Automatic Postural Responses Research Articles

H-reflex modulations during voluntary and automatic movements following upper motor neuron damage

Objectives: It is not known whether similar mechanisms account for the impairments of voluntary movement and automatic postural responses of individuals with spasticity secondary to damage to the sensorimotor cortex and its projections (i.e. upper motor neuron syndrome (UMN)). Methods: The present study examined changes in soleus H-reflexes preceding and during voluntary tibialis anterior (TA) muscle contraction of standing subjects and during balance platform induced postural perturbations that elicited similar TA muscle contractions. Twenty-two subjects (12 non-disabled; 4 with spastic-type cerebral palsy; 6 with adult-onset cerebral vascular accident) participated in the study. Data were analyzed using ANOVAs and Tukey HSD post-hoc comparison tests to assess the timing and magnitude of soleus H-reflex amplitude changes relative to the onset of TA muscle activation. Results: Results indicated that, regardless of the level of TA activation, soleus H-reflexes of subjects with UMN involvement did not demonstrate inhibition either during voluntary movements or during automatic postural perturbations. Conclusions: These findings indicate that postural reflexes, as well as volitional movements, are impaired following UMN damage and that deficits in neural pathways subserving reciprocal inhibition contribute to the impairments.

Postural responses to changing task conditions in patients with cerebellar lesions

OBJECTIVETo investigate the role of the cerebellum in postural adaptation for changes to the stimulus type of support surface displacements (backward translations v“toes up” rotations).METHODSA group of 13 patients with...

Effect of head position on postural orientation and equilibrium.

This study examined (1) how changes in head position affect postural orientation variables during stance and (2) whether changes in head position affect the rapid postural response to linear translation of the support surface in the horizontal plane. Cats were trained to stand quietly on a moveable platform and to maintain five different head positions: center, left, right, up, and down. For each head position, stance was perturbed by translating the support surface linearly in 16 different directions in the horizontal plane. Postural equilibrium responses were quantified in terms of the ground reaction forces, kinematics, dynamics (net joint torques), body center of mass, and electromyographic (EMG) responses of selected limb and trunk muscles. A change in head position involved rotation of not only the neck but also the scapulae and anterior trunk. Tonic EMG levels were modulated in several forelimb and scapular muscles but not hindlimb muscles. Finally, large changes in head orientation in both horizontal and vertical planes did not hamper the ability of cats to maintain postural equilibrium during linear translation of the support surface. The trajectory of the body's center of mass was the same, regardless of head position. The main change was observed in joint torques at the forelimbs evoked by the perturbation. Evoked EMG responses of forelimb and scapular muscles were modulated in terms of magnitude but not spatial tuning. Hindlimb responses were unchanged. Thus, the spatial and temporal pattern of the automatic postural response was unchanged and only amplitudes of evoked activity were modulated by head position.

The influence of vision on the automatic postural muscle responses of newly standing and newly walking infants.

In adults, visual inputs do not appear to contribute significantly to automatic postural muscle responses (90-100 ms latency) activated by transient support surface displacements causing threats to standing balance, but are activated through slow pathways with latencies of more than 200 ms. However, it has been shown that the postural sway behavior of early walking infants is strongly influenced by visual flow cues that falsely signal self-movement. To determine whether there also are significant contributions of vision to automatic postural muscle responses in this age group, two groups of infants were tested on a moveable platform; pre-walkers (n=6) and early walkers (n=6). Pre-walkers did not show any measurable effect of visual condition (vision vs no vision) on muscle response characteristics. However, the integrated gastrocnemius activity of early walkers increased significantly in vision versus no vision conditions (P<0.05). These results show that visual cues contribute to, or modulate, the automatic postural responses in children who are in the developmental transition to independent walking.

Postural perturbations: new insights for treatment of balance disorders.

