Yu. P. Ivanenko

Kinesthetic reference for human orthograde posture

Humans with occluded vision were subjected to superslow tilts of the supporting platform, producing the inclination of the subjects body in the sagittal plane, but subthreshold for the most vestibular and proprioceptive phasic reactions. Two types of perturbation were used: sinusoidal tilts (frequency 0.007 Hz, amplitude 1.5 degrees) and ramps (amplitude 1.0 and 0.25 degrees, angular velocity 0.04 degrees/s). During slow sinusoidal tilts of the platform, the ankle angle and body position undergo periodical changes, but these changes have significant phase lead relative to the platform movement: 119 +/- 26 for ankle angle and 55 +/- 19 degrees for body sway. Gains were about 0.9 for both parameters. Large phase shift (tens of seconds) indicated a long delay in compensation of body inclination by ankle joint. The ramp tilt produced an initial body deviation followed by a slow (seconds or tens of seconds) approach of body position to a new steady level after the termination of ramp. Large slow body movements were superimposed with small irregular oscillations (about 10% of the amplitude of large displacements) of higher frequency. These oscillations resembled normal stabilograms on a stationary support. Thus, the usual process of stabilization of body gravity center was continued, though not around a fixed set-point but relative to a slowly changing position. Data obtained support the hypothesis that, besides operative control assigned to compensate deviations from a reference position, the system of postural control includes at least one additional level, which elaborates this reference using information about mutual position of body links, muscular torques and interaction with the support on the basis of criteria taking into account the energy cost of standing and demands for stability and security.

Postural instability enhances motor responses to transcranial magnetic stimulation in humans

Does the state of postural instability require a high hierarchical level of posture control? Electromyographic (EMG) activity of leg muscles was recorded during transcranial magnetic stimulation (TMS) of the motor cortex and electrical stimulation of the tibial nerve (H-reflex) in healthy subjects standing on a rigid floor and on a rocking platform. In the soleus muscle, TMS-evoked EMG responses increased considerably (2.2+/-1.1 times) when balancing on the rocking platform, whilst the H-reflex tended to decrease. The effect of support instability was specific to the muscles participating in the posture control. The results suggest that postural instability might change the state and the role of the motor cortex in equilibrium maintenance.

Human equilibrium on unstable support: the importance of feet-support interaction.

Healthy humans maintained equilibrium on rocking supports (seesaw) of different curvatures and heights. We recorded platform tilt, horizontal displacements of the upper body, ankle joint angle and activity of ankle joint muscles. Subjects maintained balance by making seesaw rotations placing the support under the bodys centre-of-gravity. Forward displacement was balanced by compensatory plantariflexion: thus the relation between muscle activity and ankle joint angle differed from that on a rigid floor. Mechanical analysis of stability showed that standing on low seesaws requires ankle torque increase during forward body shift (as on a rigid floor) and torque decrease on high seesaws (when the seesaw height exceeded its radius). In the latter case, balancing was impossible with eyes closed. The results suggest that directionally specific torque changes in response to centre-of-gravity shifts provide important information for maintenance of orthograde posture.

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

The majority of research and therapeutic actions in Parkinsons 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.

Effects of transcranial magnetic stimulation during voluntary and non-voluntary stepping movements in humans

Here, we compared motor evoked potentials (MEP) in response to transcranial magnetic stimulation of the motor cortex and the H-reflex during voluntary and vibration-induced air-stepping movements in humans. Both the MEPs (in mm biceps femoris, rectus femoris and tibialis anterior) and H-reflex (in m soleus) were significantly smaller during vibration-induced cyclic leg movements at matched amplitudes of angular motion and muscle activity. These findings highlight differences between voluntary and non-voluntary activation of the spinal pattern generator circuitry in humans, presumably due to an extra facilitatory effect of voluntary control/triggering of stepping on spinal motoneurons and interneurons. The results support the idea of active engagement of supraspinal motor areas in developing central pattern generator-modulating therapies.

