Human postural response to lower leg muscle vibration of different duration

Body lean response to bilateral vibrations of soleus muscles were investigated in order to understand the influence of proprioceptive input from lower leg in human stance control. Proprioceptive stimulation was applied to 17 healthy subjects by two vibrators placed on the soleus muscles. Frequency and amplitude of vibration were 60 Hz and 1 mm, respectively. Vibration was applied after a 30 s of baseline. The vibration duration of 10, 20, 30 s respectively was used with following 30 s rest. Subjects stood on the force platform with eyes closed. Postural responses were characterized by center of pressure (CoP) displacements in the anterior-posterior (AP) direction. The CoP-AP shifts as well as their amplitudes and velocities were analyzed before, during and after vibration. Vibration of soleus muscles gradually increased backward body tilts. There was a clear dependence of the magnitude of final CoP shift on the duration of vibration. The amplitude and velocity of body sway increased during vibration and amplitude was significantly modulated by duration of vibration as well. Comparison of amplitude and velocity of body sway before and after vibration showed significant post-effects. Presented findings showed that somatosensory stimulation has a long-term, direction-specific influence on the control of postural orientation during stance. Further, the proprioceptive input altered by soleus muscles vibration showed significant changes in postural equilibrium during period of vibration with interesting post-effects also.     

Somatosensory influence on postural response to galvanic vestibular stimulation

We investigated how postural responses to galvanic vestibular stimulation were affected by standing on a translating support surface and by somatosensory loss due to diabetic neuropathy. We tested the hypothesis that an unstable surface and somatosensory loss can result in an increase of vestibulospinal sensitivity. Bipolar galvanic vestibular stimulation was applied to subjects who were standing on a force platform, either on a hard, stationary surface or during a backward platform translation (9 cm, 4.2 cm/s). The intensity of the galvanic stimulus was varied from 0.25 to 1 mA. The amplitude of the peak body CoP displacement in response to the galvanic stimulus was plotted as a function of stimulus intensity for each individual. A larger increase in CoP displacement to a given increase in galvanic current was interpreted as an increase of vestibulospinal sensitivity. Subjects with somatosensory loss in the feet due to diabetes showed higher vestibulospinal sensitivity than healthy subjects when tested on a stationary support surface. Control subjects and patients with somatosensory loss standing on translating surface also showed increased galvanic response gains compared to stance on a stationary surface. The severity of the somatosensory loss in the feet correlated with the increased postural sensitivity to galvanic vestibular stimulation. These results showed that postural responses to galvanic vestibular stimulus were modified by somatosensory information from the surface. Somatosensory loss due to diabetic neuropathy and alteration of somatosensory input during stance on translating support surface resulted in increased vestibulospinal sensitivity.     

Human postural responses to different frequency vibrations of lower leg muscles

We analyzed human postural responses to muscle vibration applied at four different frequencies to lower leg muscles, the lateral gastrocnemius (GA) or tibialis anterior (TA) muscles. The muscle vibrations induced changes in postural orientation characterized by the center of pressure (CoP) on the force platform surface on which the subjects were standing. Unilateral vibratory stimulation of TA induced body leaning forward and in the direction of the stimulated leg. Unilateral vibration of GA muscles induced body tilting backwards and in the opposite direction of the stimulated leg. The time course of postural responses was similar and started within 1 s after the onset of vibration by a gradual body tilt. When a new slope of the body position was reached, oscillations of body alignment occurred. When the vibrations were discontinued, this was followed by rapid recovery of the initial body position. The relationship between the magnitude of the postural response and frequency of vibration differed between TA and GA. While the magnitude of postural responses to TA vibration increased approximately linearly in the 60-100 Hz range of vibration frequency, the magnitude of response to GA vibration increased linearly only at lower frequencies of 40-60 Hz. The direction of body tilt induced by muscle vibration did not depend on the vibration frequency.     

