Effects of body turn on postural sway during symmetrical and asymmetrical standing.

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.

[The influence of the leg load and the support mobility under leg on the anticipatory postural adjustment].

Anticipatory postural adjustment is an essential part of equilibrium maintainance during standing in human. So changes in stance condition could affect both control of equilibrium and anticipatory adjustment. Anticipatory changes in the stabilogram of each leg were studied in standing subject during the early stage of quick right arm lifting while legs were on two separated supports. The center of pressure (CP) movement was analyzed in three variants of experiment: both legs on immovable support, with only right leg on the movable support and with only left leg on the moveable support. In each standing condition subject stood with symmetrical load on two legs or with the load voluntary transferred to one leg. The anticipatory CP shift depended on the mobility of the support under the leg and on loading of the leg. While standing on unmovable supports with symmetrical load on the legs before lifting of the right arm CP of right leg shifted backward and CP of left leg--forward. While standing with one leg on movable support the anticipatory CP shift of this leg was small and did not depend on the load on the leg. However the shift of CP of the leg that was placed on the unmovable support depended on the load in the same way as in the case when both legs were on unmovable supports. Results suggested that since on movable support the support and proprioceptive afferent flow from distal part of the leg that was did not supply unambiguous information about body position, the role of distal joints in posture control is reduced.

[Influence of Achilles tendon vibration on the human vertical posture during standing with asymmetrical leg loading].

The shift of center of pressure (CP) of body and CP of each leg was studied during Achilles tendon vibration of one or both legs while subject was standing with symmetrical load on the legs or with the load transferred on one leg. The CP shift of standing subject during unilateral Achilles tendon vibration depended both on the side of the tendon vibration and on the leg load. When standing with a load transferred on one leg the shift of common CP was larger than when the vibration was applied to the loaded leg. The CP shift of one leg was greater if the vibration, and the load was applied to it. Vibration of unloaded leg caused a CP shift in the contralateral loaded leg. In this case, the vibration of left unloaded leg caused no noticeable CP shift of left leg, while the vibration of the unloaded right leg caused CP shift of right foot. In the same conditions of load and vibration the CP displacement of right leg was larger than the CP shift of left foot. It can be assumed that the change in the load on the leg and unilateral vibration of leg muscles change of the internal representation of the vertical body axis, which affects the CP position of one leg during the muscles vibration.

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.

Prior history of FDI muscle contraction: different effect on MEP amplitude and muscle activity.

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.

[Influence of the movable support under one leg on human vertical posture during standing with asymmetric load on legs].

The posture in standing subjects was studied when the legs were placed on supports of different degrees of mobility, as well as, when a part of body weight was voluntary 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 influence could be changed by the load distribution between the legs during standing. When both legs were on rigid immovable supports, the posture maintaining was accomplished by control of center of pressure (CP) of both legs. When the subject transferred the weight on one foot the posture was maintained mainly due to the control of CP of loaded leg. When the legs were on the supports of different mobility, the balance was maintained by the control of CP of the leg on the immovable support. This result was observed both when the subject stood with symmetric load on the legs and when the load was transferred to one leg. Even when the leg was unloaded but was placed on immovable support its CP moved more in comparison with the CP of loaded leg on movable support. The results show that the support mobility under the legs is a factor that determines the mechanisms of the posture maintenance, and this factor is more significant than the load distribution between the legs. So, we can conclude that the upright posture is maintained accounting the physical properties of the supports under the feet.

[The dependence of motor response time in discrimination task on the type of available visual information].

The interaction of proprioceptive and visual afferentation was studied in motor task for discrimination of weights of falling objects. The availability of visual information decreased the time of motor response but to different extent depending on the kind of this information. The shortening of response time was significantly greater when subject have seen the beginning of real fall that when he simply seen the control LED signaling the release of object by electromagnet. So, subject accomplishes the task of discrimination of light and heavy object more efficiently if he sees the real falling object. This may be due to the capacity of CNS to predict the time of the impact more accurately when subject observe the initial part of real trajectory and not just an abstract visual signal.

[Characteristics of vertical posture maintenance during standing with asymmetrical legs loading].

