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Biomedical subjects

V Dietz

Publications and source records attributed to V Dietz.

At least 181 records · Page 10Linked to original sources
PubMed1989

Compensation of translational and rotational perturbations in human posture: stabilization of the centre of gravity.

EMG responses in the leg muscles, head acceleration and joint movements induced by dorsiflexing rotation of the ankle have been analysed. Perturbations were induced while subjects were standing on a platform with the ankle joints co-linear with the axis of rotation or while standing above the axis (between 5 and 25 cm), which introduced an additional translational component to the displacement. The former condition was followed by a monosynaptic gastrocnemius reflex potential and a late tibialis anterior activation (latency about 100 ms); in the latter condition the monosynaptic reflex response became smaller and a longer latency gastrocnemius response (latency about 70 ms) appeared, the strength and duration of which increased in parallel with the translational component, while the amplitude of the tibialis anterior activity decreased. Neither vestibulospinal nor muscle proprioceptive mechanisms can solely account for this effect as the biomechanical parameters were little changed in the different conditions. It is suggested that the controlled variable in the investigated task is to hold the bodies' centre of gravity over the feet which makes the change in the pattern meaningful: pure dorsiflexion of the feet is followed by a backwards sway of the body, thus a tibialis anterior activation is functionally essential to hold the centre of gravity over the feet. A backwards translation of the feet withdraws the supporting surface, thus a gastrocnemius activation is needed to restore the centre of gravity over the feet. The inhibition of the monosynaptic reflex with increasing translation is suggested to arise from a reciprocal modulation of mono- and polysynaptic gastrocnemius reflex responses.

Acceleration↗
PubMed1989

Significance of proprioceptive mechanisms in the regulation of stance.

Compensatory electromyographic (EMG) responses and several biomechanical parameters were studied following impulsive disturbance of the limbs during stance of human volunteers on a treadmill. Treadmill acceleration impulses were backwards or forwards directed, or their initial direction was reversed after 30 ms. Backwards directed impulses were followed by gastrocnemius, forwards directed ones by tibialis anterior EMG responses (latency 65 to 75 ms) whose durations depended on impulse duration. When the direction of the impulse was reversed, the respective antagonistic leg muscles were activated again with a delay of 68 to 75 ms after onset of stretch of these muscles. The behaviour of the EMG responses could best be correlated to the displacement at the ankle joint and may be described in terms of a stretch reflex response. The results indicate that these stretch reflex responses help control of the body's centre of gravity thereby preventing falling. Head movements induced by the impulses showed little correlation with the appearance of the EMG responses, suggesting that the vestibular system is unlikely to be directly involved in the generation of these responses. Vestibular signals may, however, significantly contribute to slow body sway stabilization.

Adult↗
PubMed1989

Perturbations of human posture: influence of impulse modality on EMG responses and cerebral evoked potentials.

Leg muscle EMG responses and cerebral evoked potentials (CP), elicited by perturbations of stance while on a treadmill with split belts, were analyzed in order to study the relationship between compensatory leg muscle responses and afferent input to supraspinal centers. Various conditions of perturbation were used to establish the extent to which compensatory EMG responses and CPs show congruent behavior. Four different treadmill acceleration rates were applied in three different conditions (unilateral perturbation, directed forward or backward; bilateral perturbation, directed forward or backward; and opposing bilateral perturbation). EMG responses and CPs showed parallel increases in amplitude with increasing displacement velocity. The EMG responses showed distinct differences, predominantly in the response amplitude, between the different perturbation conditions, whereas the CPs were affected only to a minor degree. Tibialis anterior EMG responses were more closely related to the CP following forward perturbation than the corresponding gastrocnemius responses were to the CP following backward perturbation. We conclude that the EMG responses are more closely related than the CPs to displacement parameters and suggest that this is due to the further spinal processing of the afferent input needed to generate an appropriate EMG response. The closer relationship between the tibialis anterior response and CP may reflect a predominant central representation and control of tibialis anterior activation in the regulation of posture. The functional implications of these findings are discussed.