Muscle resistance to slow ramp weakly depends on activation level

The mechanical response of human m. flexor pollicis longus to slow (3.2 degrees/s) linear stretch by 5.5 degrees was measured during sustained (45-60 s, 9-13.5 p.p.s.) unfused tetanus evoked by electrical stimulation. The stiffness increased during unfused tetanus. At the late phase of unfused tetanus it was 1.8 +/- 0.2 (mean +/- S.D.) times greater than at the early phase. The sensitivity of the isometric tension level to a short change in a stimulation frequency also increased. At the late phase of unfused tetanus force oscillations increased 1.2 +/- 0.2-fold during slow stretch or shortening and immediately reached a smaller amplitude after the cessation of length change. This was probably related to the friction and thixotropy in muscles. Muscle resistance to slow ramp depended only weakly on activation level. In the late phase of unfused tetanus the stiffness per unit force was 1.5 +/- 0.4 times greater at 9-13.5 p.p.s. than at 20-25 p.p.s. Thus, the relative value of muscle stiffness was greater for smaller activation levels typical for maintenance of posture. The enhancement of muscle stiffness during sustained unfused tetanus and a weak stiffness dependence on the activation level indicated a non-additivity of processes occurring in active muscle.

Eye Movements Induced by Changes in the Internal Representation of Body Posture

Oculomotor responses to body rotation were investigated in subjects standing with the eyes closed. A rotatable platform was used to provide body rotation relative to the space-stationary head or upper part of the body (fixation of the head; the head and the shoulders; and the head, the shoulders, and the pelvis). A slow rotation of the body about the longitudinal axis by ±6.5° within 10–150 s evoked an illusion of the upper part of the body turning in space, while the moving footplate was perceived as stationary in space. This illusion was accompanied by marked eye movements in the direction of the illusory rotation. In subjects grasping a rigid ground-based handle, the perception of body movements corresponded to the actual rotation of body parts. In this case, the amplitude of eye movements was substantially lower. It was concluded that the eye movement pattern depends not only on the actual relative movement of the body segments but also on the perception of this movement relative to the extrapersonal space.

Activation of walking by electrical stimulation in humans under the conditions of muscle unloading and its variations under the effect of afferent influences

The possibility of initiating an involuntary walking rhythm in a suspended human leg by electrical stimulation was studied. The subjects lay on the side with one leg suspended in an exoskeleton allowing horizontal rotation in three joints: the hip, knee, and ankle ones. To evoke involuntary walking of the suspended leg, two methods were used: continuous vibration of the quadriceps muscle of the hip and electrical stimulation of the cutaneous nerves innervating the foot of the immobile leg. The hip and ankle were involved in the involuntary movements, with reciprocal bursts of electromyographic activity being also observed in the antagonistic muscles of the hip. The application of an external load (4 N or 8 N) to the foot caused a perceptible intensification of its movements. An additional weight (0.5 kg) or a rubber band wrapped around the foot caused no substantial change in the pattern of stimulated walking. Electrical stimulation is an effective means of activating walking movements, and their characteristics confirm the assumption that the walking rhythm is of central origin. Additional afferentation from the sole’s receptors plays an important role in the modulation of the induced movements and the modification of the general walking pattern under the conditions of muscle unloading.

Study of Spinal α Motor Neuron Excitability during Standing under Normal and Complicated Conditions