Age-related changes of human balance during quiet stance

Certain aspects of balance control change with age, resulting in a slight postural instability. We examined healthy subjects between 20-82 years of age during the quiet stance under static conditions: at stance on a firm surface and/or on a compliant surface with eyes either open or closed. Body sway was evaluated from centre of foot pressure (CoP) positions during a 50 sec interval. The seven CoP parameters were evaluated to assess quiet stance and were analyzed in three age groups: juniors, middle-aged and seniors. The regression analysis showed evident increase of body sway over 60 years of age. We found that CoP parameters were significantly different when comparing juniors and seniors in all static conditions. The most sensitive view on postural steadiness during quiet stance was provided by CoP amplitude and velocity in AP direction and root mean square (RMS) of statokinesigram. New physiological ranges of RMS parameter in each condition for each age group of healthy subjects were determined. Our results showed that CoP data from force platform in quiet stance may indicate small balance impairment due to age. The determined physiological ranges of RMS will be useful for better distinguishing between small postural instability due to aging in contrast to pathological processes in the human postural control.     

Postural muscle activity patterns during standing at rest and on an oscillating floor.

Postural muscle activity pattern was examined in the eyes-closed state after adequate adaptation to floor anteroposterior oscillation. Twenty-three subjects were grouped almost evenly according to dominance of anterior or posterior postural muscles in the trunk and thigh during quiet stance. In the posterior-dominant group, this dominance was maintained at every frequency in most subjects. In the anterior-dominant group, this dominance was maintained in most subjects at 0.1 and 0.5 Hz but changed to posterior dominance at 1.0 and 1.5 Hz in about half the subjects. Periodicity of muscle activity was evaluated by EMG amplitude spectrum at the floor oscillation frequency. Periodicity of posterior-dominant muscles in the trunk and thigh increased with increasing oscillatory frequency. In the trunk, the periodicity did not differ significantly between posterior-dominant and anterior-dominant groups. However, in the thigh, periodicity was significantly lower in the anterior-dominant muscles. This was considered to be caused by nonperiodic alternating action of the anterior and posterior muscles. In the lower leg, posterior dominance was observed in quiet stance and at all oscillation frequencies. Periodicity of soleus and gastrocnemius increased at higher frequencies and was higher in gastrocnemius than in soleus. The periodicity difference between both muscles decreased with increasing oscillation frequency.     

Vestibular and somatosensory interaction during recovery of balance instability after spaceflight

The aim of our contribution was to characterize vestibular and somatosensory influence on human balance recovery during readaptation to the earth conditions after spaceflight. We tested how post-spaceflight postural reactions to galvanic stimulus (related to vestibular input) and to vibration of the lower leg muscles (somatosensory input) were changed.     

Dynamic Reweighting of Three Modalities for Sensor Fusion

We simultaneously perturbed visual, vestibular and proprioceptive modalities to understand how sensory feedback is re-weighted so that overall feedback remains suited to stabilizing upright stance. Ten healthy young subjects received an 80 Hz vibratory stimulus to their bilateral Achilles tendons (stimulus turns on-off at 0.28 Hz), a ±1 mA binaural monopolar galvanic vestibular stimulus at 0.36 Hz, and a visual stimulus at 0.2 Hz during standing. The visual stimulus was presented at different amplitudes (0.2, 0.8 deg rotation about ankle axis) to measure: the change in gain (weighting) to vision, an intramodal effect; and a change in gain to vibration and galvanic vestibular stimulation, both intermodal effects. The results showed a clear intramodal visual effect, indicating a de-emphasis on vision when the amplitude of visual stimulus increased. At the same time, an intermodal visual-proprioceptive reweighting effect was observed with the addition of vibration, which is thought to change proprioceptive inputs at the ankles, forcing the nervous system to rely more on vision and vestibular modalities. Similar intermodal effects for visual-vestibular reweighting were observed, suggesting that vestibular information is not a “fixed” reference, but is dynamically adjusted in the sensor fusion process. This is the first time, to our knowledge, that the interplay between the three primary modalities for postural control has been clearly delineated, illustrating a central process that fuses these modalities for accurate estimates of self-motion.     

Evaluation of time-to-contact measures for assessing postural stability

Postural stability has traditionally been examined through spatial measures of the center of mass (CoM) or center of pressure (CoP), where larger amounts of CoM or CoP movements are considered signs of postural instability. However, for stabilization, the postural control system may utilize additional information about the CoM or CoP such as velocity, acceleration, and the temporal margin to a stability boundary. Postural time-to-contact (TtC) is a variable that can take into account this additional information about the CoM or CoP. Postural TtC is the time it would take the CoM or CoP, given its instantaneous trajectory, to contact a stability boundary. This is essentially the time the system has to reverse any perturbation before stance is threatened. Although this measure shows promise in assessing postural stability, the TtC values derived between studies are highly ambiguous due to major differences in how they are calculated. In this study, various methodologies used to assess postural TtC were compared during quiet stance and induced-sway conditions. The effects of the different methodologies on TtC values will be assessed, and issues regarding the interpretation of TtC data will also be discussed.     