Journal Article↗
PubMed1989

Interlimb coordination of leg-muscle activation during perturbation of stance in humans.

1. Electromyographic (EMG) responses were recorded in both legs, along with corresponding joint movements, after uni- and bilateral perturbations during stance on a treadmill with split belts. Displacements were directed forward, backward, or in opposing directions. They were induced by randomly timed ramp impulses at one of four different rates of treadmill acceleration. 2. Unilateral perturbations directed backward were followed by a bilateral gastrocnemius-EMG response, forward-directed perturbations by a bilateral tibialis anterior-EMG response. The amplitude of these responses was dependent on the rate of treadmill acceleration. Relative to the response of the displaced leg, the amplitude of the EMG response on the nondisplaced side was smaller when a gastrocnemius EMG response was induced, and about equal when the tibialis anterior muscle was activated. The onset latencies were shorter on the displaced side (displaced leg 75-96 ms, non-displaced leg 93-112 ms). 3. Bilateral perturbations in one direction were followed by larger EMG responses in both legs (in the gastrocnemius for backward-directed impulses, in the tibialis anterior for forward-directed impulses). For a given acceleration rate, their amplitude was about equal to the sum of the EMG amplitude of the displaced leg and that of the nondisplaced leg obtained during unilateral displacement. The inverse result was obtained when the legs were simultaneously displaced in opposite directions: EMG responses in both legs were significantly smaller than those obtained after unilateral displacement. 4. It is concluded that a unilateral displacement evokes reflex EMG responses in the synergistic muscles of both legs, which are graded according to the size of the proprioceptive input from the primarily displaced joint. During bilateral displacements, the activity induced by the respective contralateral leg is linearly summed or subtracted, depending on whether the legs are displaced in the same or in opposite directions. In view of the short latencies of these bilateral responses, it would seem that they are mediated by a spinal mechanism. 5. Distinct differences in the behavior of the antagonistic leg muscles were observed: 1) the coactivation of the contralateral leg muscle was significantly smaller when the gastrocnemius was stretched unilaterally, whereas it was about equal for the tibialis anterior; and 2) the gastrocnemius EMG responses were closely correlated with the displacement velocity, whereas the tibialis anterior response was more closely correlated with acceleration, i.e., the tibialis anterior response was more dynamic in nature.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1988

The contribution of vestibular input to the stabilization of human posture: a new experimental approach.

An experiment was designed to evaluate the vestibular contribution to the stabilization of upright stance in normals and in two patients with loss of vestibular function. A forward or backward displacement of a load (2 kg) by a torque motor attached to the subject induced opposing movements in the head and trunk. The small linear acceleration of the head in space of about 0.1 g was followed, with a latency of 50-65 ms, by EMG responses in the tibialis anterior and rectus femoris (backward acceleration) or gastrocnemius muscles (forward acceleration). These responses were absent in patients with a vestibular deficit. It is suggested that the observed EMG responses are due to fast acting vestibulospinal reflexes involved in the regulation of upright stance. For comparable head accelerations the integrated EMG responses induced by the vestibulospinal mechanism are about ten times smaller than those induced by spinal stretch reflexes during displacement of the feet. Vestibulospinal reflexes would appear, therefore, to play only a minor role in the compensation of stumbling.

Adult↗
PubMed1988

Involvement of different receptors in the regulation of human posture.

Compensatory electromyographic (EMG) responses and several biomechanical parameters were studied following impulsive disturbance of the lower limbs during stance on a treadmill. Treadmill acceleration impulses were backwards or forwards directed, or their direction was inverted after 30 ms. Backwards directed impulses were followed by gastrocnemius and forwards directed ones by tibialis anterior EMG responses (latency 65-75 ms) whose duration depended on impulse duration. When the direction of the impulse was inverted, the respective antagonistic leg muscles were activated, with a delay of 68 to 75 ms after onset of stretch of these muscles. The behaviour of the EMG responses could best be correlated to the displacement at the ankle joint and may be described in terms of a stretch reflex response. The function of this stretch reflex mechanism is suggested to be connected with the control of the body's centre of gravity in order to prevent falling. Head movements induced by the impulses showed little correlation with the appearance of the EMG responses, suggesting that the vestibular system is unlikely to be significantly involved in the generation of these responses.