Standing on supports with different stability presents a convenient model for the study of the role of muscular proprioception and visual and vestibular information in maintaining vertical posture of humans [1, 2]. Moreover, this model allows the assessment of the involvement of various levels of the central nervous system in maintaining balance. Standing on an unstable support is known to modify many reflexes [3, 4]. This may be connected with rearrangement of low-level balance-maintaining mechanisms or with the increased role of descending cortical control. In our previous study [5], we examined the responses of femoral and crural muscles to transcranial magnetic stimulation (TMS) during standing on a hard floor and on an unstable support. It was suggested that postural control on the unstable support involves high-level supraspinal structures into sensorimotor integration. However, the increased muscular response to TMS during standing on an unstable support might be connected not only with enhanced cortical control, but also with changes in the state of the segmental apparatus of the spinal cord. Hoffman’s reflex (the H-reflex—the electromyographic response of the musculus soleus to electric stimulation of the tibial nerve in the popliteal fossa) provides us with a convenient model for the study of this question. In this work, we evaluated the effectiveness of peripheral sensory stimulation of α motor neurons during standing under normal and complicated conditions using Hoffman’s reflex. Experiments were conducted with 10 healthy volunteers aged 25–52 familiarized with the method of investigation. They all gave informed consent for this study. The subjects stood with closed eyes either on a hard floor or on an unstable support moving in the sagittal direction. The unstable support was a paperweight with a cylindrical base (height, 20 cm; base radius, 32 cm), which could perform rotational translational motion. The subjects stood in the center of the platform. A convenient posture was achieved at approximately a horizontal position of the platform. The H-reflex was induced by an ENS-01 electrostimulator. Electric stimuli (1 ms) were applied via a monopolar electrode in the right popliteal fossa, while a reference electrode was located above the knee. Electromyographic (EMG) activity of the musculus soleus was recorded by surface electrodes. Each trial lasted 5 s; an electric stimulus was applied 1 s after the beginning of the recording. The electromyogram (EMG) was digitized at a sampling rate of 1000 Hz. Each subject was underwent 20 trials under each standing condition. The H-reflex was produced by a stimulating current from 3.5 to 10 mA. The experiment was started at a stimulating current causing a small H-response, which was nearly twice as high as the baseline level of activity of the musculus soleus during standing on a level floor. Thereafter, the current amplitude was gradually increased. The M-reflex appeared against the background of the increasing H-reflex (Fig. 1); the former increased and reached its maximum while the H-reflex passed through a plateau and gradually decreased [6]. The experiment was terminated at the maximum Mresponse. Several factors might affect M-reflex amplitude during balancing on an unstable support. To avoid the influence of these factors (primarily, possible shifts of the electrodes from the nerve), the amplitudes of Mand H-reflexes were related to the maximum M-reflex as a ratio. We analyzed only those trials with M-reflexes in the range of 5–25% of the maximum M-reflex (M max ) and the H-responses from the corresponding trials, because the H-reflex is the index of excitability of spinal α motor neurons in precisely this range. The response amplitude was determined as the peak-topeak distance in each trial. Since the responses to stimulation depend on the level of muscular activity during the stimulation, we applied different methods of data analysis. First, H-responses in each trial were normalized to the level of baseline activity before the stimulus. Second, trials with approximately equal levels of baseline activity during standing on the level floor and the unstable support were selected and the corresponding H-reflexes were compared. The level of baseline activity was determined for 0.5 s before the stimulus. The statistical significance of the results was estimated using the paired t test. Study of Spinal a Motor Neuron Excitability during Standing under Normal and Complicated Conditions

[Investigation of muscle tone in patients with Parkinson's disease in unloading conditions].

Parkinson’s disease (PD) is a progressive neurodegenerative disorder, the main symptoms of which are hypertonicity and difficulties emerging during performance of stepping movements due to increased muscle stiffness. Biomechanical (stiffness) and electrophysiological (shortening reaction, SR) characteristics of hip and shank muscles were examined in 25 patients with mild and moderate stages of PD (1 to 3 of Hoehn and Yahr Rating Scale, 61 ± 9 years) and 22 age-matched healthy controls in unloading leg conditions during passive flexion/extension of hip, knee, and ankle joints, as well as the changes in the tonic state of muscles under the influence of levodopa. The data obtained were compared with similar findings in healthy subjects. Essentially greater stiffness in all leg muscle groups (except foot extensors) was observed in patients with PD as compared to the healthy subjects. In patients with PD, SR values in hip and shank extensors as well as in foot flexors and extensors were essentially greater then in the healthy subjects. The medicine essentially reduced the stiffness of hip flexors and knee flexors and extensors. The SR persisted, although the frequency of its occurrence decreased in half of studied muscles, and a significant decrease in the SR value was observed in foot extensors. The medicine had no marked effect on the SR in the proximal muscles. Thus, the increased muscle stiffness in patients with PD manifests itself as distorted reactions to external disturbances and increased reflectory reactions of muscles.