Postural control and stability are critical to the execution of almost any voluntarymovement. In fact, "posture is the foundation upon which movement rides" (Cram &Criswell, 2011, p. 182). Posture is actively controlled by the neuromuscular system in concertwith somatosensory, visual and vestibular informational inputs. To maintain a desirableposture under a wide range of individual, task and environmental constraints, the centralnervous system (CNS) minimizes deflections of the body from a desirable orientationthrough the use of postural adjustments that fall into two general categories, compensatoryand anticipatory (Santos et al., 2010). Compensatory postural adjustments (CPAs) are feedbackbased mechanisms that are initiated by sensory events following the loss of desirableposture (Alexandrov et al., 2005; Park et al., 2004). CPAs act to restore stability after apostural perturbation has occurred (Henry et al., 1998; Macpherson et al., 1989; Maki &McIlroy, 1996). For example, immediately after a trip, reflexive and/or consciouslycontrolled CPAs could help prevent a fall.The second category of adjustments, and the focus of this chapter, are anticipatory posturaladjustments (APAs). APAs are feed-forward mechanisms elicited by expected posturaldisturbances that produce preemptive muscle responses that help maintain stability. Forexample, when reaching for an object such as a book on a shelf, muscles in the trunk andlegs activate in advance of muscle activity and movement in the shoulder and arm (Aruin &Latash, 1995a). These contractions in trunk and leg muscles constitute APAs because theyprecede the principal (focal) movement of the arm1.APAs were first described in 1967 when Belen’kii et al. reported the existence of electricalactivity in the sacrolumbar muscles of the trunk and upper leg prior to fast shoulder flexionin standing. The presumed functional roles of APAs are to predict the stability-perturbingforces to be generated by an imminent focal movement or external perturbation, andproduce a preparatory muscular contraction (i.e., APA) to stabilize the body in advance ofthe perturbation (Aruin & Latash, 1995a; Bouisset & Zattara, 1987; Massion, 1992; Zattara &Bouisset, 1988). Research has demonstrated that APAs are tailored to specific characteristics
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