**2. Afferent contributions to control posture**

The maintenance of bipedal stance is characterized by continuous, small deviations around the actual upright. Depending on sensory context and neuromuscular constraints, the nervous system can adjust the relative afferent contributions to maintain stability. Evidence supports that, as we move about a changing environment, the nervous system continually integrates multisensory information, with the need for continual online updating of estimates of the centre of mass [19].

Balance corrections imply the interaction among several sensory inputs. Somatosensory systems respond early to motion and muscle stretch at the ankle, knee and trunk, as does the vestibular system, which senses head accelerations [20]. Visual inputs mainly influence later stabilizing reactions to the initial balance corrections [20-21]. Each sensory modality makes a unique contribution to control posture. However, the information sent by discrete receptors is not as relevant as the integrated information sent by receptors distributed throughout the body. Descending postural commands are multivariate in nature, and the motion at each joint is affected uniquely by input from multiple sensors. In different sensory environments, the nervous system is able to re-weight its available afferents in order to optimize stance control. For example, with increasing stance width, lateral body motion is detected more easily by proprioceptors and less readily by vision or the vestibular system [22].

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].

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 dependence on vision [26].

#### **2.1. Somatosensory systems**

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

126 Peripheral Neuropathy - A New Insight into the Mechanism, Evaluation and Management of a Complex Disorder

with vertical gravity provide a strong spatial invariant used to control balance [13].

task being performed [15,17].

centre of mass [19].

**2. Afferent contributions to control posture**

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

The maintenance of bipedal stance is characterized by continuous, small deviations around the actual upright. Depending on sensory context and neuromuscular constraints, the nervous system can adjust the relative afferent contributions to maintain stability. Evidence supports that, as we move about a changing environment, the nervous system continually integrates multisensory information, with the need for continual online updating of estimates of the

Balance corrections imply the interaction among several sensory inputs. Somatosensory systems respond early to motion and muscle stretch at the ankle, knee and trunk, as does the vestibular system, which senses head accelerations [20]. Visual inputs mainly influence later stabilizing reactions to the initial balance corrections [20-21]. Each sensory modality makes a unique contribution to control posture. However, the information sent by discrete receptors is not as relevant as the integrated information sent by receptors distributed throughout the body. Descending postural commands are multivariate in nature, and the motion at each joint is affected uniquely by input from multiple sensors. In different sensory environments, the nervous system is able to re-weight its available afferents in order to optimize stance control. For example, with increasing stance width, lateral body motion is detected more easily by

proprioceptors and less readily by vision or the vestibular system [22].

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

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 available [28].

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

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, subthreshold mechanical noise may enhance the detection of pressure changes on the sole of the feet [38].

**2.3. Vestibular system**

quiet stance [64].

information are attenuated or absent [60].

**3. Motor contribution to control posture**

the subjects concerning the task to be performed

Vestibular inputs tonically activate the anti-gravity leg muscles during normal standing

During dynamic tasks, vestibular information contributes to head stabilization to enable successful gaze control [57] and, during active tasks, it provides a stable reference frame from which to generate postural responses [13]. In children, evidence on the development of sensory organization to control posture, according to each sensory component in relation to age suggests that the vestibular afferent system reach adult level at 15 to 16 years of age [27].

Postural Balance and Peripheral Neuropathy http://dx.doi.org/10.5772/55344 129

Since loss of vestibular information may lead to deficits in trunk control but had less effect on the legs, vestibulo-spinal control may act primarily to stabilize the trunk in space and to facilitate intersegmental dynamics [58]. Vestibular influences are earlier for the sagittal plane and are directed to leg muscles, whereas roll control, in the frontal plane, is later and focused on trunk muscles [59]. Vestibular reflexes and perceptual signals appear to have a specific role in the maintenance of upright stance, under conditions in which other sources of postural