Specific and Nonspecific Visual Influences on the Stability of the Vertical Posture in Humans

We studied physiological mechanisms of vision-related stabilization of the vertical posture in humans using a stabilographic technique; spontaneous deviations of the projection of the center of gravity during quiet stance and magnitudes of the postural response to vibratory stimulation of proprioceptors of the lower leg muscles under varied conditions of visual control were measured. The stability of quiet stance, as estimated according to the root mean square value of the sagittal component of the stabilogram, was the best with eyes open. Vibration-induced postural responses were the smallest also under these conditions. Spontaneous postural sway and the amplitude of response to vibratory stimulation increased when only a central sector of visual field (20 ang. deg) was preserved and, especially, under conditions of closed eyes and horizontal inversion of visual perception using prismatic spectacles. Parallel changes in the quantitative stabilographic indices and amplitude of vibration-induced postural responses show that the intensity of the latter is probably determined by the background stiffness of the musculoskeletal system. We tried to estimate separately the contributions of the stiffness factor, on the one hand, and specific visual influences, on the other hand, by testing the parameters of quiet stance and postural responses under conditions of standing while lightly touching a support with the index finger. We found that the influence of the conditions of visual control on the stability of quiet stance while touching the support was eliminated. At the same time, the magnitude of postural responses to vibratory stimulation decreased but, nonetheless, changed with visual conditions in the same manner as when standing without additional support. We conclude that vision performs a dual function in the control of the vertical posture; it forms the basis for the spatial reference system and serves the source of information on the movements of one's body.     

Effects of Near-Threshold Stimulation of the Vestibular Apparatus on Postural Responses Evoked by Displacements of the Visualized Surrounding

In healthy subjects in the relaxed upward stance and perceiving a virtual visual environment (VVE), we recorded postural reactions to isolated visual and vestibular stimulations or their combinations. Lateral displacements of the visualized virtual scene were used as visual stimuli. The vestibular apparatus was stimulated by application of near-threshold galvanic current pulses to the proc. mastoidei of the temporal bones. Isolated VVE shifts evoked mild, nonetheless clear, body tilts readily distinguished in separate trials; at the same time, postural effects of isolated vestibular stimulation could be detected only after averaging of several trials synchronized with respect to the beginning of stimulation. Under conditions of simultaneous combined presentation of visual and vestibular stimuli, the direction of the resulting postural responses always corresponded to the direction of responses induced by VVE shifts. The contribution of an afferent volley from the vestibular organ depended on the coincidence/mismatch of the direction of motor response evoked by such a volley with the direction of response to visual stimulation. When both types of stimulations evoked unidirectional body tilts, postural responses were facilitated, and the resulting effect was greater than that of simple summation of the reactions to isolated actions of the above stimuli. In the case where isolated galvanic stimulation evoked a response opposite with respect to that induced by visual stimulation, the combined action of these stimuli of different modalities evoked postural responses identical in their magnitude, direction, and shape to those evoked by isolated visual stimulation. The above findings allow us to conclude that the effects of visual afferent input on the vertical posture under conditions of our experiments clearly dominate. In general, these results confirm the statement that neuronal structures involved in integrative processing of different afferent volleys preferably select certain type of afferentation carrying more significant or more detailed information on displacements (including oscillations) of the body in space.     

Velocity of head movements and sensory-motor adaptation during and after short spaceflight

To investigate to time course of sensory-motor adaptation to microgravity, we tested spatially-directed voluntary head movements before, during and after short spaceflight. We also tested the re-adaptation of postural responses to sensory stimulation after space flight. The cosmonaut performed in microgravity six cycles of voluntary head rotation in pitch, roll and yaw directions. During the first days of weightlessness the angular velocity of head movements increased. Over the next days of microgravity the velocity of head movements gradually decreased. On landing day a significant decrease of head rotation velocity was observed compared to the head movement velocity before spaceflight. Re-adaptation to Earth condition measured by body sway on soft support showed similar time course, but re-adaptation measured by postural responses to vestibular galvanic stimulation was prolonged. These results showed that the angular velocity of aimed head movements of cosmonauts is a good indicator of sensory-motor adaptation in altered gravity conditions.     