Biomechanical Phenomena↗
PubMed1988

Posture in Parkinson's disease: impairment of reflexes and programming.

The leg muscle electromyographic responses induced during stance by impulsive displacements of a treadmill belt (directed forward or backward and at different rates) were studied in a group of patients with Parkinson's disease and a group of age-matched healthy subjects. Young normal subjects were also studied both before and after intake of a dopamine antagonist (haloperidol). Compensatory gastrocnemius electromyographic responses resulting from backward-directed displacements were significantly smaller in both the patients and the young normal subjects following intake of haloperidol. The reduced sensitivity of the gastrocnemius muscle to stretch correlated with an inability to compensate for the perturbations. In the patients, the gastrocnemius response was followed by enhanced activation of the tibialis anterior muscle. This was not the case in the normal subjects after intake of dopamine antagonist and is probably not, therefore, the consequence of acute dopamine deficiency. In the patients the angular rotation at the ankle joint induced during faster backward-directed displacements was slower than that in normal subjects, despite identical amounts of gastrocnemius electromyographic activity. This supports earlier findings of changes in intrinsic muscle stiffness in Parkinson's disease. None of these differences were seen when the tibialis anterior muscle was stretched. This differential behavior of the antagonist leg muscles can best be explained by the different function fulfilled by these muscles in regulation of stance and gait.

Adult↗
PubMed1988

Fast head tilt has only a minor effect on quick compensatory reactions during the regulation of stance and gait.

Sudden tilts of the head to the front or rear were induced during stance, balancing, gait and during perturbations of gait. The most prominent response in the leg muscle electromyogram (e.m.g.) to head tilt occurred in the tibialis anterior muscle (latency about 55 ms) following a backward tilt induced during balancing. During stance and gait, the e.m.g. activity related to head tilt was only a minor component of the leg muscle activity normally occurring during gait. When the head tilt was induced shortly after a perturbation of gait (treadmill acceleration impulse), the compensatory reaction in the leg muscles did not significantly differ from that seen after the gait perturbation alone. In addition, the rate of acceleration of the head was tested against the compensatory e.m.g. responses: No correlation of influence could be discerned. The results indicate that sudden head tilts and the resulting head acceleration have little influence on the e.m.g. patterns that occur during gait and perturbations of gait. It is assumed that these patterns are regulated by central programs, and that the compensation for leg perturbation is achieved mainly by spinal reflex mechanisms. It is discussed whether the lack of head tilt responses is the result of an antagonistic vestibular-neck interaction, or whether it indicates a reduced effectiveness of vestibulo- and cervico-spinal reflexes during gait.

Adult↗
PubMed1988

Reproducibility and adaptation of the EMG responses of the lower leg following perturbations of upright stance.

The electromyographic (EMG) response of gastrocnemius, soleus and anterior tibialis muscles to a backwardly directed perturbation of stance was recorded in 12 normal subjects using surface electrodes and studied with regard to its reproducibility (test-retest reliability coefficients, variability coefficients) and to adaptational effects. (1) Reproducibility was shown to be uniformly high and can be interpreted as an index for the high intraindividual constancy of the results. (2) Adaptational effects have been found and should be circumvented, either by pre-adapting the subjects to the motor task, or by restriction of the period of measurement. (3) Variation of the position of the electrodes produced only small effects. The impedance of the surface electrodes was not critical if kept below 5 k omega. EMG investigations with surface electrodes during stance and perturbations of stance provide highly reliable results with respect to intraindividual changes but interindividual variability of the results clearly marks the limits of this method. The interindividual variability observed with surface electrodes is of the same order as that reported in the literature for inserted needle recording.

Adaptation, Physiological↗
PubMed1988

Spastic paresis: impaired spinal reflexes and intact motor programs.

