Increasing muscle activity correlations during spontaneous movements in the first six months of life.

Spontaneous muscle activity in the first months of life is an important prerequisite to developing voluntary motor skills and to adapting sensorimotor circuits and muscle tone to body and environmental changes. Even though high variability is a characteristic of early development, several studies have reported significant correlations of limb movements. These assessments were typically made based on kinematics, while the analysis of lower and upper limb muscle activity may provide additional information about maturation of the neuromuscular control. To this end, we examined the electromyographic activity of 12 muscles of the upper and lower limbs in full-term healthy infants (n = 40) aged from 1 week to six months. An increase of ipsilateral and contralateral limb muscle activity correlations with age was found in both flexors and extensors and may reflect a progressive emergence of elements of coordinative neuromuscular behaviour. Correlations between arm and leg muscle responses also increased during passive leg movements. Overall, the findings are consistent with maturation of physiologically relevant neuromuscular network connections during the course of transition from spontaneous-like to voluntary goal-directed movements during early development.

Synergistic influences of sensory and central stimuli on non-voluntary rhythmic arm movements.

In recent years, neuromodulation of the cervical spinal circuitry has become an area of interest for investigating rhythmogenesis of the human spinal cord and interaction between cervical and lumbosacral circuitries, given the involvement of rhythmic arm muscle activity in many locomotor tasks. We have previously shown that arm muscle vibrostimulation can elicit non-voluntary upper limb oscillations in unloading body conditions. Here we investigated the excitability of the cervical spinal circuitry by applying different peripheral and central stimuli in healthy humans. The rationale for applying combined stimuli is that the efficiency of only one stimulus is generally limited. We found that low-intensity electrical stimulation of the superficial arm median nerve can evoke rhythmic arm movements. Furthermore, the movements were enhanced by additional peripheral stimuli (e.g., arm muscle vibration, head turns or passive rhythmic leg movements). Finally, low-frequency transcranial magnetic stimulation of the motor cortex significantly facilitated rhythmogenesis. The findings are discussed in the general framework of a brain-spinal interface for developing adaptive central pattern generator-modulating therapies.

Effects of spinal cord stimulation on motor functions in children with cerebral palsy.

Is it possible to regulate the functional properties of abnormally developed spinal neuronal locomotor networks using transcutaneous spinal cord stimulation? This question has been studied in twenty-eight participants (∼9 yrs) with spastic cerebral palsy, and mainly Gross Motor Function Classification System for Cerebral Palsy level III. The participants were randomly assigned to two groups. The experimental group received transcutaneous spinal cord stimulation at two spinal levels (over T11 and L1 spinous processes), combined with locomotor treadmill training, whereas the participants of the control group received locomotor treadmill training only. After spinal cord stimulation in the experimental group we found an incremental increase in knee torque whereas in the control group this effect was absent. The amplitude of hip motion increased in both groups. A decrease of co-activation of hip and muscles of the lower extremities was observed in the experimental group while in the control group co-activation decreased only in hip muscles. The results support the idea that locomotor function can be improved significantly with the combination of training and transcutaneous spinal cord stimulation than with training alone.

[The Activation of Interlimb Interactions Increase the Motor Output in Legs in Healthy Subjects under the Conditions of Arm and Leg Unloading].

We studied the effect of arm movements and movements of separate arm joints on the electrophysiological and kinematic characteristics of voluntary and vibration-triggered stepping-like leg movements under the conditions of horizontal support of upper and lower limbs. The horizontal support of arms provided a significantly increase in the rate of activation of locomotor automatism by non-invasive impact on tonic sensory inputs. The addition of active arm movements during involuntary rhytmic stepping-like leg movements led to an increase in EMG activity of hip muscles and was accompanied by an increase in the amplitude of hip and shin movements. Passive arm movements had the same effect on induced leg movements. The movement of the shoulder joints led to an increase in the activity of hip muscles and an increase in the amplitude of movements of the knee and hip joints. At the same time, the movement of forearms. and wrists had similar facilitating effect on electrophysiological and kinematic characteristics of rhytmic stepping-like movements, but influenced the distal segments of legs to a greater extent. Under the conditions of sub-threshold vibration of leg muscles, voluntary arm movements led to the activation of involuntary rhytmic stepping movements. During voluntary leg movements, the addition of arm movements had a significantly smaller impact on the parameters of rhytmic stepping than during involuntary leg movements. Thus, the simultaneous movements of upper and lower limbs are an effective method of activation of neural networks connecting the rhythm generators of arms and legs. Under the conditions of arm and leg unloading, the interactions between the cervical and lumbosacral segments of the spinal cord seem to play the major role in the impact of arm movements on the patterns of leg movements. The described methods of activation of interlimb interactions can be used in the rehabilitation of post-stroke patients and patients with spinal cord injuries, Parkinson's disease and other neurological diseases.

