AGE-SPECIFIC PECULIARITIES OF THE IA AFFERENT INNERVATION OF HUMAN  M. SOLEUS

AGE-SPECIFIC PECULIARITIES OF THE IA AFFERENT INNERVATION OF HUMAN M. SOLEUS

A. Chelnokov

Velikiye Luki State Academy of Physical Education and Sports, Velikiye Luki, Russia

The aim was to study age-specific peculiarities of the afferent Ia innervation of human m. soleus.

The experiment was conducted on 24 males, 9 to 18 years old (9-12 year-old boys, 14-15 and 17-18 year-old teenagers). The method of recording of Ia afferent presynaptic inhibition (PI) m. soleus under homonymous vibrating stimulation at tendo calcaneus (N.P. Anissimova et al., 1987, Figure 1) was used to evaluate the afferent input m. soleus. The method is based on the evaluation of m. soleus H-reflex amplitude suppression and its recovery in the aftereffect period. The more the m. soleus H-reflex suppression, the higher the PI of the n. tibialis Ia afferent fibers. The persons being tested laid down on a special couch, in the back-lying position, with their legs outstretched and their feet drooping freely over the couch edge. A DC vibrator motor DMP-30-H1-01 with an eccentric was used. The vibrator was attached to the right shin above t. calcaneus with a special rubber band. Moderate vibration was applied, for selective activation of the Ia afferent fibers of m. soleus. The stimulation frequency was 65 Hz, the oscillation amplitude was 0.25 mm irrespective of the vibrator pressing force. The m. soleus H-reflex was recorded during a 60-second period before the vibration stimulation, during a 30-second period of vibration stimulation, and during a 60-second aftereffect period. The interval between the testing stimuli applied to n. tibialis was 10 seconds. The m. soleus biopotentials were taken using bipolar electrodes. Signals from the biopotential amplifier output were supplied through an analog-to-digital converter to one of the PC ports. For processing, the Myo software (ANO Vozvrascheniye, St. Petersburg, Russia, 2003) was used.

Figure 1. Vibration stimulation method of determining presynaptic inhibition of Ia fibers (N. P. Anissimova et al., 1987).

Different human ontogenesis periods are described with different intensities of the PI of m. soleus Ia afferent fibers. The research results have shown that, under homonymous vibration stimulation of t. calcaneus, the PI of m. soleus afferent fibers is considerably higher in 17-18 year-old teenagers than in 9-12 year-old boys and 14-15 year-old teenagers (Figure 2).

Analysis of Figure 2 shows that the same tendency in dynamics of m. soleus H-reflex amplitude suppression under vibration stimulation of t. calcaneus is observed in all age groups. The maximum PI values are observed at the 30^th second of vibration stimulation of t. calcaneus, while the minimum values, as compared to rest, are observed at the 1^st second of vibration. Thus, at the 30^th second of vibration stimulation, the m. soleus H-reflex decrease, as compared to rest, was 1.10 mV (89.02%) in 9-12 year-old children, 2.67 mV (78.96%) in 14-15 year-old teenagers and 0,75 mV (89,91%) in 17-18 year-old teenagers.

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Figure 2. Dynamics of m. soleus H-reflex amplitude during vibration stimulation of t. calcaneus (A) and aftereffect period (B) in 9-12 year-old, 14-15 year-old teenagers boys and 17-18 year-old teenagers.

The research results demonstrate different rates of recovery of PI of m. soleus primary afferent fibers after vibration stimulation of t. calcaneus in studying age groups. Thus, 17-18 year-old teenagers showed faster PI recovery after the vibration stimulation as compared to 9-12 year-old boys and 14-15 year-old teenagers. An evidence of this is faster increase of m. soleus H-reflex amplitude to the background values after vibration stimulation in 17-18 year-old teenagers than in other age groups. In 17-18 year-old teenagers, the recovery of PI of the m. soleus Ia afferent fibers was observed at the 30^th second (113,92%) after the vibration end, while with 14-15 year-old boys, this was observed at the 40^thsecond (96,70%), while with 9-12 year-old boys, this was observed at the 60^th second (91,50%) (Figure 2).

In our opinion, the change of intensity of the PI of the m. soleus primary afferent fibers, under homonymous vibration stimulation, is caused by the development of reflectory functions of the neuromotor apparatus, which is associated with the level of the morphofunctional maturation of its links and their anatomic changes in the process of the human age-related development.