This article reviews the neural control of posture as understood through studies of automatic responses to mechanical perturbations. Recent studies of responses to postural perturbations have provided a new view of how postural stability is controlled, and this view has profound implications for physical therapy practice. We discuss the implications for rehabilitation of balance disorders and demonstrate how an understanding of the specific systems underlying postural control can help to focus and enrich our therapeutic approaches. By understanding the basic systems underlying control of balance, such as strategy selection, rapid latencies, coordinated temporal spatial patterns, force control, and context-specific adaptations, therapists can focus their treatment on each patient's specific impairments. Research on postural responses to surface translations has shown that balance is not based on a fixed set of equilibrium reflexes but on a flexible, functional motor skill that can adapt with training and experience. More research is needed to determine the extent to which quantification of automatic postural responses has practical implications for predicting falls in patients with constraints in their postural control system.

Conditioned and unconditioned forelimb reflex systems in the cat: involvement of the intermediate cerebellum.

Temporary inactivation of the cerebellar interposed nuclei was used to assess the role of the intermediate cerebellum in the performance of forelimb cutaneo-muscular reflexes in the cat. The following types of reflexive responses were evaluated: the classically conditioned and unconditioned forelimb withdrawal responses and the forelimb tactile placing, hopping and magnet responses. The experiments tested the hypothesis that the intermediate cerebellum is involved in the performance of all the above forelimb reflexes. The forelimb withdrawal reflex was classically conditioned in a newly developed paradigm in which animals were first operantly conditioned to stand on four elevated platforms. Trained animals were microinjected with a gamma-aminobutyric acid (GABA) agonist, muscimol, in the interposed nuclei, and the effects of inactivation of the intermediate cerebellar output on the forelimb reflexes were examined. The main findings of these experiments are that unilateral muscimol inactivation of the interposed nuclei in the cat abolished the expression of the classically conditioned limb flexion reflex, suppressed the performance of the unconditioned withdrawal reflex and, in parallel, down-regulated the tactile placing, hopping and magnet postural responses in the ipsilateral forelimb. These observations are inconsistent with concepts indicating exclusive involvement of the intermediate cerebellum in the classically conditioned reflexes elicited by aversive stimuli. On the contrary, they support the hypothesis of a more global involvement of this structure in learned and unlearned defensive flexion reflexes and in automatic postural response systems.

Practice modifies the developing automatic postural response.

The purpose of this study was to examine effects of experience with a postural task on components of the automatic postural response including: (1) probability of activation of functionally appropriate postural muscles; (2) number of functionally appropriate postural muscles activated; and (3) onset latencies of functionally appropriate postural muscles in infants. Infants (n=15; age 36-48 weeks old) able to pull themselves into a standing position but not able to walk independently were tested using a postural task requiring the infant to stand and balance, with support, following a forward or backward movement of the support surface (platform perturbation). Infants were tested twice at 5-day intervals. One-half of the infants, the training group, were given intense platform perturbation training on the days between test sessions. Infants in the second group were also brought into the laboratory on the days between test sessions but were not exposed to platform perturbations during those days. Electromyograms of six leg and trunk muscles were recorded during test sessions to provide muscle onset latencies, probability of muscle activation data, and the number of postural muscles activated following a perturbation. Training infants demonstrated significant increases in probability of activating functionally appropriate muscles with tibialis anterior, quadriceps, and abdominal muscles activated in response to backward sway and gastrocnemius muscle in response to forward sway. The number of functionally appropriate postural muscles activated in a single trial also increased in the training group. There were no significant changes in mean postural muscle onset latencies or number of trials with antagonist muscle coactivation for either training or control groups. These findings suggest that during development selective parameters of the automatic postural response are affected by experience with the postural task.

Two functional muscle groupings during postural equilibrium tasks in standing cats.

