**3. Motor contribution to control posture**

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 the subjects concerning the task to be performed

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, other segments are disturbed, producing instability. Thus the precise movement of distal segments can be realized only by stabilizing more proximal segments [66]. Just before a voluntary movement, the stretch reflex response in agonist muscles is enhanced [67], consistent with a stabilizing effect. In healthy subjects, axial tone is modulated sensitively and dynamically, this control originates, at least in part, from tonic lengthening and shortening reactions, and a similar type of control appears to exist for postural tone in the proximal muscles of the arm [68].

**4. Clinical assessment of balance**

In order to assess instability or walking difficulty, it is essential to identify the affected movements and circumstances in which they occur (i.e. uneven surfaces, environmental light, activity) as well as any other associated clinical manifestation that could be related to balance, postural control, motor control, muscular force, movement limitations or sensory deficiency. The clinical evaluation should include a detailed assessment of long tracts, cranial nerves, motor control, motor strength, the eyes and the ears. To evaluate the vestibular system and its relationships with other sensory inputs and the oculo-motor system, specific tests have to be

Postural Balance and Peripheral Neuropathy http://dx.doi.org/10.5772/55344 131

Standardized scales and questionnaires may be helpful to evaluate and to follow-up deficits that may be evident on daily life activities (e.i. Berg's Balance Scale [81]; Tinetti scale [82]; balance symptoms questionnaire by Jáuregui-Renaud et al.[83]), as well as falls [84]. Some clinical test include the "Get up and go test" [85], the five-step test and the Functional Reach [86], the Mobility Fall Chart [87] and the evidence based risk assessment tool [88], among others. However, before choosing a tool the clinician should consider the purpose of its design

To evaluate balance, a neurological examination should be performed, including an examina‐ tion of motor and sensory function; care should be taken to assess static and dynamic postural control and gait, as well as to identify visual and vestibular disorders. During static upright stance, it is important to observe the width of the stance, the symmetry of the stance, the balance at the level of the joints as well as the trunk posture, while changing the sensory conditions (i.e. visual input and the surface of support). The sensory conditions may include at least: standing with the eyes open and closed, on a hard and a compliant surface, standing with the feet together and balancing on the two legs. To clinically asses the response to simple pertur‐ bations, the clinician may observe the reaction to push gently the patient while standing.

To measure balance, different aspects may be analysed: electric potentials due to muscle activation, kinematics that is concerned with movement itself and kinetics, concerned with the forces and the moments of forces that are developed during movements. To record kinetics, force platforms are used. The centre of pressure is recorded over a period of time, while standing on the force platform (wearing a safety harness) under different sensory conditions. Several moving force platforms have been designed in order to create dynamic conditions, while maintaining a constant angle between the foot and lower leg and moving the visual enclosure of the platform, which can be coupled to the body sway. Regardless of the technique of measurement used, to interpret any recording of body sway, several factors have to be considered, including the fact that body sway increase with age, with an increased dependence on vision [53, 89-90], and may be affected by body weight and gender [90-91]. In patients with polyneuropathy, special care should be taken in considering adaptive compensation to

To evaluate gait, a sensory-motor evaluation should be performed, as well as a postural and skeletal examination [92]. To asses walking it is necessary to analyse the initiation, the stepping,

performed, including eye movement recordings and vestibular reflexes.

and the purpose for which the tool is to be applied, as well as its reliability.

changes in biomechanical factors as well as sensory deficits.

To preserve balance, postural adjustments are made through flexible synergies, in which the activity of the participating muscles is set to task-specific conditions [5]. The most rapid postural reactions are a class of motor activities mediated primarily by inputs derived from the forces and motions of the feet upon the surface of support [69]; the supporting reactions and placing reactions (tactile, visual and vestibular) adapt the activity of the postural muscles of the limbs to their function of body support. Perturbations to balance imply that the central nervous system select patterns of muscle activation that are appropriate for a variety of perturbations [70], in agreement with biomechanical constraints such as those imposed by inter-segmental dynamics and musculoskeletal geometry [71]. A confluence of proprioceptive and vestibular modulation to the basic centrally initiated template of activity may establish the amplitude pattern of the muscle response synergy [70, 72-73]. The confluence of sensory inputs presumably permits the proprioceptive and vestibular inputs to reinforce each other righting effects, and prevent to fall.

It has been suggested that postural adjustments can be described as a single feedback control scheme, with scalable heterogenic gains that are adjusted according to biomechanical constraints [74]. In addition, muscle weakness and muscle fatigue have to be considered. Clinical evidence have shown that patients with polyneuropathy who have ankle weak‐ ness are more likely to experience multiple and injurious falls than are those without specific muscle weakness [75]. Also, an altered posture, which is common in patients with muscle weakness, may interfere with the position of the centre of mass, and there by also cause balance problems [76].

In healthy subjects, inducing localized muscle fatigue at various musculatures has been shown to adversely affect postural control. Plantar-flexor muscle fatigue may impair the effectiveness of postural control and increase the amount of postural regulatory activity required to control unperturbed bipedal posture when the quality of the postural support surface information is altered (by standing on a foam support surface) to a greater extent than when it is not [77]. In healthy subjects who perform fatiguing exercises, acute effects of fatigue may differ between joints, with the most substantial effects evident at the lower back, followed by the ankle and recovery of postural control [78]. Also, during quiet standing, fatigue of trunk muscles may increase reliance on somatosensory inputs from the foot soles and ankles for controlling posture [79], while lumbar fatigue impairs the ability to sense a change in lumbar position [80].
