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Summary
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.
Introduction
The postural system consists of several sensory systems (proprioceptive, visual and vestibular), the motor system, and a central integrating control system which involves complex interactions among multiple neural systems (Horak and MacPherson 1996). Tendon or muscle vibration generates proprioceptive information which is not congruent with actual body position. The proprioceptive input induced by muscle vibration has been shown to alter spatial body orientation within seconds (Lackner 1988, Roll et al. 1989). The primary muscle spindle endings that are highly sensitive to mechanical stimuli are responsible for the response to
muscle vibration (Roll et al. 1989). Vibration of lower leg muscles induces spatially oriented body tilts, with a tendency to fall during bilaterally applied vibration (Eklund 1969). Unilateral muscle vibration of the tibialis anterior muscle generates body tilt forwards and in the direction of the stimulated leg, while unilateral muscle vibration of the triceps surae (lateral and medical gastrocnemius and the soleus muscle) causes a backward tilt which is opposite the side of the stimulated leg
The functional character of two lower leg muscles - tibialis
anterior (TA) and gastrocnemius (GA) - differs. While the TA is a muscle which shows very little activity during normal standing, GA is a muscle with more or less constant activity during stance. We predicted that these differences would affect the response characteristics of each muscle to variations in frequency of vibration. A linear dependence between induced body tilt magnitude and vibration frequency was found for vibrations applied to both plantar sole zones (Kavounoudias et al. 1999). Likewise, the enhanced motor evoked potentials to vibration applied to extensor carpi radialis muscle varied with changes in vibration frequency (Siggelkow et al. 1999). We can expect a similar dependence of magnitude of postural response on vibration frequency of lower leg muscles. The aim of our research was to characterize the human postural sway response (forward-backward and left-right stabilograms) to vibrations of different
frequency (40-100 Hz) applied to lower leg muscles
(tibialis anterior, gastrocnemius). Methods Fifteen healthy volunteers (8 females and 7 males) aged 19-28 years (mean age 23.3 2.6) served as the subjects. Informed consent was obtained from all of them. To ensure that the subjects had normal balance control, we performed a balance screening test with them before the experiment. The data from the screening test were compared to the physiological range of
stabilometric parameters in the human upright posture
(Hlavačka et al. 1990). Vibrations were generated by a small DC motor with an eccentric weight of 5 g. The DC motor was enclosed in a plastic tube 9 cm high and 5 cm in diameter to allow safe application to the skin overlying the
muscles. The vibrator was fixed in place with an elastic
sling and was applied directly on the muscle (not tendon),
for a more adequate response of muscle receptors to the
vibration frequency (Roll et al. 1989). Vibration was applied to lower leg muscles (TAand GA) of standing subjects at four different frequencies
Discussion
The results of our experiment show that the magnitude of postural responses (body tilt, leaning) to leg muscle vibration can be modulated by the frequency of the vibratory stimulus (Figs 1 and 2). The frequency of vibration does not influence the direction of body slope (Fig. 3). The direction of the postural response was determined only by the muscle stimulated. Body tilting occurred in all our subjects with lower leg muscle vibrations which is consistent with prior reports
concerning the postural orientation during muscle or Achilles tendon vibration (Eklund 1972, Hayashi et al.1981, Roll and Vedel 1982). Recent work on human postural control
during combined leg muscle vibration and galvanic vestibular
stimulation (Hlavačka et al. 1995, 1996) suggests that the
maintenance of vertical orientation of the body in an upright standing position with eyes closed is under the continuous control of vestibular and leg proprioceptive inflow.
Each sensory system contributes to the establishment of a reference system for maintaining the vertical position of the body. The control of human upright posture requires both operative control assigned for compensation of body deviations from a reference position and a conservative control system which elaborates this reference using kinesthetic inputs with participation of body scheme mechanisms (Gurfinkel etal. 1995). It is reasonable to assume from our results that the magnitude of the body tilt determined by the frequency of muscle vibration is involved in operative postural control. The reference setting is always updated as sensory conditions change. In our experiment, a new body position was set each time when the muscle vibration (proprioceptive input) started or the vibration frequency was changed.
Body tilts evoked by vibration of each muscle maintained the same direction. This indicates some special and defined role of the muscle as an element of human body scheme. As a response to proprioceptive stimulation of a freely standing person on a stable support, each muscle involved in the human body scheme is always given the same special direction of evoked body tilt.
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