O. V. Kazennikov

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.

Locomotor‐like movements evoked by leg muscle vibration in humans

We attempted to elicit automatic stepping in healthy humans using appropriate afferent stimulation. It was found that continuous leg muscle vibration produced rhythmic locomotor‐like stepping movements of the suspended leg, persisting up to the end of stimulation and sometimes outlasting it by a few cycles. Air‐stepping elicited by vibration did not differ from the intentional stepping under the same conditions, and involved movements in hip and knee joints with reciprocal electromyogram (EMG) bursts in corresponding flexor and extensor muscles. The phase shift between evoked hip and knee movements could be positive or negative, corresponding to ‘backward’ or ‘forward’ locomotion. Such an essential feature of natural human locomotion as alternating movements of two legs, was also present in vibratory‐evoked leg movements under appropriate conditions. It is suggested that vibration evokes locomotor‐like movements because vibratory‐induced afferent input sets into active state the central structures responsible for stepping generation.

Postural instability enhances motor responses to transcranial magnetic stimulation in humans

Does the state of postural instability require a high hierarchical level of posture control? Electromyographic (EMG) activity of leg muscles was recorded during transcranial magnetic stimulation (TMS) of the motor cortex and electrical stimulation of the tibial nerve (H-reflex) in healthy subjects standing on a rigid floor and on a rocking platform. In the soleus muscle, TMS-evoked EMG responses increased considerably (2.2+/-1.1 times) when balancing on the rocking platform, whilst the H-reflex tended to decrease. The effect of support instability was specific to the muscles participating in the posture control. The results suggest that postural instability might change the state and the role of the motor cortex in equilibrium maintenance.

Time structure of a goal-directed bimanual skill and its dependence on task constraints.

The aim of the study was to elucidate the underlying principles of bimanual coordination and to establish quantitative coordination criteria. Healthy human subjects were instructed to open a loaded drawer with the left hand and to grasp, lift and reinsert with the right hand a small peg in the drawer recess. This bimanual goal-oriented task was executed promptly and consistently after a few trials. The temporal structure of the individual limb actions was assessed for computing interlimb synchronization and temporal correlation. In all subjects, both hands were well synchronized at the goal with high intermanual correlation in reaching the goal (event times of drawer opening and grasping the peg). This temporal goal-invariance was independent of movement speed and of the highly variable timing of the individual hands and persisted when subjects were blindfolded. Unilateral loading of the pulling hand and cutaneous anesthesia of the left index finger and thumb used for grasping the drawer handle significantly increased the pull-phase. This slowing of the left hand was matched by an adaptive delay of the right non-disturbed hand, thus preserving goal invariance. As a working hypothesis, we propose that multimodal sensory signals generated in the leading arm be transmitted centrally to re-parameterize the non-disturbed arm.

Effects of transcranial magnetic stimulation during voluntary and non-voluntary stepping movements in humans

Here, we compared motor evoked potentials (MEP) in response to transcranial magnetic stimulation of the motor cortex and the H-reflex during voluntary and vibration-induced air-stepping movements in humans. Both the MEPs (in mm biceps femoris, rectus femoris and tibialis anterior) and H-reflex (in m soleus) were significantly smaller during vibration-induced cyclic leg movements at matched amplitudes of angular motion and muscle activity. These findings highlight differences between voluntary and non-voluntary activation of the spinal pattern generator circuitry in humans, presumably due to an extra facilitatory effect of voluntary control/triggering of stepping on spinal motoneurons and interneurons. The results support the idea of active engagement of supraspinal motor areas in developing central pattern generator-modulating therapies.

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.

Influence of preliminary information about the mass on anticipatory muscle activity during the catching of a falling object

A heavy or light object fell into the cup held between the thumb and the index finger of a sitting subject. The anticipatory muscle activity and the grip force applied to the cup depended on the object mass, whereas the temporal parameters, such as the moment of the start and the duration of muscle activity and the moment of the maximum grip force remained unchanged. Preliminary verbal information about the object mass sufficed for the predictive programming of adequate muscle activity and grip force. Without this information, i.e., when the mass of the falling object was unknown, the anticipatory activity was planned in expectation of a heavy weight.

Goal synchronization of bimanual skills depends on proprioception

The present experiments in Human subjects were designed to test whether proprioceptive feedback plays a role in optimising bimanual synchronization in a goal-oriented familiar task. Goal-synchronization is a typical feature of bimanual everyday skills. The purpose of the study was to disturb proprioceptive signalling by means of vibrating the leading left limb while subjects performed a bimanual task on a drawer manipulandum. Blindfolded subjects reached for and opened the drawer with the left hand while the right hand was reaching for grasping an object as the drawer was fully opened. Discrete events of the task were used to measure movement onset times of pulling and grasping hands and of goal arrival times. A spatial--temporal goal invariance was still present despite asymmetrical limb assignments and subjects were blindfolded. In contrast, when vibration (80 Hz) was applied to the forearm flexors of the leading pulling limb, we found that the interval between the hands at goal reaching was significantly prolonged. This suggests that synchronization is not predetermined entirely by feedforward commands and that proprioceptive feedback is necessary for updating an internal forward model and perhaps also for lower-level corrections in order to ensure covariant limb movements for optimal goal-synchronization.

Characteristics of the maintenance of the vertical posture during standing with an asymmetrical load on the legs

Movements of the common center of pressure (CP) and the CPs of the right and left legs separately were studied during the maintenance of the vertical posture by subjects standing with symmetrical load on their legs or with the shift of the load to the right or left leg. It was shown that standing with a symmetrical load on the legs was accompanied by the movement of the CP of an individual leg along the straight line with small deviations aside, whereas movement of the common CP represented the curve with frequent changes in direction and filling up some space. The shift of the load to one leg resulted in the movement of the CP of the loaded leg that was similar to that observed during a symmetrical load on the legs. The movement of the CP of the unloaded leg was chaotic. The shift of the load to one leg decreased the correlation between the movements of the CPs of the left and right legs compared to standing with a symmetrical load on the legs. The velocity of movement of the CP of the leg loaded increased in the sagittal direction but remained stable in the frontal direction. The velocity of movement of the CP of the unloaded leg remained stable in the sagittal direction but increased in the frontal direction. We suppose that during standing with an asymmetrical load on the legs the role of the single in the maintenance of the vertical posture depend on the load on the leg.

Characteristics of the maintenance of the upright posture in subjects touching an external object while standing on a moving or immobile platform

Postural sway was compared for humans touching an external object while standing on an immobile or slowly moving posturographic platform. When the platform moves, the central nervous system may interpret the movement of the point of the contact with the external object as the movement of the body relative to the support or as the movement of the support itself. Thus, the information concerning the body position that is provided by the touch becomes ambiguous. It was demonstrated that contact with an external object during standing on an unstable support leads to a decrease in support sway. When a subject stands on a moving platform, this decrease is smaller than in the case of an immobile platform. Contact with an external object causes a decrease in postural responses to shank muscle vibrations on an immobile platform. On a moving platform, this decrease is nonsignificant. The change in postural sway depending on the unambiguity of afferent information is discussed in terms of the interaction between afferent signals of different modalities on the basis of the body scheme in subjects maintaining balance.