1. This study examined the relation between electromyographic (EMG) activation and the contact force and joint torques of the left hindlimb during postural equilibrium tasks in the standing cat. It is the appropriate application of force by the limbs against the support surface that allows the animal to control its center of mass and maintain equilibrium. 2. Cats were trained to stand quietly on a moveable force platform. During quiet stance, the cat was perturbed by a platform translation in each of 12 directions evenly spaced in the horizontal plane. EMG activity of mono- and biarticular thigh muscles, three-dimensional ground reaction force under the paw (contact force), and kinematics of the hindlimb segments were recorded Net joint torques were computed using inverse dynamics. The analysis focused on the functional organization of the rapid, automatic postural response in relation to the sagittal plane contact force and joint torques. 3. The muscles of the thigh were subdivided into two functional groups, based on the relationship of the evoked response to the various components of the sagittal plane contact force or joint torques. The first group, consisting of the monoarticular and some biarticular muscles, was correlated with the vertical force component, Fz. The second group, consisting of a separate group of biarticular muscles, was correlated with the difference between knee and hip torque. This torque difference is a function of both sagittal plane force components, Fz and Fy, and is related to contact force direction. 4. It is suggested that this subdivision of muscle activations reflects a neural strategy of parallel control of the two muscle groups in relation to their influence on Fz and Fy. Such a control mechanism could be a strategy for simplifying the control of the multisegmented limb in contact force tasks such as maintaining postural equilibrium.

Neural and musculoskeletal contributions to the development of stance balance control in typical children and in children with cerebral palsy.

Studies on the development of automatic postural responses in both typically developing children and children with cerebral palsy were performed. With the appearance of "pull-to-stand" behavior, typically developing children first began to show muscle responses to platform movements in mainly the ankle muscles. With increased development, additional agonist muscles were added to the response pattern and a consistent distal to proximal sequence began to emerge. Well-organized responses were seen with the onset of independent stance and walking, along with the reduction of antagonist muscle co-activation. The older children with cerebral palsy who were pre-walkers had immature muscle activation patterns like those seen in the typically developing children at the pull-to-stand stage of development. These included disorganized muscle responses and increased frequency of coactivation of both proximal-distal and agonist-antagonist muscles. In order to determine if musculoskeletal constraints contributed to these response patterns, normal children were asked to stand in a crouched posture similar to that of children with CP. This caused postural muscle response patterns to more closely approximate those of children with spastic diplegia.

Longitudinal Development of the Automatic Postural Response in Infants.

In this longitudinal study, the development of automatic postural responses elicited during stance following perturbation of the support surface was investigated. Infants (N = 9) unable to stand without support were tested initially: follow-up tests were performed until the infants were able to stand and walk independently. Surface electromyographic recordings of leg and trunk muscle activity following a postural perturbation induced by a forward or backward translation of the support surface were made for each infant. Muscle onset latencies following the perturbation and the proportion of trials in which muscle activity was recorded were determined. First, infants activated appropriate muscles either in isolation or in pairs and then combined these muscles into functional synergies. Although activation of all three postural muscles was recorded in infants before they were able to stand and walk independently, the three-muscle response was not consistent in the youngest children. The proportion of trials eliciting muscle activity continued to increase (p <.O5) after infants began to walk independently, with postural muscle activity recorded in virtually every trial by late independent walking. Thus. the automatic postural response elicited during stance was shown to begin with activity in single or paired muscles, followed by activation of the postural muscles in functional synergies. These data illustrate the progressive development of an effective sensory-motor organization.

Bilateral labyrinthectomy in the cat: effects on the postural response to translation.

1. This study examined the role of vestibular afferent information on the postural responses of four cats, evoked by movements of the support surface during stance. Animals were exposed to linear translations of the supporting surface in eight evenly spaced directions in the horizontal plane, before and after bilateral labyrinthectomy. Postural responses were quantified in terms of the ground reaction forces under each paw and the evoked activity in selected muscles. 2. The cats were able to stand on the platform within 1-3 days after labyrinthectomy and were able to maintain balance during all perturbations of stance, even when they stood in total darkness, completely deprived of visual information. After lesion, postural responses were characterized by normal latency and normal spatial and temporal patterning of electromyographic (EMG) response. The pattern of force response showed the force constraint strategy that characterizes postural responses in the intact animal. 3. The only deficit in the postural response after lesion was a hypermetria, or active over-response that caused the animals to overbalance somewhat but did not impair their ability to remain upright. Analysis of the trajectory of the animal's center of mass during the trials indicated that the hypermetria was due to an abnormally large, active response on the part of the animal and could not be attributed to changes in the passive stiffness of the musculoskeletal system. The hypermetria was transient, and response amplitude returned to control levels after the rapid compensation phase of 10-15 days. 4. It is concluded that vestibular information is not essential for triggering the rapid, automatic postural response to translations of the support surface, nor is it necessary for the selection or shaping of the evoked response. Instead, somatosensory information appears to predominate in these postural adjustments. However, vestibular afferent input does influence the scaling of the postural response.

