Kathrine Jáuregui-Renaud

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/55344

**1. Introduction**

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#### **1.1. Control of posture**

Postural control can be defined as the control of the body's position in space for the purposes of balance and orientation [1]. A definition of balance is the ability to maintain or return the body's centre of gravity within the limits of stability that are determined by the base of support (i.e., the area of the feet) [2]; while spatial orientation defines our natural ability to maintain our body orientation in relation to the surrounding environment, in static and dynamic conditions.

Maintaining balance encompasses the acts of preserving, achieving or restoring the body centre of mass relative to the limits of stability that are given by the base of support [3], which implies the control of posture in preventing falling. Then, in order to modify motor responses on the basis of sensory input, the appropriate responses to any external or internal perturbation have to be chosen [4]. Nevertheless, to maintain stability, all movements that affect the static and dynamic position of the center of mass of the body must be preceded or accompanied by adjustments of other segments.

During upright stance, balance corrections appear to be triggered by signals presumably located within the lower trunk or pelvis [5], and sensory feedback is required from vestibular, visual and somatosensory origin. A bipedal stance position that provides good stability is maintained mainly by efferent ankle mechanisms; to minimize the effect of perturbations when segmental oscillation is allowed, hip mechanisms are used. Locations of the centre of gravity at the borders of the limits of stability correspond to the region where balance cannot be maintained without moving the feet [6].

In order to orient the body, while keeping balance, visual, vestibular and somatosensory modalities are also involved. Every directed activity implies that the body was previously

© 2013 Jáuregui-Renaud; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

oriented [7]. Although, any part of the body surface can influence the control and perception of body orientation [8], evidence suggest that the representation of the body's static and dynamic geometry may be largely based on muscle proprioceptive inputs that continuously inform the central nervous system about the position of each part of the body in relation to the others [9-11]. The muscle innervation patterns necessary to produce particular body relative movements depend on body orientation to gravity [12]. To oppose the acceleration of gravity, there are contact forces of support on the body surface, the otolith organs provide information about head orientation with respect to the gravitoinertial force. If the head or both the head and the trunk are aligned with the vertical, the gravitational or egocentric reference associated with vertical gravity provide a strong spatial invariant used to control balance [13].

Allum et al. have suggested that a confluence of knee, trunk and vestibulo-spinal inputs triggers human balance corrections depending on the mode of movement the body is forced into by a perturbation, and on the differential weighting of proprioceptive and vestibulo-spinal inputs in the triggered muscle's balance correcting response [5]. A combined deficit of vestibular and somatosensory input may preclude adjustments to postural control [23].

Postural Balance and Peripheral Neuropathy http://dx.doi.org/10.5772/55344 127

Normal postural coordination of the trunk and legs also requires both somatosensory and visual information [24]; older adults may be less stable under conditions in which peripheral vision is occluded and ankle somatosensation is limited, only remaining foveal vision and vestibular input [25]. However, evidence suggest different selection of sensory orientation references depending on the personal experience of the subjects, leading to a more or less heavy

When healthy subjects stand on a solid base of support, in a lightened environment, they rely on their somatosensory systems, the proprioceptive and the tactile systems. For this purpose, the proprioceptive system provides information on joint angles, changes in joint angles, joint position and muscle length and tension; while the tactile system is associated mainly with sensations of touch, pressure and vibration. In children, studies on the development of sensory organization to control posture according to each sensory component in relation to age

During upright stance, somatosensory information from the legs may be utilized for both, direct sensory feedback and use of prior experience in scaling the magnitude of automatic postural responses [28]. Reduced somatosensory information from the lower limbs alters the ability to trigger postural responses and to scale the magnitude of these responses [29-31]. Even if the input from skin, pressure and joint receptors of the foot may be of minor importance for the compensation of rapid displacements, it may play a major role at low frequencies [32]. In patients with diabetic neuropathy, sensory conduction in the lower legs results in the late onset of an otherwise intact, centrally programmed response; along with this finding, a different relationship between the severity of the neuropathy and the quality of amplitude and velocity scaling suggests that the role of this peripheral sensory information may differ depending both, on the postural control task and on the quality of the sensory information

The foot sole and ankle muscle inputs contribute jointly to posture regulation [33]. Foot sole sensation is an important component of the balance system [34]. Cutaneous afferent messages from the main supporting zones of the feet may have sufficient spatial relevance to induce adapted regulative postural responses [35]. After perceptual training for hardness discrimi‐ nation of the support surface, the ability of healthy subjects to regulate their standing posture

In healthy subjects, increased severity of experimentally induced loss of plantar cutaneous sensitivity may be associated with greater postural sway; such an association could be affected by the availability of visual input and the size of the support surface [37]. Additionally, sub-

may improve with improvement of the perceptive ability of the soles [36].

suggests that the proprioceptive function seems to mature at 3 to 4 years of age [27].

dependence on vision [26].

**2.1. Somatosensory systems**

available [28].

The attentional demands of postural control vary according to the postural task [14], the age of individuals and their balance abilities [15-17]. Teasdale et al. (1993) [18], examined the extent to which reduction in available sensory inputs may increase the attentional demands of postural control in healthy aduls; both young and old adults showed delays in reaction time as the postural task complexity increased, with an increase on attentional demands when sensory inputs were reduced. Studies using dual task paradigms to examine attention requirements of balance control when performing a secondary task, in both healthy and older adults with balance impairment, suggest that these are important contributions to instability, depending on the complexity of the task as well as the type of the second task being performed [15,17].