Leg muscle EMG responses evoked by short treadmill acceleration impulses applied during stance were analysed in patients with spastic hemiparesis. The compensatory reactions on the unaffected side consisted of a diphasic pattern of leg muscle activation. The first response could best be described as a polysynaptic spinal stretch reflex response. This response was absent on the spastic side, except for its later, declining component. This remainder of the first response and the following activation of the antagonistic muscle was identical on both the unaffected and the spastic side. This part of the pattern is assumed to be centrally programmed (at the spinal level) and triggered by the termination of the acceleration impulse.

Adolescent↗
PubMed1987

Task-dependent gating of somatosensory transmission in two different motor tasks in man: falling and writing.

The cerebral potentials induced by an electrical stimulus (median nerve or finger) were recorded over the central region of the scalp and were analysed during falling onto the extended arms or during writing to investigate the influence of different motor tasks on the transmission of a synchronous afferent volley to the brain. During both falling (before landing) and writing, the first peaks (20-40 ms) were reduced. Later peaks (60-200 ms) were enhanced during writing but reduced during falling. A reduction of the first peak was also obtained after ischaemic blockade of group I afferents, suggesting that the cerebral transmission of group I afferents is inhibited during falling and writing. The subjects reported a corresponding reduction in the perception of the stimulus during falling. During writing, however, the large late waves indicate a task specific processing of the remaining afferent volley. Such a gating of sensory information to the brain is assumed to play a functional role in the respective motor tasks.

Evoked Potentials, Somatosensory↗
PubMed1987

Afferent and efferent control of stance and gait: developmental changes in children.

Comparisons were made between the cerebral potentials (CPs) and EMG responses of leg muscles evoked by perturbation impulses during stance and gait in normal children aged from 1 to 10 years. Changes in the efferent arm of the reflex systems during development were reflected in parallel changes with age of the afferent system, expressed in the CP: in the youngest children (1-2 years of age) monosynaptic stretch reflex potentials appeared following perturbations during both stance and gait, together with a reduced level of longer latency EMG responses. The CP, too, had a profile that did not, at this early stage, differ in either condition. In children from 6 to 10 years of age, the adult pattern was reached, with the suppression of monosynaptic stretch reflexes and the early part of the CP during gait perturbation. This is interpreted as an inhibition of group I afferents at both segmental and supraspinal levels, involving suppression of both segmental stretch reflexes and group I signals to supraspinal centres. This control of afferent information had yet to be established in early infancy. The age group from 2 to 6 years showed progressive changes, with an increase in both the level and phasic nature of polysynaptic EMG responses and a corresponding transformation of the latency and shape of the CP. It is suggested that maturation of compensatory EMG responses during gait is achieved by the establishment of descending inhibition of group I afferents and facilitation of polysynaptic spinal reflexes via group II afferents.

Afferent Pathways↗
PubMed1987

Stumbling reactions in man: significance of proprioceptive and pre-programmed mechanisms.

1. Electromyogram (e.m.g.) responses of the leg musculature and the corresponding joint movements were studied following a perturbation of the limb during walking on a treadmill, produced by a randomly timed treadmill acceleration impulse, either predictable, or unpredictable in its amplitude and rate of acceleration. 2. The rate of rise of ipsilateral gastrocnemius e.m.g. response following a perturbation was dependent on the rate of treadmill acceleration. For a given acceleration rate the amplitude of the e.m.g. response and the timing of its peak was dependent on the amplitude of the impulse and the rate of rise of the gastrocnemius response was the same for impulses of both small and large amplitude. The onset latency was shorter (65 ms) for high accelerations and longer (85 ms) for lower ones. 3. The amplitude of the ipsilateral biceps femoris response was much smaller than the gastrocnemius response but was larger following unpredictable than predictable impulses. 4. The initial gastrocnemius response was followed by a tibialis anterior activation associated with a gastrocnemius depression and sometimes with a second, weak gastrocnemius activation. The gastrocnemius depression ended within a fixed time range relative to the onset of the response. The tibialis anterior activation was most pronounced when unpredictable impulses with high acceleration but a small amplitude were induced. 5. It is concluded that generation of the first gastrocnemius response is obviously under continuous control by muscle proprioceptive information and can be best described in terms of a stretch reflex response. It is suggested that, on the evidence of the diphasic or triphasic e.m.g. pattern, a close interaction occurs between a central programme and muscle proprioceptive input in order to generate the appropriate e.m.g. pattern. 6. On the basis of earlier work (Berger, Dietz & Quintern, 1984a) and on the present results it is suggested that the e.m.g. responses may be mediated mainly by muscle proprioceptive input from group II afferents. This input is modulated and processed by spinal interneuronal circuits, closely connected with spinal locomotor centres. The mode of processing depends on various factors, such as the predictability of the nature of the impulse.