Effect of galvanic vestibular stimulation on human postural responses during support surface translations

1. We investigated the role of the vestibular system in postural control by combining galvanic vestibular stimulation (0.2-0.5 mA) with platform translations in standing subjects. Vestibular stimulation delivered 500 ms before and continuously during the platform translation produced little change in the earliest center of pressure (COP) and center of mass (COM) movements in response to platform translations, but resulted in large changes during the execution of the postural movement and in the final equilibrium position. 2. Vestibular stimulation produced anterior or posterior shifts in the position of COP and COM, depending on the polarity of the galvanic current. These shifts were larger during platform translations than during quiet stance. The peak of these shifts in COP and COM occurred at 1.5-2.5 s after the onset of platform translation, and increased in magnitude with increasing platform velocity. The final equilibrium positions of COP and COM were also shifted, but these shifts were smaller and not dependent on platform velocity. 3. These results imply that a tonic step of galvanic current to the vestibular system can change the final equilibrium position for an automatic postural response. Furthermore, these results indicate that the vestibular system may play a larger role in interpreting sensory reafference during postural movements, and especially during fast postural movements, than in controlling quiet stance. Finally, these results indicate that the vestibular system does not play a critical role in triggering the earliest postural responses, but it may be critical in establishing an internal reference for verticality.

The importance of somatosensory information in triggering and scaling automatic postural responses in humans.

To clarify the role of somatosensory information from the lower limbs of humans in triggering and scaling the magnitude of automatic postural responses, patients with diabetic peripheral neuropathy and age-matched normal controls were exposed to posterior horizontal translations of their support surface. Translation velocity and amplitude were varied to test the patients' ability to scale their postural responses to the magnitude of the translation. Postural response timing was quantified by measuring the onset latencies of three shank, thigh, and trunk muscles and response magnitude was quantified by measuring torque at the support surface. Neuropathy patients showed the same distal-to-proximal muscle activation pattern as normal subjects, but the electromyogram (EMG) onsets in patients were delayed by 20-30 ms at all segments, suggesting an important role for somatosensory information from the lower limb in triggering centrally organized postural synergies. Patients showed an impaired ability to scale torque magnitude to both the velocity and amplitude of surface translations, suggesting that somatosensory information from the legs may be utilized for both direct sensory feedback and use of prior experience in scaling the magnitude of automatic postural responses.

Activity of neuromuscular compartments in lateral gastrocnemius evoked by postural corrections during stance.

1. The electromyographic (EMG) activity of the four neuromuscular compartments in lateral gastrocnemius (LG) of cats was investigated to determine whether these intramuscular subdivisions could be activated differentially during automatic postural corrections. EMG electrodes were surgically implanted into each of the four compartments of left LG-LG1, LG2, LG3, and LGm--in two cats. Electrodes were also implanted into soleus and gluteus medius for comparative purposes. 2. Quiet quadrupedal stance was disturbed first by linearly translating the cats on a movable platform in each of 16 different horizontal directions. Mechanical events during corrections were characterized in terms of the three-dimensional forces exerted by each paw on the platform. EMG and force traces were quantified (area under the curve) and normalized, and tuning curves were constructed that relate muscle response and force change to direction of platform movement. 3. In a second series of trials, translations were presented along one direction only over a series of six velocities ranging from 5 to 16 cm/s. The third series of perturbations, termed the pop-up, consisted of a rapid upward displacement of the support under the left hindlimb only over a series of six amplitudes ranging from 1 to 10 mm. Evoked EMG activity and average change in force were normalized and regressions were computed onto velocity and amplitude, respectively. The slopes of the regressions were compared. 4. EMG tuning curves associated with the multidirectional horizontal translations revealed no differential activity across LG compartments. Similarly, there was no statistical difference among the slopes of the regressions within LG. In contrast, soleus exhibited significantly different slopes from LG for the regressions. Thus it is concluded that LG compartments are not differentially activated during automatic postural responses to perturbations of the support surface.

