Vera L. Talis

Support stability influences postural responses to muscle vibration in humans

We studied the effect of support stability on postural responses to the vibration of Achilles tendons and of neck dorsal muscles in healthy humans. For this purpose we compared postural responses on a rigid floor and on 6 cm high rocking supports (see‐saws) of different curvatures (different radii: 30, 60 and 120 cm). The subject stood with eyes closed, the centre of the feet coincided with the centre of the see‐saw. We recorded platform tilt, horizontal displacements of the upper body, ankle joint angle and activity of ankle joint muscles. On the rocking platform subjects maintained balance in a sagittal direction by making see‐saw rotations placing the support under the bodys centre of gravity. Equilibrium maintenance requires that the torque in the ankle joint increases during forward body displacements, as on the rigid floor, and be accompanied by a plantar flexion (not by a dorsiflexion) in the ankle joint. The directional dependence of vibration‐induced reactions on the see‐saw was the same (relative to space) as on the rigid floor: backward body displacement during Achilles tendon vibration and forward body displacement during neck muscle vibration. A decrease of support stability (with a decrease of the radius from 120 to 30 cm) diminished significantly the effect of Achilles tendon vibration and to a lesser extent the effect of neck muscle vibration. In contrast, the increase of platform stability by hand contact with a stable external object gave rise to prominent body sway in response to Achilles tendon vibration. Neck muscle vibration on the movable support provoked a quick initial forward body sway. This initial quick response was absent during vibration of the Achilles tendons. We conclude that postural responses to muscle vibration reflect the participation of different muscles in posture control and depend on the support properties. Support instability changes the role of proprioceptive information and the state of the system of equilibrium maintenance.

Asymmetric leg loading during sit-to-stand, walking and quiet standing in patients after unilateral total hip replacement surgery

BACKGROUND Asymmetric limb loading persists well after unilateral total hip replacement surgery and represents a risk of the development of osteoarthritis in the non-operated leg. Here we studied bilateral limb loading in hip arthroplasty patients for a variety of everyday activities. METHODS Twenty-seven patients and 27 healthy age-matched control subjects participated in the study. They were asked to stand up from a chair, to stand quietly, to perform isometric maximal voluntary contractions and to walk along a 10 m path at a natural and fast speed. Two force platforms measured vertical forces under each foot during quiet standing and sit-to-stand maneuver. Temporal variables of gait were measured using footswitches. FINDINGS In all tasks patients tended to preferentially load the non-operated limb, though the amount of asymmetry depended on the task being most prominent during standing up (inter-limb weight bearing difference exceeded 20%, independent of speed or visual conditions). In contrast, when performing maximal voluntary contractions, or during walking and quiet standing, the inter-limb difference in the maximal force production, stance/swing phase durations or weight bearing was typically less than 10%. INTERPRETATION The results suggest that the amount of asymmetry might not be necessarily the same for different tasks. Asymmetric leg loading in patients can be critical during sit-to-stand maneuver in comparison with quiet standing and walking, and visual information seems to play only a minor role in the control of the weight-bearing ability. The proposed asymmetry indices might be clinically significant for development of post-surgical rehabilitation.

Human equilibrium on unstable support: the importance of feet-support interaction.

Healthy humans maintained equilibrium on rocking supports (seesaw) of different curvatures and heights. We recorded platform tilt, horizontal displacements of the upper body, ankle joint angle and activity of ankle joint muscles. Subjects maintained balance by making seesaw rotations placing the support under the bodys centre-of-gravity. Forward displacement was balanced by compensatory plantariflexion: thus the relation between muscle activity and ankle joint angle differed from that on a rigid floor. Mechanical analysis of stability showed that standing on low seesaws requires ankle torque increase during forward body shift (as on a rigid floor) and torque decrease on high seesaws (when the seesaw height exceeded its radius). In the latter case, balancing was impossible with eyes closed. The results suggest that directionally specific torque changes in response to centre-of-gravity shifts provide important information for maintenance of orthograde posture.

Anticipatory postural adjustment: the role of motor cortex in the natural and learned bimanual unloading

Anticipatory postural adjustment (APA) during bimanual action is observed when participants hold an object in one hand and then lift that object with the other hand. The decrease in activity of a forearm flexor muscle prior to an active forearm unloading acts to stabilize the forearm position. Recent studies have investigated the influence of the corticospinal system on muscle activity during APA through transcranial magnetic stimulation. It was shown that at different times during APA, the amplitude of motor-evoked potentials in the forearm flexors decreased in conjunction with the decrease of muscle activity. If the unloading is triggered via an electromagnet by lifting an equal weight by the other arm, the anticipatory postural adjustment is learned through the repetition of unloading (three series of 20 trials). Using the transcranial magnetic stimulation, we examined changes in the motor-evoked potential in the forearm flexors before and after APA learning. Motor-evoked potential amplitude did not significantly change as forearm flexor activity decreased. The motor-evoked potential/background electromyogram ratio, however, increased in the final learning session in comparison to the initial learning session and stationary loading. The present results corroborate a hypothesis on the fundamental role of the motor cortex in the suppression of synergies that interfere with the execution of the new coordination in the process of motor learning.

