We studied hand movement during imitation of cello bowing while the rectilinear movement of the right bowing arm should occur in parallel with the bow orientation along the arm trajectory. Musically untrained individuals moved the bow across the bar that imitated the cello. Motion analysis was used to investigate the influence of a variety of experimental conditions: (1) bow motion on the bar surface, (2) on the left hand lying on the bar, and (3) in the air without touching the bar. It was found that the trajectory of the marker on the index finger at the bow frog differed significantly from the marker trajectory at the bow tip. In all conditions the marker on the index finger moved along a trajectory close to a straight line with an orientation slightly deviating from perpendicular to the bar. The marker trajectory at the bow tip deviated more from the perpendicular direction. Differences in the trajectories of markers at the bow frog and the bow tip depended on the condition of bow movement. The smallest differences were observed when the bow was moved on the left hand. It is suggested that sensation from the contralateral hand was used to create the internal representation of the relative position of the bow and the bar.

Vertical posture of a standing subject with different orientations of one or both feet was studied. Symmetrical turn of both feet in a standing person led to an increase in the movement of the common center of pressure (CCP) and center of pressure (CP) of each foot. It is likely that, for the vertical position maintenance with symmetrical feet turn, the movement of the CPs of both feet was controlled simultaneously. During standing with the turn of one foot, the CP movement increased for the foot that retained orientation in the sagittal direction. For the turned foot the CP movement did not change. Thus, the turn of the foot of one leg changed the involvement of this leg in the upright position maintenance. The vertical posture was maintained mainly due to the position control of CP of the foot oriented in the sagittal direction. In addition, there was a change in the predominant direction of the CP movement relative to the foot orientation. When the foot was oriented in the sagittal direction, the predominant movement direction of the CP was turned outward. With the foot rotated outward, the predominant movement direction of the CP was turned inward. This change in the CP movement direction was observed in the cases of simultaneous turn of both feet as well as the turn of one foot. It can be assumed that, in standing, the balance-maintaining system controls the CP position of each leg and takes into account the torsional torque in the leg that occurs when the foot is rotated.

The interaction of anticipatory and reflexive changes in the grip force of the right hand was tested for the effect on the grip force developed by the thumb and index finger of the left hand. The test task for the right hand was the same in all test variants, to hold a cup where a weight fell (with the thumb and index finger). Three different tasks were chosen for the left hand. In the first task, the left fingers touched a sensor with a negligible grip force. In the second task, the subject had to hold a force sensor loaded with a 200-g load. In the third task, the subject was instructed to slowly increase the grip force by fingers of both hands. In response to the impact of the falling weight, an involuntary increase was observed in the grip force of the right-hand fingers, depending on the available visual information of the falling weight and being independent of the motor task performed by left hand. When the subject’s eyes were closed, the grip force increased in all tasks as a response to the impact of the weight fall. When the subject’s eyes were open, an automatic increase in the right-hand grip force occurred 200–300 ms before the impact. The left-hand grip force changed in different ways. In the first and second tasks, the grip force of the left hand did not change despite the changes in the grip force of the right hand in response to the impact of the falling weight. In case of a coordinated slow increase in the grip forces of both hands, an increase in the grip force was observed in the unaffected left hand before the impact and during the response to the impact, like in the right hand. Thus, a common motor task for both hands led to similar changes in grip force, which were recorded in both right and left hands not only during the response to an external stimulus, but also during anticipation. Movement planning was assumed to involve the organization of left–right interaction at the supraspinal level, probably, at the level of motor cortex interactions between the left and right hemispheres.

Body sway has usually been studied during symmetrical standing while lower limbs equally contributed in upright posture. The present study aimed to examine the dynamics of body sway while turning during standing with symmetrical and asymmetrical weight distribution between the legs. Subjects performed a body turn of 30° to the right and left during quiet standing and standing with the right or left foot loaded with 70% of body weight. We found that body turn in the symmetrical posture induced weight increase on the foot contralateral to the turning direction and the common center of pressure (COP) velocity increase. Body turn in the asymmetrical posture induced further loading of the foot that was initially overloaded; also turning in the direction of the unloaded foot affects weight distribution more than turning in the direction of the overloaded foot. The posture transition from symmetrical to asymmetrical induced the common COP velocity increase and forward protraction of the unloaded foot COP. Turning in the asymmetrical posture produced further increase of the common COP velocity and further forward protraction of the unloaded foot COP. Moreover, when the left leg was unloaded this resulted not only in left leg's COP forward, but also in left leg's COP lateral protraction, and left leg's COP frontal velocity increase. These findings reveal that body position and weight distribution between the feet interact to stabilize upright posture and show the effect of footedness during turning in asymmetrical standing.