Effect of knee joint laxity on long-loop postural reflexes: evidence for a human capsular-hamstring reflex.

The onset latency and discharge amplitude of preprogrammed postural responses were evaluated in order to determine if the structure of synergistic activation could be altered by ligamentous laxity at the knee joint. Twelve subjects with unilateral and one subject with bilateral anterior cruciate ligament (ACL) insufficiency were tested while standing on a moveable platform. External balance perturbations (6 cm anterior or posterior horizontal displacements of the platform) were presented at velocities ranging from 15 to 35 cm/s. Perturbations were presented under the following experimental conditions: unilateral and bilateral stance, knees fully straight or flexed, and with ankle motion restricted or free. These stance, knee position, and ankle motion conditions were introduced to alter the stress transmitted to the knee joint during movement of the support surface. The automatic postural response was recorded from the tibialis anterior (T), quadriceps (Q), and medial hamstrings muscles (H) bilaterally. The normal response to an externally induced backward sway involved the automatic activation of T and Q at latencies of 80 ms and 90 ms respectively. Activation of the hamstrings in the non-injured extremity was not coupled with the postural response. Hamstrings are not typically involved in the correction posterior sway because H activation would tend to pull the center of mass further backwards. However, when the response in the ACL-deficient extremity was compared to the non-injured limb: (1) the automatic postural response in the ACL-deficient extremity was restructured to include hamstrings activation (100 ms latency), (2) H activation time was faster and less variable in the ACL-deficient limb, and (3) the ratio of H/Q discharge amplitude integrated over 100 ms and 200 ms from the onset of EMG activation showed a dominance of hamstring activity during unilateral stance on the lax limb. In addition, H/Q ratios integrated over 200 ms showed dominant hamstring activity in the ACL-deficient limb during bilateral stance. (4) Cross-limb comparisons showed greater normalized IEMG amplitudes for T, H, and Q during unilateral stance on the lax limb. These results suggest that a capsular-hamstring reflex is integrated into the existing structure of a preprogrammed postural synergy in order to compensate for ligamentous laxity. Furthermore, the generalized increase of response gain observed during perturbations of unilateral stance on the lax limb indicates that joint afference can modulate central programming to control localized joint hypermobility. A concept of postural control is discussed with respect to the capsular reflex, joint loading and displacement of the center of gravity.

Influences of object weight and instruction on grip force adjustments

This study investigated the influence of object weight and instructions on grip force responses in humans. Using a precision grip, subjects lifted a small instrumented test object to a predetermined height. Prior to each set of 40 trials, subjects were verbally instructed to either "hold" or "let go" of the object in response to any change in weight. Unpredictably on some trials (less than 20%), a sudden sustained increase (load) or decrease (unload) in vertical load was applied to the object. Grip responses to these induced weight changes were evaluated by measuring grip force, object position, and associated electromyographic (EMG) activity. Grip force changes for a load were over three times greater than those for an unload. Such asymmetry may reflect everyday grasp and manipulation in a gravity-influenced world. Grip force adjustments to loads following "hold" instructions were on the average somewhat larger than those following "let go" instructions, but there was no influence of instructions on responses to unloads. These findings contrast with more robust influences of verbal instruction on automatic postural and proximal upper limb responses and also may suggest that grip force adjustments are influenced to a greater extent by intrinsic task variables than by extrinsic volitional intent. Such organization appears tailored to functional task requirements in natural environmental contexts.

Head acceleration following linear translations in the freely-standing cat

The aim of this study was to determine whether vestibular information related to head acceleration is available for triggering postural responses to perturbations of stance in the freely-standing cat. Linear accelerations of the head were recorded during postural responses evoked by linear translations of the support surface. A consistent initial peak of acceleration was observed at an average latency of 22 ms and magnitude of 0.03 g (g is acceleration due to gravity, 9.8 m/s/s). The acceleration peak preceded the first evoked EMG activity by an average of 24 ms. It was concluded that stimulation of the vestibular apparatus was both adequate and early enough for the vestibular system to have triggered the automatic postural response.

