The investigation of locomotor activity control system in humans under the air-stepping conditions during passive and voluntary leg movements |
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Authors: | I A Solopova V A Selionov A A Grishin |
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Institution: | 1.Kharkevich Institute for Problems of Data Transmission,Russian Academy of Sciences,Moscow,Russia |
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Abstract: | The degree of activation of the central stepping program during passive leg movement was studied in healthy subjects under
unloading conditions; the excitability of spinal motoneurons was studied during passive and voluntary stepping movements.
Passive stepping movements with characteristics maximally close to those during voluntary stepping were accomplished by the
experimenter. The bursts of muscular activity during voluntary and imposed stepping movements were compared. In addition,
the influence on the leg movement of artificially created loading onto the foot was studied. The excitability of spinal motoneurons
was estimated by the amplitude of modulation of the m. soleus H reflex. Changes in the H reflex (Hoffmann’s reflex) after fixation of the knee and hip joints were also studied. In most subjects, passive movements
were accompanied by bursts of electromyographic (EMG) activity in the hip muscles (sometimes in shank muscles); the timing
of the EMG burst during the step cycle coincided with the burst’s timing during voluntary stepping. In many cases, the bursts
in EMG activity exceeded the activity of homonymous muscles during voluntary stepping. Simulation of foot loading influenced
significantly the distal part of the moving extremity during both voluntary and passive movements, which was expressed in
the appearance of movements in the ankle joint and an increase in the phasic EMG activity of the shank muscles. The excitability
of motoneurons during passive movements was higher than during voluntary movements. Changes and modulation of the H reflex throughout the step cycle were similar without restriction of joint mobility and without hip joint mobility. Fixation
of the knee joint was of great importance. It is supposed that imposed movements activate the same mechanisms of rhythm generation
as supraspinal commands during voluntary movements. During passive movements, presynaptic inhibition depends mostly on the
afferent influences from the moving leg rather than on the central commands. Under the conditions of “air-stepping,” the afferent
influences from the foot pressure receptors are likely to interact actively with the central program of stepping and to determine
the final activity pattern irrespective of the movement type (voluntary or passive). |
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