Human cervical spinal cord circuitry activated by tonic input can generate rhythmic arm movements.

The coordination between arms and legs during human locomotion shares many features with that in quadrupeds, yet there is limited evidence for the central pattern generator for the upper limbs in humans. Here we investigated whether different types of tonic stimulation, previously used for eliciting stepping-like leg movements, may evoke nonvoluntary rhythmic arm movements. Twenty healthy subjects participated in this study. The subject was lying on the side, the trunk was fixed, and all four limbs were suspended in a gravity neutral position, allowing unrestricted low-friction limb movements in the horizontal plane. The results showed that peripheral sensory stimulation (continuous muscle vibration) and central tonic activation (postcontraction state of neuronal networks following a long-lasting isometric voluntary effort, Kohnstamm phenomenon) could evoke nonvoluntary rhythmic arm movements in most subjects. In ∼40% of subjects, tonic stimulation elicited nonvoluntary rhythmic arm movements together with rhythmic movements of suspended legs. The fact that not all participants exhibited nonvoluntary limb oscillations may reflect interindividual differences in responsiveness of spinal pattern generation circuitry to its activation. The occurrence and the characteristics of induced movements highlight the rhythmogenesis capacity of cervical neuronal circuitries, complementing the growing body of work on the quadrupedal nature of human gait.

[Are Interlimb Interactions Disturbed in Patients with Parkinson's Disease: a Study in Unloading Condition?]

During natural human locomotion neural connections characteristic for the control of quadrupedal walking are activated. The degree of interaction between the neural networks generating rhythmic movements of the upper and lower extremities, depends on the tonic state of each of these networks, adjustable by motor command in the brain. The distortion of such command in patients with Parkinson's disease (PD) may lead to disturbance of interlimb interactions. In conditions of arms and legs unloading in 17 patients with PD and 16 healthy subjects investigated the effect of limb movement of one girdle on the parameters of motor activity of the limbs of the other girdle in their combined cyclic movements. In patients the weakening of influences of active or passive arm movements, and active movements of the distal parts of the arms on voluntary leg movements was shown, while in healthy subjects the effect of arm movements on the parameters of voluntary movements was significant. The effect of arm movements on the possibility of involuntary activation of air- stepping by vibratory stimulation of the leg muscles in patients was absent, while in healthy subjects the motor activity of the arms increased the opportunity of involuntary rhythm activation. The differences in the influence of leg movements to the rhythmic movement of the upper extremities in patients and healthy subjects were revealed. The medication exhibited interlimb interaction, but it was insufficient for normalization of the state of neural networks in patients. It is concluded that in patients with PD, neural networks generating stepping rhythm are in a state of heightened tonic activity. This does not allow not only activating involuntary rhythmic movements, but also showing facilitation effect with arms to legs.

Lack of non-voluntary stepping responses in Parkinson's disease.

The majority of research and therapeutic actions in Parkinson's disease (PD) focus on the encephalic areas, however, the potential involvement of the spinal cord in its genesis has received little attention. Here we examined spinal locomotor circuitry activation in patients with PD using various types of central and peripheral tonic stimulation and compared results to those of age-matched controls. Subjects lay on their sides with both legs suspended, allowing low-friction horizontal rotation of the limb joints. Air-stepping can be used as a unique and important model for investigating human rhythmogenesis since its manifestation is largely facilitated by the absence of external resistance. In contrast to the frequent occurrence of non-voluntary stepping responses in healthy subjects, both peripheral (muscle vibration) and central (Jendrassik maneuver, mental task, Kohnstamm phenomenon) tonic influences had little if any effect on rhythmic leg responses in PD. On the other hand, a remarkable feature of voluntary air-stepping movements in patients was a significantly higher frequency of leg oscillations than in age-matched controls. A lack of non-voluntary stepping responses was also observed after dopaminergic treatment despite the presence of prominent shortening reactions (SRs) to passive movements. We argue that the state and the rhythmogenesis capacity of the spinal circuitry are impaired in patients with PD. In particular, the results suggest impaired central pattern generator (CPG) access by sensory and central activations.