We studied the motion of the common center of pressure (CP) and of right and left foot CP, while maintaining a vertical posture when standing with symmetrical legs loading and with the transfer a part of weight to the right and left leg. It was shown that when standing with symmetrical legs loading the single leg CP motion occurred along a straight line with small side deviations in contrast to common CP motion, which was along an irregular curve, which often changed the direction and covered some area. When weight was transferred on one leg the motion of CP of loaded leg was similar to the motion of CP of that leg during standing with symmetrical load. The motion of CP of unloaded leg was along an irregular curve. During standing with weight transferred on one leg, the correlation between the CP movements of the left and right foot was reduced compared to standing with symmetrical load on thelegs. The velocity of loaded leg CP increased in the sagittal direction, but did not change inthe frontal direction. The velocity of unloaded leg CP motion did not change in the sagittal direction, and increased in the frontal. It was suggested that during standing with asymmetrical load on legs the involvement of the leg in maintaining the vertical posture depends on the load supported by this leg.

[Control of different electromyogram levels of M. abductor pollicis brevis by means of visual feedback in healthy subjects].

We studied voluntary control of integrated electromyogram (IEMG) in the range of 20 +/- 5% and 40 +/- 5% of the IEMG of m. abductor pollicis brevis during its maximum voluntary contraction with and without visual feedback. Healthy subjects performed IEMG control with visual feedback in 5 trials; IEMG control with visual feedback in 5 trials for 5 days, and the reproduction of memorized IEMG value without visual feedback after 5 trials of IEMG under the visual control. The accuracy of IEMG control was estimated by the following parameters: time of IEMG being out of the required 10% range (ERROR); IEMG variability (VARIABILITY), and the bias of IEMG mean level (BIAS) during 30-sec trials. The IEMG control in the range of 20 +/- 5% with visual feedback improved in all subjects over the course of 5 trials. Within 5-day training, ERROR and VARIABILITY reduced on the first day only; during the last 4 days there was no accuracy increase. ERROR increased more than twice when the 20% IEMG level was reproduced without vision. The IEMG control in the range of 40 +/- 5% improved neither during 5 trials, nor during 5 days of training with visual feedback. ERROR increased for about 1.5 times when the 40% IEMG level was reproduced without vision. It was concluded that the motor system, particularly the motor cortex, could control the given level of muscle activity using the visual feedback.

Load detection and discrimination in a catching task: role of haptic and auditory information.

The aim of this study was to verify the contribution of haptic and auditory cues in the quick discrimination of an object mass. Ten subjects had to brake with the right hand the movement of a cup due to the falling impact of an object that could be of two different masses. They were asked to perform a quick left hand movement if the object was of the prescribed mass according to the proprioceptive and auditory cues they received from object contact with the cup and did not react to the other object. Three conditions were established: with both proprioceptive and auditory cues, only with proprioceptive cue or only with an auditory cue. When proprioceptive information was available subjects advanced responses time to the impact of the heavy object as compared with that of the light object. The addition of an auditory cue did not improve the advancement for the heavy object. We conclude that when a motor response has to be chosen according to different combinations of auditory and proprioceptive load-related information, subjects used mainly haptic information to fast respond and that auditory cues do not add relevant information that could ameliorate the quickness of a correct response.

[Participation of the primary motor cortex in programming of muscle activity during catching of falling object].

Object fell into the cup that sitting subject held between thumb and index fingers. Transcranial magnetic stimulation (TMS) of the primary motor cortex was performed early before and during anticipatory grip force increasing. Comparison of current EMG activity of adductor pollicis brevis and first dorsal interosseous muscles and responses of these muscles on TMS showed that responses were increased before the raising of muscle activity. From the other side only slight augmentation of responses was observed during subsequent strong muscle activation. It is assumed that the increasing of the TMS responses that occurred before the initiation of muscle activity reflects the enhancement ofthe motor cortex excitability associated to specific processes related to the motor cortex participation in programming of the muscles activities.

[Dependence of anticipatory changes of hand muscle activity and grip force on the height of the fall during catching of the object].