Postural Responses and Effector Factors in Persons with Unexplained Falls: Results and Methodologic Issues

Automatic postural responses and effector factors were examined in 10 persons with unexplained falls after clinical examination and 24 older controls. Fallers were more unstable than controls on clinical tests of balance (20% of fallers vs 79.2% of controls were able to stand on one foot (P less than .005), 40% of fallers and 100% of controls were stable while turning in place (P less than .001), postural stress test median score was 12 for fallers and 20 for controls (P less than 0.001). We found prolonged tibialis anterior latency (fallers 158.8 +/- 23.7, controls 143.2 +/- 15.7 milliseconds, P = 0.03), marked losses in ankle strength (dorsiflexion: faller 3.2 +/- 2.9, controls 8.9 +/- 4.2 foot-pounds, P less than 0.001; plantarflexion: fallers 7.9 +/- 5.3, controls 21.4 +/- 11.1 foot-pounds, P less than 0.001), and decreases in range of motion (ankle plantarflexion: fallers 29.2 +/- 7.0, controls 37.8 +/- 12.4 degrees, P = 0.02). Gastrocnemius latency and electromyographic (EMG) measures of sequence showed no differences between fallers and controls. Sequence measures were not symmetric between the lower extremities in either fallers or controls. In this population of persons with unexplained falls who demonstrated substantial impairments in functional balance, effector factors appear more impaired than automatic postural responses. Alternatively, contemporary analysis of automatic postural responses may require further development before it can be useful to study balance problems in older persons.

Direction and amplitude precuing has no effect on automatic posture responses

Automatic postural responses of leg muscles to the sudden displacement of standing support were investigated under four different conditions of information given to subjects in advance. Results from three groups of subjects were compared: 6 normal subjects, 10 patients with cerebellar disease, and 9 patients with Parkinson's disease. Specifically, each subject was provided with visual information about the direction and/or the amplitude of an upcoming platform tilt. For the control situation no advance information on the characteristics of platform tilt was provided. Neither the latencies nor the integrals of postural EMG-responses showed alterations with advance information. In contrast, in a control experiment in which 3 normal subjects had to perform large or small forward or backward voluntary movements of the body around the ankle joint, shorter onset-latencies of leg muscle EMG responses were observed with increasing complexity of the advance information. These results suggest that, unlike voluntary movements, postural responses to rapid surface tilts do not benefit from advance visual information on direction or amplitude of a postural disturbance.

Stance dependence of automatic postural adjustments in humans.

This study investigated the effect of initial stance configuration on automatic postural responses in humans. Subjects were tested in both bipedal and quadrupedal stance postures. The postural responses to horizontal translations of the supporting surface were measured in terms of the forces at the ground, movement of the body segments, and electromyographic (EMG) activity. Postural responses to the same perturbations changed with initial stance posture; these responses were biomechanically appropriate for restoring centre of mass. A change in stance configuration prior to platform movement led to a change in both the spatial and temporal organization of evoked muscle activation. Specifically, for the same direction of platform movement, during bipedal stance muscles on one side of the lower limb were activated in a distal to proximal sequence; during quadrupedal stance, muscles on the opposite side of the lower limb were activated and in a proximal to distal sequence. The most significant finding was an asymmetry in the use of the upper limbs and the lower limbs during postural corrections in quadrupedal stance. Whereas antagonists of the upper limb were either co-activated or co-inhibited, depending on the direction of translation, lower limb antagonists were reciprocally activated and inhibited. Human subjects in a quadrupedal stance posture used the lower limbs as levers, protracting or retracting the hips in order to propel the trunk back to its original position with respect to the hands and feet. Postural responses of the subjects during quadrupedal stance were remarkably similar to those of cats subjected to similar perturbations of the supporting surface. Furthermore, the same predominance of lower limb correction is characteristic of both species, suggesting that the standing cat is a good model for studying postural control in humans.