We studied the vertical posture in subjects in the standing position with different turns of the body and different distributions of the load between the legs. We recorded the motion of the projection of the common center of pressure (CCP) and of the center of pressure (CP) for the left and right legs. The predominant direction of CP movement was determined from a stabilogram, and then changes of this direction were analyzed during body turns and with different load distributions between the legs. Body turn led to the shift of the predominant direction of CCP movement towards the turn side. This change in the direction of CCP movement was observed with any load distribution between the legs. At the same time, weight transfer to one leg also led to the direction shift of CCP movement towards the loaded leg. The direction of CP movement of the loaded leg did not change, but the direction of CP movement of the unloaded leg shifted clockwise upon unloading both right and left legs. We assume that the changes in the mechanisms of maintaining the vertical position with asymmetric distribution of the leg load may be associated not only with the change in the force interaction with the supporting surface, but also with asymmetry of axial muscle tone.

Maintenance of a vertical posture was studied in standing subjects with a fixed knee joint of one leg and a different weight distribution between the legs. Knee fixation on one leg did not affect the speed of movements of the common center of pressure (CP) at any weight distribution between the legs, and the stability of vertical posture was therefore unchanged. However, the relative contributions of the legs to the posture control changed when knee movements of one leg were restricted. The speed of CP movements of the free leg was independent of the weight loading on the leg. The speed of CP movements of the leg with the knee fixed depended on the weight distribution and was higher when the leg was loaded. Thus, the leg with the fixed knee joint made a greater contribution to maintaining vertical posture when the leg was loaded. Yet its contribution was comparable with that of the unloaded free contralateral leg even in this case, as was evident from lack of differences in CP movements between the two legs. It was assumed that the leg with the free knee joint played a major role in maintaining equilibrium of vertical posture, while the leg with the fixed knee joint mostly acted to more finely adjust the body position.

The vertical posture was studied during standing with fееt on the support surfaces of different structures. The movements of the center of pressure (CP) of each leg and the common CP (CCP) were recorded while the subject stood with a support on a smooth floor and with the support of one foot on a spike mat (SM) with different load distributions between the legs. When the body weight was transferred to one leg during standing under ordinary conditions on a smooth floor, the CP of the loaded leg moved more than the CP of the unloaded leg; i.e., the posture sway was compensated mainly due to the activity of the loaded leg, which created a larger torque. When the subject stood with one foot on the SM, the CP movement of this leg did not depend on the leg load and was about 60% of the CP movement of the leg on the smooth floor. Apparently, the CP displacement of the unloaded leg on smooth support was larger than the CP displacement of the loaded leg creating the torque necessary for compensating the body sway. Thus, maintaining the vertical posture was carried out mainly by the leg standing on the smooth support. It is assumed that additional stimulation of different surface and deep receptors of the foot caused by foot support on the SM hampered the perception of its CP position, and the vertical posture was maintained mainly by the leg afferent signals from which more precisely reflected the CP position.

Упреждающие изменения позной активности являются важным элементом поддержания равновесия при стоянии у человека. Изменение механических условий поддержания равновесия, вероятно, должно приводить и к изменению упреждающих настроек. У стоящего на раздельных опорах человека исследовали упреждающие изменения в стабилограмме обеих ног при быстром подъеме правой руки до горизонтального уровня. В разных вариантах эксперимента подъем руки производился во время стояния с обеими ногами на неподвижных опорах или, когда только правая или только левая нога была на подвижной опоре. В каждом варианте эксперимента испытуемый стоял с симметричной нагрузкой на ноги или с произвольным перераспределением веса на одну ногу. Упреждающее смещение центра давления (ЦД) ноги в начале подъема руки зависело от распределения нагрузки между ногами и от подвижности опоры под ногами. При стоянии на неподвижных опорах с симметричной нагрузкой на ноги перед подъемом правой руки ЦД правой ноги смещался назад, а ЦД левой ноги – вперед. При стоянии на неподвижных опорах с нагрузкой на одну ногу упреждающее смещение ЦД этой ноги уменьшалось. При стоянии с подвижной опорой под ногой упреждающее смещение ЦД этой ноги было небольшим и не зависело от нагрузки на нее. При этом упреждающее смещение ЦД ноги на неподвижной опоре зависело от нагрузки на нее так же, как при стоянии с обеими ногами на неподвижной опоре. Предполагается, что на подвижной опоре из-за того, что опорная и проприоцептивная афферентация от дистальных звеньев ноги не дает однозначной информации о положении тела, ведущая роль в поддержании равновесия смещается от дистального к проксимальному уровню.

