Analysis of Adaptation in Human Postural Control
Abstract: The ability to obtain correct balance control information and adjust motor performance appropriately are essential elements in effective postural control. However, functional disorders may affect the reliability of receptor information and ability to carry out corrective motions needed to maintain balance. This emphasizes the importance of rapid and accurate adaptation processes that can optimize the use of available sensor and motor resources. The presented studies aimed to investigate the role of adaptation in human postural control during upright stance, while submitted to balance perturbations evoked by vibratory proprioceptive or galvanic vestibular stimulation, and when different constraints affected the sensory input or motor output. The postural control performance was assessed with posturography and analyzed with respect to variance, correlation, GARCH or MIMO methods, or by a special new method designed to quantify adaptation in postural control.Postural control could, both quantitatively and qualitatively, substantially change movement patterns when submitted to balance perturbations. Three different methods seemed to be used to improve balance control: 1) Suppression of the balance disturbances evoked by the perturbations, 2) Adoption of a favorable posture that had better properties to withstand the disturbances, 3) Use of the gained experiences acquired from previous occasions of exposure to the balance perturbations. These adaptive methods could be manipulated separately and act at different time scales.Postural control adapted the body posture and responses to balance perturbations similarly to repetitive galvanic vestibular and vibratory proprioceptive stimulation. The location of the disturbed somatosensory receptors, e.g., calf or neck muscle receptors, were of minor importance for the movement control strategy used. Moreover, elderly people used similar adaptive methods as middle-aged people, by altering posture and suppressing the effects of balance perturbation.Even though adaptation could have a substantial effect on postural control, the findings imply that adaptation could not entirely compensate for the differences imposed by internal or external constraints such as biomechanics or effects on sensory information. Although the adaptive adjustments were similar during vibration and galvanic stimulation, the two kinds of stimulation induced different movement responses. Moreover, the findings suggest that deficits found of postural control in elderly were not associated with inactive or inappropriate adaptation ability, but probably associated with sensory or motor deficits in their original balance control.Access to visual information had marked effect on the balance control during all trial conditions investigated, in terms of reduced influence of balance perturbations and decreased body movements above 0.1 Hz. Reduced sensory information from the cutaneous mechanoreceptors in the feet substantially increased the body movements, reflected by significantly increased torque variance. However, the effects of disturbed mechanoreceptors information were rapidly compensated for within 100 seconds. Postural control was affected substantially both by decreased muscle force in proportion to body weight, and by fatigued triceps surae muscles. Decreased muscle force significantly increased the body sway induced by balance perturbations compared with fatigued triceps surae muscles. Postural control adaptation was able to reduce but not fully compensate for the changed muscular efficiency caused by decreased muscle force or fatigued muscles. To conclude, in the evaluation of postural control, it is important to consider sensory deficits, decreased motor control efficiency as well as adaptation insufficiencies, and regard such factors in future therapeutic methods.
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