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Alexander S. Aruin

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Alexander S. Aruin

A number of factors are likely to play a major role in the process of generation of anticipatory postural adjustments (APAs). Among them are the magnitude and direction of an expected perturbation, properties of a voluntary action associated with the perturbation, and features of the postural task such as a body's configuration prior to the action. The aim of this study was to analyze the effect of body configuration on APAs. Experiments were performed on 8 healthy subjects performing fast bilateral shoulder extension movements while standing. Body configuration was modified by instructions to the subjects to stand vertically or with a forward upper body bend varying from 15 to 60°. The electrical activity of postural muscles and displacements of the center of pressure were recorded. Results indicated that APAs were modified with changes in the angular position of the upper body. Decreased anticipatory activation was seen in rectus abdominis and rectus femoris, while increased anticipatory inhibition was observed in erecteor spinae and biceps femoris across conditions with forward bend. As a result, the total anticipatory activity of muscles in a muscle pair in series with a forward bend showed only slight modulation as compared to vertical posture. These results suggest that the CNS uses reorganization of the anticipatory activity of postural muscles by compensating for the changes in APAs of individual muscles in a muscle pair in such a way that the overall anticipatory activity of the muscle pair stays unchanged. Such compensation in counteracting the expected mechanical effects of the perturbation is used to accommodate both changes in the length of postural muscles and diminished stability of the body due to forward bend.

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Swati Shenoy and Alexander S. Aruin

The objective of this study was to determine if a forward-tilted seat and the resultant semi-kneeling body position associated with sitting on the Balans Multi Chair (BMC) affect postural control in sitting. Nine healthy subjects were seated on either the BMC or a regular (REG) chair with their arms extended. They were instructed to induce self-initiated body perturbations in four different directions by exerting brief pulses of force against a stationary frame positioned in front of them. Electromyographic (EMG) activities of trunk and leg muscles were recorded before and during the perturbations. The results show that sitting on both types of chairs was associated with anticipatory activation of trunk and upper leg muscles. In contrast, anticipatory activation of distal muscles was observed while sitting only on the REG chair and was absent while sitting on the BMC. The outcome of the study suggests that although the forward-tilting seat and semi-kneeling body position might help in preserving a normal lordosis, it is not associated with anticipatory activation of lower leg muscles, which might reduce the ability of an individual to counteract self-initiated body perturbations. These findings stress the important role of chair designs in the control of sitting posture.

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Etem Curuk, Yunju Lee and Alexander S. Aruin

The authors investigated anticipatory postural adjustments in persons with unilateral stroke using external perturbations. Nine individuals with stroke and five control subjects participated. The electromyographic activity of 16 leg and trunk muscles was recorded. The onsets of muscle activity during the anticipatory phase of postural control were analyzed. The individuals with stroke did not show an anticipatory activation of leg and trunk muscles on the affected side; instead, the muscle onsets were seen after the perturbation, during the balance restoration phase. However, an anticipatory activation of muscles on the unaffected side was seen in individuals with stroke, and it was observed earlier compared with healthy controls (p < .05). The individuals with stroke showed a distal to proximal order of anticipatory activation of muscles on the unaffected side. The outcome of the study provides a basis for future investigations regarding ways of improving balance control in people with stroke.

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Alexander S. Aruin and Gil L. Almeida

The hypothesis that persons with Down syndrome are likely to trade mechanical efficacy of motor patterns for safety was tested in a study of postural adjustments associated with fast voluntary movements. Control subjects and subjects with Down syndrome performed bilateral shoulder flexion and extension movements “as fast as possible” while standing on a force platform. Anticipatory changes in the background activity of postural muscles were seen in both groups of subjects. Subjects with Down syndrome demonstrated simultaneous EMG bursts in postural “agonist-antagonist” pairs. Subjects with Down syndrome also demonstrated patterns of deviations in the major postural joints, suggesting that they were using a different strategy which may be a correlate of “clumsiness.” It was concluded that practice in conditions of changing components of a motor task is the most promising way to encourage the central nervous system of persons with Down syndrome to search for alternative strategies and improve these individuals' ability to solve everyday motor problems.

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Mohan Ganesan, Yun-Ju Lee and Alexander S. Aruin

The use of a footrest while performing activity in standing is frequently associated with improvement of a user’s well-being however no information exists on the role of a footrest in improving postural stability. The aim of the study was to evaluate the effects of using a footrest in postural control. Twenty healthy young volunteers were tested using three experimental conditions: standing with two feet on the force platform and standing on the force platform when one foot was placed on a 15 cm footrest positioned in front or laterally. The mean and root mean square distance, range and velocity of the center of pressure (COP) were calculated in the anterior-posterior (AP) and medio-lateral (ML) directions using the force platform data. The COP displacements in AP and ML directions increased in conditions of standing with one foot placed on the footrest regardless of its location. Standing with eyes closed increased COP displacements further. The outcome of the study suggests the importance of using COP measures for evaluation of postural stability and provides additional information needed for optimization of working conditions involving standing with a footrest.

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Veena Iyengar, Marcio J. Santos and Alexander S. Aruin

We investigated whether slower velocity of arm movement affects grip-force generation in conditions with the finger touch provided to the wrist of the target arm. Nine subjects performed the task of lifting and transporting an object at slow, intermediate, and fast velocities with a light finger touch from the contralateral arm and without it. There was an effect of velocity of arm movement on grip-force generation in both conditions. However, when the no touch and touch trials performed with similar velocity were matched, the effect of touch on grip-force reduction was statistically significant (p < .001). The observed decrease in grip force could not be explained by slower movement execution in the touch conditions and underlines the importance of using a contralateral touch in the performance of activities of daily living. It also points to a possibility of the development of therapeutic advances for the enhancement of grip-force control in patients with neurological impairments.