Enhancing human balance control with galvanic vestibular stimulation

 With galvanic vestibular stimulation (GVS), electrical current is delivered transcutaneously to the vestibular afferents through electrodes placed over the mastoid bones. This serves to modulate the continuous firing levels of the vestibular afferents, and causes a standing subject to lean in different directions depending on the polarity of the current. Our objective in this study was to test the hypothesis that the sway response elicited by GVS can be used to reduce the postural sway resulting from a mechanical perturbation. Nine subjects were tested for their postural responses to both galvanic stimuli and support-surface translations. Transfer-function models were fit to these responses and used to calculate a galvanic stimulus that would act to counteract sway induced by a support-surface translation. The subjects' responses to support-surface translations, without and with the stabilizing galvanic stimulus, were then measured. With the stabilizing galvanic stimulus, all subjects showed significant reductions in both sway amplitude and sway latency. Thus, with GVS, subjects maintained a more erect stance and followed the support-surface displacement more closely. These findings suggest that GVS could possibly form the basis for a vestibular prosthesis by providing a means through which an individual's posture can be systematically controlled. Received: 11 May 2000 / Accepted in revised form: 20 November 2000     

Modulation of soleus H reflex due to stance pattern and haptic stabilization of posture

The quiet stance is a complicated motor act requiring sophisticated sensorimotor integration to balance an artificial inverted pendulum with the ankle musculature. The objective of this study was to characterize the effects of stance pattern (bilateral stance vs. unilateral stance) and directional influence of light finger touch (medial–lateral vs. anterior–posterior) in unilateral stance upon responsiveness of the soleus H reflex. Sixteen healthy volunteers (mean age, 24.25 ± 1.77 years) participated in four postural tasks with the eyes open, including the bilateral stance (BS), the unilateral stance without finger touch (USNT), and with finger touch in the medial–lateral direction (USML) and anterior–posterior direction (USAP). Meanwhile, the soleus H reflex, the pre-stimulus background activity of ankle antagonist pairs, and center of pressure (CoP) sway were measured. In reference to the BS, the USNT resulted in a significant stance effect on suppression of the soleus H reflex (H/M_max) associated with enhancement of CoP sway. Among the conditions of unilateral stance, there was a marked directional effect of finger touch on modulation of the H/M_max. A greater disinhibition of the H/M_max in consequence to light touch in the ML direction than in the AP direction was noted (H/M_max: USML > USAP > USNT). This directional modulation of the soleus H reflex concurred with haptic stabilization of posture in unilateral stance, showing a more pronounced reduction in CoP sway in the USML condition than in the USAP condition. However, alteration in postural sway and modulation of the soleus H reflex were not mutually correlated when stance pattern or touch vector varied. In conclusion, gating of the soleus H reflex indicated adaptation of an ankle strategy to stance pattern and haptic stabilization of posture. Relative to bilateral stance, postural maintenance in unilateral stance relied less on reflexive correction of the soleus. When finger touch was provided in line with prevailing postural threat in the lateral direction, postural stability in unilateral stance was better secured than finger touch in anterior–posterior direction, resulting in more pronounced disinhibition of the monosynaptic reflex pathway.     

Influence of artificial vestibular feedback on posture instability caused by asymmetric proprioceptive stimulation

Subjects kept a vertical posture, standing on a rigid support. Stability of a posture was estimated by the sizes of standard deviations (sigma) from average amplitudes of the subject's head fluctuation in respect to zero coordinates. To create a feedback on the vestibular input, transmastoidal bipolar galvanic stimulation was used. Changes of current in contour of feedback looked as linear function considering amplitude and velocity of the subject's head displacements. Varying the factors of feedback function, it was possible to reduce sigma for lateral sways increased (in comparison with their values at the quiet stance in the darkness) as a result of unilateral vibrating stimulation of m. gluteus medialis. The results specify inequality of "velocity" and "position" information for maintenance of vertical posture in different subjects. The results specify also the ability of the central nervous system (CNS) to revalue weights of various kinds of information entering via the same channel. The data confirm the hypothesis according to which galvanic vestibular input is capable to deliver in CNS and adequate information on the current orientation of the body. This information can be used for stabilization of a posture.     