Adolescent↗
PubMed1987

Role of peripheral afferents and spinal reflexes in normal and impaired human locomotion.

For many years, electrophysiological investigations of locomotion were restricted to animals, largely the cat. They concentrated on and emphasized the role of spinal interneuronal networks responsible for the generation of the locomotor pattern. Following the introduction of perturbation impulses and electrical nerve stimulation during stance and gait, information became increasingly available concerning the role of the reflex systems involved in the regulation of gait, their afferent pathways and their control by supraspinal motor centres. During gait monosynaptic stretch reflexes are inhibited. From a knowledge of the behaviour of the cerebral potentials evoked during stance and gait, it can be deduced that during gait the signals of group I afferents are blocked at both segmental and supraspinal levels. Polysynaptic reflex responses are mainly responsible for the compensation of perturbations introduced during gait. They are most probably mediated by group II afferents via a spinal pathway closely connected with the spinal locomotor centres. The functioning of these responses depends on an intact supraspinal control. They are suggested to be incorporated in a more complex e.m.g. pattern mainly determined by central mechanisms. In contrast to the gait condition, segmental stretch reflex activity does contribute to activation of extensor muscles of the leg during fast movements, such as running and hopping. In children at an early stage in the development of gait (around 1 to 2 years of age), as well as in patients with spastic paresis, the polysynaptic reflex responses are reduced or absent, and isolated monosynaptic reflex potentials are present. This suggests a reciprocal modulation of mono- and polysynaptic reflex mechanisms, both being dependant on supraspinal control. When this control is either not yet matured (small children) or impaired (spastic paresis), inhibition of monosynaptic stretch reflexes is absent and associated with a reduced facilitation of polysynaptic spinal reflexes. In spastic muscle hypertonia, the tension developed at the Achilles tendon during gait cannot be explained by gastrocnemius activation alone. In patients with spastic hemiparesis gastrocnemius e.m.g. activity is reduced in the spastic leg as compared to the unaffected one. It can be concluded that the paretic muscle undergoes changes in its mechanical properties, secondary to the supraspinal lesion, which results in the development of spastic muscle hypertonia.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1986

Obstruction of the swing phase during gait: phase-dependent bilateral leg muscle coordination.

We have investigated bilateral leg muscle activation following an obstruction of the forward swinging leg during gait. When the holding impulse was released at the beginning of the swing phase, weak gastrocnemius (GM) and biceps femoris (BF) responses appeared in the contralateral, standing leg. When the holding impulse was released at the end of the swing phase, strong tibialis anterior (TA) and rectus femoris (RF) responses appeared in the swinging leg, and GM and BF responses in the standing leg. In the latter condition the TA response was followed by a premature impact associated with a GM activation. The latency between onset of impulse and appearance of the responses was 65-70 ms in both legs. While in the first condition the duration of GM and BF response corresponded to the duration of the impulse, it was independent for the duration of GM and BF response in the latter. It is concluded that different strategies for compensation are at work. In the first condition, body stability is maintained during the obstruction by the standing leg. In the second condition, body support is provided by a premature touchdown of the swinging leg. It is suggested that the appropriate pattern is, in part, released by a spinal generator.

Adult↗