DOES THE STABILITY OF ELBOW SUPPORT INFLUENCE THE ELBOW JOINT MATCHING ACCURACY

The aim of the present work was to determine if the elbow joint instability and translatory movements during elbow flexion lead to significant errors in elbow angle perception. The matching elbow was fixed on a rocking platform of two different heights so that the elbow flexion was associated with tilting movement of the support and the angle/torque relationship changed depending on the height of the platform. The matching on any of the rocking supports did not cause an over-flexion constant error but it did increase the error variance, especially in the high rocking support condition. An adaptation to the rocking support condition was revealed by an after-effect resulting in an overestimation of the reference angle in the final testing on a rigid support. It is concluded that the elbow angle perception is modified as a result of adaptation to the rocking support, which is associated with recalibration of position sense during the experimental session. The results are consistent with the hypothesis that the estimation of the elbow joint angle depends on the internal representation of the arms dynamics.

Anticipatory postural adjustment before bimanual unloading reactions: the role of the motor cortex in motor learning.

The role of the motor cortex in forming a learned coordination (stabilization of the forearm on unloading) was studied in humans. Subjects maintained a 1-kg weight with the right (postural) forearm, the weight being attached via an electromagnet. Unloading of the postural arm was initiated by the subjects by lifting a similar load with the left arm. In control experiments, lifting of the load did not lead to unloading of the postural arm. Changes in motor cortex excitability were studied by transcranial magnetic stimulation applied to the representation area of the right biceps muscle in the motor cortex at the beginning and end of the experiments. Repeated unloading tests showed progressive decreases in the amplitude of the movement of the unloaded forearm, which were accompanied by increases in the anticipatory inhibition of the electromyogram of the biceps muscle of the unloaded arm (learning). Muscle responses to transcranial magnetic stimulation during the learning process showed no significant changes. Analysis of normalized muscle responses to transcranial magnetic stimulation (response/baseline) showed that these increased at the end of training and reached a significantly higher level than seen at the beginning of training. These results lead to the conclusion that the motor cortex plays a fundamental role in inhibiting synergies and coordinations which would interfere with the formation of the new coordination during motor learning.

New Pages in the Biography of Nikolai Alexandrovich Bernstein

Nikolai Alexandrovich Bernstein (1896–1966) is well known today primarily for formulating the problem of redundant degrees of freedom and their elimination in motor control, as well as his hierarchical theory of movement coordination. This paper aims to uncover new pages in the biography of N.A. Bernstein, based on materials from the archive of his nephew Alexander Sergeevich Bernstein, as well as recent interviews with the former pupils of N.A. Bernstein. Concentrated around several interdisciplinary seminars, they grew into a young generation of physiologists in the late sixties and made remarkable contributions inspired by Bernstein’s new principles of neuroscience. These include the discovery of the spinal automatism of stepping in the cat, the “equilibrium point” hypothesis, the hindlimb wiping reflex of the frog as an example of a targeted trajectory organized at the spinal level, and the probabilistic prognosis in human activity.

Changes in Corticospinal Excitability in the Reactions of Forearm Muscles in Humans to Vibration

Transcranial magnetic stimulation was used to study changes in corticospinal excitability during vibration of the flexor and extensor muscles of the wrist in healthy humans. The ratios of muscle stimulation responses to activity levels in these muscles on contraction associated with vibration (the tonic vibratory reflex, TVR) and after vibration of antagonist muscles in isometric conditions (the antagonist vibratory reflex, AVR) were analyzed. The normalized muscle response in the wrist flexors was found to increase by 66% compared with threshold values in the direct vibratory response (TVR), by 75% in the relayed vibratory response (AVR), and by 18% on voluntary contraction. However, increases in the motor response in vibratory responses as compared with those on voluntary contraction did not reach significance, which contrasted with the responses in the forearm flexors. These results are discussed from the point of view that the motor cortex plays different roles in vibratory responses in the distal and proximal muscles.

Involvement of the motor cortex in the bimanual unloading reaction: a transcranial magnetic stimulation study.

The responses of the biceps brachii muscle of the upper arm to magnetic stimulation of the motor cortex during the postural pretuning and forearm unloading tasks were studied in humans. On active unloading, the amplitude of the evoked response decreased in parallel with a decrease in muscle activity. During stationary holding of the load, the muscle response changed in proportion to the load. When, on the background of stationary holding of the load, the other arm took on the same load, the amplitude of the evoked response in the biceps muscle of the arm holding the load decreased without any change in the muscle activity. Passive unloading was accompanied by similar changes in the response evoked by magnetic stimulation as seen with active unloading. The question of whether the decrease in muscle activity (postural pretuning) in active unloading may be associated with both direct corticospinal influences and influences mediated via subcortical structures is discussed.

Elbow Matching Accuracy in Young and Elderly Humans under Unusual Mechanical Constraints

Experiment was carried out to study the proprioception accuracy of elderly (61–83 years old) and young (22–36 years old) subjects during contralateral elbow matching in sagittal plane. The subjects performed the task under ordinary condition and under experimental condition (matching forearm attached to the rocking cylindrical platform of low (LS), or high (HS) height, so that the elbow flexion was associated with tilting movement of the support and with backward movement of the upper arm). Control matching of young and elderly subjects does not differ significantly in terms of constant and absolute error. First block of LS and HS induced absolute error increase and matching arm velocity decrease in both groups, but in the second block of matching on rocking supports both arms velocity of elderly subject decreased and absolute error of elderly subjects toward the second block of rocking condition appeared lower than those of young subjects. Aftereffect of the restricted matching could be observed in elderly as a significant increase of matching arm velocity and corresponding constant error increase. It could be concluded that under unusual mechanical constraints elderly subjects tended to use “conservative” strategy followed by significant aftereffect toward the final ordinary support condition.