Gait Initiation in Children with Rett Syndrome

Rett syndrome is an X-linked neurodevelopmental condition mainly characterized by loss of spoken language and a regression of purposeful hand use, with the development of distinctive hand stereotypies, and gait abnormalities. Gait initiation is the transition from quiet stance to steady-state condition of walking. The associated motor program seems to be centrally mediated and includes preparatory adjustments prior to any apparent voluntary movement of the lower limbs. Anticipatory postural adjustments contribute to postural stability and to create the propulsive forces necessary to reach steady-state gait at a predefined velocity and may be indicative of the effectiveness of the feedforward control of gait. In this study, we examined anticipatory postural adjustments associated with gait initiation in eleven girls with Rett syndrome and ten healthy subjects. Muscle activity (tibialis anterior and soleus muscles), ground reaction forces and body kinematic were recorded. Children with Rett syndrome showed a distinctive impairment in temporal organization of all phases of the anticipatory postural adjustments. The lack of appropriate temporal scaling resulted in a diminished impulse to move forward, documented by an impairment in several parameters describing the efficiency of gait start: length and velocity of the first step, magnitude and orientation of centre of pressure-centre of mass vector at the instant of (swing-)toe off. These findings were related to an abnormal muscular activation pattern mainly characterized by a disruption of the synergistic activity of antagonistic pairs of postural muscles. This study showed that girls with Rett syndrome lack accurate tuning of feedforward control of gait.     

Age-related changes in postural responses to backward platform translation

The aim of the study was to investigate age-related changes in postural responses to platform translation with 3 various velocities. We focused on the influence of linear velocity using the smoothed profile of platform acceleration (till 100 cm.s(-2)). Eleven healthy young (20-31 years) and eleven healthy elderly (65-76 years) subjects were examined. The subjects stood on the force platform with their eyes closed. Each trial (lasting for 8 sec) with different velocity (10, 15, 20 cm.s(-1)) of 20 cm backward platform translation was repeated 4 times. We have recorded displacements of the centre of pressure (CoP) and the EMG activity of gastrocnemius muscle (GS) and tibialis anterior muscle (TA). The results showed increased maximal values of CoP responses to the platform translation. There was also observed a scaling delay of CoP responses to platform translation with different velocities in elderly. The EMG activity of GS muscle during backward platform translation was of about similar shape in both groups during the slowest platform velocity, but it increased depending on rising velocity. EMG activity of TA was not related to the platform velocity. Early parts of postural responses showed significant co-activation of TA and GS muscles of elderly. It is likely that elderly increased body stiffening in order to help their further balance control.     

Chaos in Balance: Non-Linear Measures of Postural Control Predict Individual Variations in Visual Illusions of Motion

Visually-induced illusions of self-motion (vection) can be compelling for some people, but they are subject to large individual variations in strength. Do these variations depend, at least in part, on the extent to which people rely on vision to maintain their postural stability? We investigated by comparing physical posture measures to subjective vection ratings. Using a Bertec balance plate in a brightly-lit room, we measured 13 participants'' excursions of the centre of foot pressure (CoP) over a 60-second period with eyes open and with eyes closed during quiet stance. Subsequently, we collected vection strength ratings for large optic flow displays while seated, using both verbal ratings and online throttle measures. We also collected measures of postural sway (changes in anterior-posterior CoP) in response to the same visual motion stimuli while standing on the plate. The magnitude of standing sway in response to expanding optic flow (in comparison to blank fixation periods) was predictive of both verbal and throttle measures for seated vection. In addition, the ratio between eyes-open and eyes-closed CoP excursions during quiet stance (using the area of postural sway) significantly predicted seated vection for both measures. Interestingly, these relationships were weaker for contracting optic flow displays, though these produced both stronger vection and more sway. Next we used a non-linear analysis (recurrence quantification analysis, RQA) of the fluctuations in anterior-posterior position during quiet stance (both with eyes closed and eyes open); this was a much stronger predictor of seated vection for both expanding and contracting stimuli. Given the complex multisensory integration involved in postural control, our study adds to the growing evidence that non-linear measures drawn from complexity theory may provide a more informative measure of postural sway than the conventional linear measures.     

Is the attenuation effect on the ankle muscles activity from the EMG biofeedback generalized to – or compensated by – other lower limb muscles during standing?