Timing of changes of hand muscle activity and grip force before the impact of the object that fell from different height into the cup held between thumb and index fingers of sitting subject was studied in three variants of experiment: 1) the subject has seen the fall of the object, 2) the subject has not seen the movement of the object but has initiated the fall, 3) the subject has no information about the object fall. In the third variant changes of muscle activity and grip force has occurred in response to the impact of the falling object. In the second variant changes of muscle activity and grip force preceded the impact the falling object by 200-280 MC and this time interval that did not depend on the height of the fall. In the first variant of experiment changes of the muscle activity and grip force has occurred in 150 ms after the start of fall independently of the height of the fall when the height was 30-50 cm. When the height of the fall was 70-105 cm the changes has preceded the impact by the time interval that did not depend on the height of fall. Thus when the height of fall was small the time of the start of the changes of muscle activity and grip force was related to the start of the fall, but when the height was large the start of changes was related to the presumed impact of falling object.

[Influence of an additional movement task for the upper extremities on equilibrium maintenance in frontal and sagittal level in standing man].

The postural oscillations of standing man were studied during additional manual motor task that consisted of maintaining of the moving ball in the center of flat box. The movement of a center of pressure (CP) in frontal and sagittal plane were analyzed during standing on stable rigid support and on moving unstable support. The influence of the additional motor task on CP movement depend on level of support stability. Sagittal CP movement increased while the additional task was executed during standing on moving support but it did not when the support was stable. Frontal CP movement decreased when the additional task was executed during standing on stable support but it did not while the support was unstable. Thus execution of the additional motor task execution led to the reduction of efficacy of the postural control on the moving unstable support. This result suggests that the cortical influence on the postural mechanism was stronger during standing on moving support in comparison to the standing on the stable support.

[Dependence of anticipatory grip force in catching task on previous trial].

Heavy or light object fell into the cup that sitting subject held between thumb and index fingers. A grip force applied to the cup was measured at the moment of impact. The grip force was stronger in trials following the trial with heavy object than after the trial with light object independently of object weight in the current trial. It means that the anticipatory grip force was planned accounting the result of previous trial.

Corticospinal influences on the distal muscles of the hand in conditions of inertial loading.

Electromyographic activity and synchronous discharges in the muscles of the wrist induced by transcranial magnetic stimulation of the motor cortex as the thumb and index finger were used to hold a handle bearing a weight were studied during performance of a number of motor tasks. When the subject increased grip force, for example, in response to increases in the weight of the attached load or by voluntarily squeezing the handle, the evoked response increased proportionally to muscle activity. If the subject moved the hand holding the handle up and down with an amplitude of 10 cm and a frequency of 0.5-1 Hz, grip force changed in accordance with the predicted inertial loading. The muscle response in the adductor pollicis muscle increased to a greater extent than the activity in the muscle. The response to sudden inertial loading consisted of a reflex increase in grip force, the muscle response increasing to a lesser extent than activity in the muscle. This suggests that larger increases in evoked muscle responses on up and down movement of the hand with a load are associated with anticipatory changes in grip force. These results are assessed from the point of view of the involvement of the motor cortex in generating anticipatory changes in muscle activity in the distal muscles.

[Influence of preliminary information about mass on anticipatory muscle activity during catching of falling object].

Heavy or light object fell into the cup held between thumb and index fingers of sitting subject. The anticipatory muscle activity and the grip force applied to cup depended on the mass of object while the temporal parameters (time of beginning of muscle activity, duration of the activity, the time of grip force maximum) were constant. The preliminary verbal information about mass of the falling object was enough for predictive force programming. Without such information, i.e. during fall the object of unknown mass the anticipatory activity was planned in expectation of heavy weight.

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.

[Corticospinal influence on the hand distal muscles under weight load].

Baseline and synchronous evoked EMG activity were studied in wrist muscles of human subjects under conditions of transcranial magnetic stimulation of the motor cortex during the performance of different motor tasks. During recording, a subject had to hold a loaded handle with the thumb and index finger. When a subject increased the grip force (intentionally or under conditions of increasing handle load), the synchronous EMG response increased proportionally to the muscle activity. If the subject moved the hand with the handle up and down with the amplitude of 10 cm and frequency of 0.5-1.0 Hz, the grip force changed in accordance with the expected inertia load. The EMG response in the abductor pollicis brevis increased to a larger degree than the baseline activity. In case of a grip force increase in response to a sudden unexpected rise of the inertia load, the evoked EMG response increased to a lesser degree than the muscle baseline activity. The results suggest that the increase in the evoked response during voluntary periodical movement of the handle is related to the anticipatory grip force adjustment. The involvement of the motor cortex in the anticipatory activity of distal muscles is discussed.