Patients with chronic unilateral peripheral loss may vary widely in the amount they could use their remaining vestibular function and show an increased reliance on proprioceptive infor‐ mation [61]. In patients with bilateral loss of vestibular function, postural compensation depends upon the ability to increase reliance on the remaining sensory systems for postural orientation [62]. During visually induced sway, patients with loss of vestibular function may not utilize somatosensory cues to a greater extent than normal subjects; that is, changes in somatosensory system gain may not be used to compensate for their vestibular deficit [63}. However, precision contact of the index finger at mechanically non-supportive force levels may serve as a substitute in subjects with vestibular loss, when they are attempting to maintain

According to the review by Massion (1984) [65], posture is built up by the sum of several basic mechanisms. First the tone of the muscles gives them a rigidity that helps to maintain the joints in a defined position; the postural tone is added to this basic tonus, mainly in the extensor muscles. Postural fixation maintains the position of one or several joints against an internal force (eg. body weight), by co-contraction of the antagonistic muscles around the joints. Coordination between movement and posture is observed with the voluntary movements of body segments. Postural adjustments accompanying voluntary movements show three main characteristics [65]: they are anticipatory with respect to movement, they are adaptable to the condition in which the movement is executed and they are influenced by instructions given to

During upright stance, compensatory torques must be generated to oppose the destabiliz‐ ing torque due to gravity. Then, spontaneous sway is generated by the continuous body deviations countered by corrective torques. During movement of one segment of the body,

Clinical studies have shown that patients with large-fibre neuropathy do not show abnormal body sway during stance [39-40]. Studies in patients with Charcot-Marie-Tooth type 1A disease suggest that functional integrity of the largest afferent fibres may not be necessary for appropriate equilibrium control during quiet stance [39]; also, in this group of patients, postural instability correlates significantly with decreased vibration [41].

Contact of the index finger with a stationary surface can greatly attenuate postural instability during upright stance, even when the level of force applied is far below that necessary to provide mechanical support [42]. In healthy subjects, standing in the dark, spatial information about body posture derived from fingertip contact with a stationary surface greatly improves stability [42]. However, haptic information about postural sway derived from contact with other parts of the body can also increase stability [43].

### **2.2. Visual system**

Visual influence on postural control results from a complex synergy that receives multimodal inputs, and may have similar effects on the leg and trunk segments [44]. In order to stabilize the head in space, visual information of the environment must be definite [45]. Healthy subjects show decreased stability in the dark [46], and to compensate for large postural instabilities, visual information is required.

In infants, a cephalo-caudal developmental gradient may be observed as children develope from 3 to 14 months of age, while a wide variety of response patterns may be seen in the 3- to 5-month-olds, indicating that postural responses are not functional prior to experience with stabilizing the center of mass [47]. In children, the peripheral visual contribution to dynamic balance control increase from 3 to 6 years of age, with a maximum in 6-year-old children.; then it decreases in the 7-year-old children and increase again from 8–9 years of age to adulthood [48]. Evidence on the development of sensory organization to control posture, according to each sensory component, in relation to age suggests that the visual afferent system reach adult level at 15 to 16 years of age [27].

Epidemiological studies have shown that visual impairment is strongly associated with falls in the elderly [49-50]. Among older adults with glaucoma, greater visual field loss or thinner retinal nerve fiber layer thickness is associated with reduced postural stability [51]. These findings could be explained by several factors, including poor visual acuity, reduced visual field, impaired contrast sensitivity, and the presence of cataract [49, 52]. However, the role of vision in posture control may be evident even in subjects between 40 and 60 years old [53].

Visual inputs distinguish between translation and rotation of the head. Static visual cues may slowly control re-orientation or displacement, whereas dynamic visual cues may contribute to fast stabilization of the body [54]. Optical motions, like those produced when an observer moves through an environment, have an effect on postural stability [55}. However, flow structure apparently interacts with the exposed retinal area in controlling stance [56].