Biofeedback based on electromyograms (EMGs) has been recently proposed to reduce exaggerated postural activity. Whether the effect of EMG biofeedback on the targeted muscles generalizes to – or is compensated by – other muscles is still an open question we address here. Fourteen young individuals were tested in three 60 s standing trials, without and with EMG-audio feedback: (i) collectively from soleus and medial gastrocnemius and (ii) from medial gastrocnemii. The Root Mean Square (RMS) of bipolar EMGs sampled from postural muscles bilaterally was computed to assess the degree of activity and postural sway was assessed from the center of pressure (CoP). In relation to standing at naturally, EMG-audio feedback from soleus and medial gastrocnemii decreased plantar flexors’ activity (∼10 %) but at the cost of increased amplitude of tibialis anterior (∼5%) and vasti muscles (∼20 %) accompanied by a posterior shift of the mean CoP position. However, EMG-audio feedback from medial gastrocnemii reduced only plantar flexors’ activity (∼5%) when compared to standing at naturally. Current results suggest the EMG biofeedback has the potential to reduce calf muscles’ activity without loading other postural muscles especially when using medial gastrocnemii as feedback source, with implications on postural training aimed at assisting individuals in activating more efficiently postural muscles during standing.     

Vestibular-dependent spinal reflexes evoked by brief lateral accelerations of the heads of standing subjects

An impulsive acceleration stimulus, previously shown to activate vestibular afferents, was applied to the mastoid. Evoked EMG responses from the soleus muscles in healthy subjects (n = 10) and patients with bilateral vestibular dysfunction (n = 3) were recorded and compared with the effects of galvanic stimulation (GVS). Subjects were stimulated while having their eyes closed, head rotated, and while tonically activating their soleus muscles. Rectified EMG responses were recorded from the leg contralateral to the direction of head rotation. Responses were characterized by triphasic potentials that consisted of short-latency (SL), medium-latency (ML), and long-latency (LL) components beginning at (mean ± SD) 54.2 ± 4.8, 88.4 ± 4.7, and 121 ± 7.1 ms, respectively. Mean amplitudes for the optimum stimulus rise times were 9.05 ± 3.44% for the SL interval, 16.70 ± 4.41% for the ML interval, and 9.75 ± 4.89% for the LL interval compared with prestimulus values. Stimulus rise times of 14 and 20 ms evoked the largest ML amplitudes. GVS evoked biphasic responses (SL and ML) with similar latencies. Like GVS, the polarity of the initial interval was determined by the polarity of the stimulus and the evoked EMG response was attenuated when subjects were seated. There was no significant EMG response evoked when subjects were stimulated using 500-Hz vibration or in patients with bilateral vestibular dysfunction. Our study demonstrates that a brief lateral acceleration, likely to activate the utricle, can evoke spinal responses with properties similar to those previously shown for vestibular activation by GVS. The triphasic nature of the responses may allow the nervous system to respond differently to short compared with long-duration linear accelerations, consistent with their differing significance.     

The influence of vibration on spinal alpha-motoneurons excitability in static conditions and during evoked stepping in human

In healthy human the excitability of spinal alpha-motoneurons under application of vibrostimulation (20-60 Hz) to different leg muscles was investigated both in stationary condition and during stepping movements caused by vibration in the condition of suspended leg. In 15 subjects the amplitude of H-reflex were compared under vibration of rectus femoris (RF) and biceps femoris (BF) muscles of left leg as well during vibration of rectus femoris of contralateral, motionless leg in three spatial positions: upright, supine and on right side of body with suspended left leg. In dynamic conditions the amount of H-reflex was compared during evoked and voluntary stepping at 8 intervals of step cycle. In all body positions the vibration of each ipsilateral leg muscles caused significant suppression of H-reflex, this suppression was more prominent in the air-stepping conditions. The vibration of contralateral leg RF muscle had a weak influence on the amplitude of H-reflex. In 7 subjects the muscle vibration of ipsilateral and contralateral legs generated stepping movements. During evoked "air-stepping" H-reflex had different amplitudes in different phases of step cycle. At the same time the differences between responses under voluntary and non-voluntary stepping were revealed only in stance phase. Thus, different degree of H-reflex suppression by vibration under different body position in space depends on, it seems to be, from summary afferent inflows to spinal cord interneurons, which participate in regulation of posture and locomotion. Seemingly, the increasing of spinal cord neurons excitability occurs under involuntary air-stepping in swing phase, which is necessary for activation of locomotor automatism under unloading leg conditions.