CoP of standing individuals in the frontal plane synchronous with the vibration trains [25]. Furthermore, several manoeuvres can produce striking postural post-effects [26]. An intense voluntary isometric contraction of a muscle or group of muscles produces a sustained involuntary contraction of the same muscles lasting several seconds or minutes [27]. Neck unilateral vibration induces persistent directional post-effects on the subjective straight ahead and on the self-motion perception of vestibular origin [28]. Another example of a postural post-effect occurs during the podokinetic adaptation induced by prolonged stepping with concurrent body yaw rotation [29; 30]. On a different note, a six-minute period of treadmill walking induces a post-effect consisting in an antero-posterior tilt of the body when standing quietly afterwards, both eyes open (EO) and eyes closed (EC) [31; 32]. These cases show that the continuous activation/stimulation of the proprioceptive system may not be necessary and can be substituted by conditions eliciting prolonged rhythmic muscle activities. Hence, these post-effects are not necessarily connected to a prolonged voluntary muscle tonic contraction but can also emerge when the muscle contractions (and their sensory feedback) are repeated for a certain period.