#### **2.3. Vestibular system**

threshold mechanical noise may enhance the detection of pressure changes on the sole of the

128 Peripheral Neuropathy - A New Insight into the Mechanism, Evaluation and Management of a Complex Disorder

Clinical studies have shown that patients with large-fibre neuropathy do not show abnormal body sway during stance [39-40]. Studies in patients with Charcot-Marie-Tooth type 1A disease suggest that functional integrity of the largest afferent fibres may not be necessary for appropriate equilibrium control during quiet stance [39]; also, in this group of patients,

Contact of the index finger with a stationary surface can greatly attenuate postural instability during upright stance, even when the level of force applied is far below that necessary to provide mechanical support [42]. In healthy subjects, standing in the dark, spatial information about body posture derived from fingertip contact with a stationary surface greatly improves stability [42]. However, haptic information about postural sway derived from contact with

Visual influence on postural control results from a complex synergy that receives multimodal inputs, and may have similar effects on the leg and trunk segments [44]. In order to stabilize the head in space, visual information of the environment must be definite [45]. Healthy subjects show decreased stability in the dark [46], and to compensate for large postural instabilities,

In infants, a cephalo-caudal developmental gradient may be observed as children develope from 3 to 14 months of age, while a wide variety of response patterns may be seen in the 3- to 5-month-olds, indicating that postural responses are not functional prior to experience with stabilizing the center of mass [47]. In children, the peripheral visual contribution to dynamic balance control increase from 3 to 6 years of age, with a maximum in 6-year-old children.; then it decreases in the 7-year-old children and increase again from 8–9 years of age to adulthood [48]. Evidence on the development of sensory organization to control posture, according to each sensory component, in relation to age suggests that the visual afferent system reach adult

Epidemiological studies have shown that visual impairment is strongly associated with falls in the elderly [49-50]. Among older adults with glaucoma, greater visual field loss or thinner retinal nerve fiber layer thickness is associated with reduced postural stability [51]. These findings could be explained by several factors, including poor visual acuity, reduced visual field, impaired contrast sensitivity, and the presence of cataract [49, 52]. However, the role of vision in posture control may be evident even in subjects between 40 and 60 years old [53].

Visual inputs distinguish between translation and rotation of the head. Static visual cues may slowly control re-orientation or displacement, whereas dynamic visual cues may contribute to fast stabilization of the body [54]. Optical motions, like those produced when an observer moves through an environment, have an effect on postural stability [55}. However, flow

structure apparently interacts with the exposed retinal area in controlling stance [56].

postural instability correlates significantly with decreased vibration [41].

other parts of the body can also increase stability [43].

feet [38].

**2.2. Visual system**

visual information is required.

level at 15 to 16 years of age [27].

Vestibular inputs tonically activate the anti-gravity leg muscles during normal standing

During dynamic tasks, vestibular information contributes to head stabilization to enable successful gaze control [57] and, during active tasks, it provides a stable reference frame from which to generate postural responses [13]. In children, evidence on the development of sensory organization to control posture, according to each sensory component in relation to age suggests that the vestibular afferent system reach adult level at 15 to 16 years of age [27].

Since loss of vestibular information may lead to deficits in trunk control but had less effect on the legs, vestibulo-spinal control may act primarily to stabilize the trunk in space and to facilitate intersegmental dynamics [58]. Vestibular influences are earlier for the sagittal plane and are directed to leg muscles, whereas roll control, in the frontal plane, is later and focused on trunk muscles [59]. Vestibular reflexes and perceptual signals appear to have a specific role in the maintenance of upright stance, under conditions in which other sources of postural information are attenuated or absent [60].

Patients with chronic unilateral peripheral loss may vary widely in the amount they could use their remaining vestibular function and show an increased reliance on proprioceptive infor‐ mation [61]. In patients with bilateral loss of vestibular function, postural compensation depends upon the ability to increase reliance on the remaining sensory systems for postural orientation [62]. During visually induced sway, patients with loss of vestibular function may not utilize somatosensory cues to a greater extent than normal subjects; that is, changes in somatosensory system gain may not be used to compensate for their vestibular deficit [63}. However, precision contact of the index finger at mechanically non-supportive force levels may serve as a substitute in subjects with vestibular loss, when they are attempting to maintain quiet stance [64].
