Effect of Mechanical Stimulation Applied to the Lower-Limb Musculature on Stability and Function of Stair Climbing

Mechanical muscle-tendon vibration affects musculature and the nervous system. As the vibrations used in previous studies were varied, consistently determining the effect of mechanical vibration was challenging. Additionally, only a few studies have applied vibrations to dynamic motion. This study investigated whether the vibration based on the sensorimotor response could affect the stability and function of stair climbing. Electroencephalogram (EEG) signals were recorded from the sensorimotor area, and mu rhythms, dependent on the vibration frequencies, were analyzed. Based on the analysis, the vibratory stimulus conditions were set and applied to the Achilles tendon of the lower limb during stair climbing. Simultaneously, electromyogram (EMG) signals from the gastrocnemius lateralis (GL), gastrocnemius medialis (GM), soleus (SOL), and tibialis anterior (TA) were recorded. Activations and co-activations of the shank muscles were analyzed according to the phases of stair climbing. When vibration was applied, the TA activation decreased in the pull-up (PU) phase, and calf muscle activations increased during the forward continuous (FCN) phase. These changes and their degrees differ significantly between stimulus conditions (p < 0.05). Co-activation changes, which differed significantly with conditions (p < 0.05), appeared mostly in the PU. These results imply that the vibration affects stability and function of stair climbing, suggesting that the vibration characteristics should be considered when they are applied to dynamic movement.

On the reflex mechanisms of cervical transcutaneous spinal cord stimulation in human subjects

Transcutaneous and epidural electrical spinal cord stimulation techniques are becoming more valuable as electrophysiological and clinical tools. Recently, remarkable recovery of the upper limb sensorimotor function during cervical spinal stimulation was demonstrated. In the present study, we sought to elucidate the neural mechanisms underlying the effects of transcutaneous spinal cord stimulation (tSCS) of the cervical spine. We hypothesized that cervical tSCS can be used to selectively activate the sensory route entering the spinal cord and transsynaptically converge on upper limb motor pools. To test this hypothesis, we applied cervical tSCS using paired stimuli (homosynaptic depression) and during passive muscle stretching of the wrist flexor (presynaptic inhibition via Ia afferents), voluntary hand muscle contraction (descending facilitation of motoneuron pool), and muscle-tendon vibration of the wrist (presynaptic inhibition via afferent occlusion). Our results demonstrate significant inhibition of the second evoked response during paired stimulus delivery, inhibition of responses during passive muscle stretching and muscle-tendon vibration, and facilitation during voluntary muscle contraction, which share similarities with responses evoked during lumbosacral tSCS. These results indicate that the route of the stimulation current transmission passes via afferents in the dorsal roots through the spinal cord to activate the motor pools and potentially interneuronal networks projecting to upper limb muscles. Using a novel stimulation paradigm, our study is the first to present evidence of the sensory neuronal pathway of the cervical tSCS propagation. Overall, our work demonstrates the utility and sensitivity of cervical tSCS to engage the sensory pathway projecting to the upper limbs. NEW & NOTEWORTHY Despite therapeutic effects that have been demonstrated previously using noninvasive cervical spinal stimulation, it has been unclear whether, and to what degree, the stimulation can activate the sensory afferent system. Our study presents evidence that cervical transcutaneous spinal cord stimulation can engage the sensory pathways and transsynaptically converge on motor pools projecting to upper limb muscles, demonstrating the utility and sensitivity of cervical spinal stimulation for electrophysiological assessments and neurorehabilitation.

Inducing Any Virtual Two-Dimensional Movement in Humans by Applying Muscle Tendon Vibration

In humans, tendon vibration evokes illusory sensation of movement. We developed a model mimicking the muscle afferent patterns corresponding to any two-dimensional movement and checked its validity by inducing writing illusory movements through specific sets of muscle vibrators. Three kinds of illusory movements were compared. The first was induced by vibration patterns copying the responses of muscle spindle afferents previously recorded by microneurography during imposed ankle movements. The two others were generated by the model. Sixteen different vibratory patterns were applied to 20 motionless volunteers in the absence of vision. After each vibration sequence, the participants were asked to name the corresponding graphic symbol and then to reproduce the illusory movement perceived. Results showed that the afferent patterns generated by the model were very similar to those recorded microneurographically during actual ankle movements ( r = 0.82). The model was also very efficient for generating afferent response patterns at the wrist level, if the preferred sensory directions of the wrist muscle groups were first specified. Using recorded and modeled proprioceptive patterns to pilot sets of vibrators placed at the ankle or wrist levels evoked similar illusory movements, which were correctly identified by the participants in three quarters of the trials. Our proprioceptive model, based on neurosensory data recorded in behaving humans, should then be a useful tool in fields of research such as sensorimotor learning, rehabilitation, and virtual reality.