Anticipatory adjustments of postural activity are an essential element of vertical body position maintenance during human standing. Changes in mechanical conditions of standing must affect anticipatory adjustments. Anticipatory changes in the stabilogram of each leg were studied in a subject standing on two separate supports during quick right arm lifting up to the horizontal level. In different experimental variants, the arm was lifted during standing with both legs on unmovable support or with only right or only left leg on movable support. In each standing condition, the subject stood with symmetrical load on two legs or with the load voluntarily transferred to one leg. The anticipatory leg center of pressure (CP) shift at the early stage of arm lifting depended on load distribution between the legs and support mobility under legs. During standing on unmovable supports with symmetrical load on the legs before the right arm lifting, the right leg CP shifted backward while the left leg CP shifted forward. When the subject stood on unmovable supports with load on one leg, the anticipatory CP shift of this leg decreased. While standing with one leg on a movable support, the anticipatory CP shift of this leg was small and did not depend on the leg load. However, the anticipatory shift in the CP of the leg placed on unmovable support depended on the load in the same way as in the case when both legs were on unmovable supports. The results suggest that, since support and proprioceptive afferentation from distal parts of the leg does not supply unambiguous information about body position on movable support, the leading role in balance maintenance is shifted from the distal to proximal level.

Motor evoked potentials (MEPs) in the right first dorsal interosseous (FDI) muscle elicited by transcranial magnetic stimulation of left motor cortex were assessed in ten healthy subjects during maintenance of a fixed FDI contraction level. Subjects maintained an integrated EMG (IEMG) level with visual feedback and reproduced this level by memory afterwards in the following tasks: stationary FDI muscle contraction at the level of 40 ± 5 % of its maximum voluntary contraction (MVC; 40 % task), at the level of 20 ± 5 % MVC (20 % task), and also when 20 % MVC was preceded by either no contraction (0–20 task), by stronger muscle contraction (40–20 task) or by no contraction with a previous strong contraction (40–0–20 task). The results show that the IEMG level was within the prescribed limits when 20 and 40 % stationary tasks were executed with and without visual feedback. In 0–20, 40–20, and 40–0–20 tasks, 20 % IEMG level was precisely controlled in the presence of visual feedback, but without visual feedback the IEMG and force during 20 % IEMG maintenance were significantly higher in the 40–0–20 task than those in 0–20 and 40–20 tasks. That is, without visual feedback, there were significant variations in muscle activity due to different prehistory of contraction. In stationary tasks, MEP amplitudes in 40 % task were higher than in 20 % task. MEPs did not differ significantly during maintenance of the 20 % level in tasks with different prehistory of muscle contraction with and without visual feedback. Thus, in spite of variations in muscle background activity due to different prehistory of contraction MEPs did not vary significantly. This dissociation suggests that the voluntary maintenance of IEMG level is determined not only by cortical mechanisms, as reflected by corticospinal excitability, but also by lower levels of CNS, where afferent signals and influences from other brain structures and spinal cord are convergent.

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.

The shift of the common center of pressure (CCP) and the center of pressure (CP) of one leg was studied during the Achilles tendon vibration of one or both legs while the subject was standing with symmetrical load on the legs or with the load transferred to one leg. The CP shift of the standing subject during unilateral Achilles tendon vibration depended on both the side of application of vibration and on the distribution of the leg load. During standing with a asymmetrical load on the legs, the shift of the CCP was larger than when the vibration was applied to the loaded leg. The CP shift of one leg was greater if both vibration and the load were applied to it. Vibration of the unloaded leg caused a CP shift in the loaded contralateral leg. In this case, vibration of the left unloaded leg did not cause any noticeable CP shift of the left leg, while vibration of the unloaded right leg caused a CP shift of the right leg. Under the similar conditions of loading and vibration, the displacement of the CP of the right leg was larger than the displacement of the CP of the left leg. It may be suggested that postural asymmetry and unilateral vibration of the leg muscles change the internal representation of the position of the body axis in relation to the vertical, which affects the displacement of the CP of one leg in response to afferent stimulation of the leg muscles.

Changes in the vertical posture maintenance were studied when the legs were placed on supports of different degrees of mobility and part of the body weight was voluntarily transferred to one leg. The aim of these experiments was to explore how the mobility of support under the feet affects the balance and how this effect depends on the load distribution between the legs during standing. When both legs were on rigid immovable supports, the vertical posture was maintained by control of the center of pressure (CP) on both legs. When the subject transferred the weight to one foot, the posture was maintained mainly due to the control of CP of the loaded leg. When the legs were on supports of different degrees of mobility, the balance was maintained by the leg on the immovable support. This result was observed both when the subject stood with symmetrical load on the legs and when the load was transferred to one leg. Even when the leg was unloaded but placed on the immovable support, its CP moved more compared to the CP of the loaded leg on a movable support. The results obtained show that the support mobility is a factor that determines the mechanisms of posture maintenance, and this factor is more significant than load distribution between the legs. Thus, the upright posture is maintained with the physical properties of support under the feet taken into account.