n addition, there are also specialized pathwaysto the spinal cord (Kuypers, 1964; Szokol and Perreault, 2009;Deliagina et al., 2014) and specialized activation of the trunkmusculature during various postural and motor tasks (Urquhartet al., 2005; Falgairolle et al., 2006, 2013; Tsao et al., 2011;Beliez et al., 2015). For instance, descending pathways to theaxial musculature (that links all parts of the body together andprovides axial muscle tone and trunk stabilization) via somaticdescending brain stem and monoaminergic pathways are distinctfrom the descending tracts to limb motoneurons (Kuypers, 1964;Szokol et al., 2008; Sivertsen et al., 2014).

Small movements accompany the maintenance of any posture.Typically, unless human posture is unstable, body segmentoscillations do not exceed 1–2◦ of joint movements and the CoPoscillations are about 1–2 cm. The fact that postural oscillationsare small supports the assumption that the system is linearwithin a limited range of movements and, therefore, linearcomputational models and analyses can be applied (Winter et al.,2003; Mergner, 2007; Kiemel et al., 2008; Assländer and Peterka,2014). While this assumption is valid to some extent and manystudies provided very important information about posturalstrategies and the contribution of different sensory inputs tobalance control, one should have in mind that there is alsosubstantial non-linearity in the postural control system, which isoften overlooked

Changes in the gaze direction may also modulatepostural responses (Ivanenko et al., 1999), consistent withsupraspinal or cognitive influences on posture control, likelybecause the gaze represents an important reference frame forthe internal model of spatial orientation. Thus, the fact thatautomatic postural reactions are accomplished in accordancewith internal representation of body scheme (Popov et al.,1986; Smetanin et al., 1988; Gurfinkel, 1994) indicates thatit does not only serve for conscious perception of positionbut it is also the basis for planning and implementing motoractivity.