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PubMed1999

Age differences in postural sway during volitional head movement.

OBJECTIVE: To examine the role of a volitional self-paced head-turn movement on the postural sway characteristics of healthy young and elderly subjects. DESIGN: Cross-sectional design. SETTING: Motor control research laboratory. SUBJECTS: Ten young adults and 10 elderly subjects. MAIN OUTCOME MEASURES: Postural sway characteristics of each subject were examined using a Kistler force platform. Each subject was tested under four experimental conditions: (1) static postural sway with vision; (2) static postural sway without vision; (3) postural sway with vision and self-paced head-turn movement; and (4) postural sway with no vision and a self-paced head-turn movement. Subjects performed six 15-second trials in each experimental condition. Dependent variables analyzed on each trial were mean sway amplitude (in millimeters), sagittal sway standard deviation, lateral sway standard deviation, and frequency of sway (in hertz). RESULTS: During the static conditions (e.g., no voluntary movement), the young subjects produced significantly less postural sway than the elderly in both the vision condition (sway amplitude in the young, 3.80 mm; in the elderly, 4.89 mm) and the no-vision condition (young, 5.44 mm; elderly, 5.95 mm). This increased sway was the result of greater lateral sway in the elderly for the vision condition (3.73 vs. 2.68 mm), and greater sagittal sway for the elderly in the no-vision condition (5.55 vs. 4.70 mm). There were no significant differences between the groups in the frequency of sway. When asked to initiate and complete the head-turn, elderly subjects significantly increased their mean sway amplitude and decreased their frequency of sway, whereas the young subjects did not significantly alter their postural sway profiles. CONCLUSIONS: These results demonstrate different postural sway control strategies for young and elderly subjects when asked to perform volitional movements.

Accidental Falls↗
PubMed1999

Anterior ankle-foot orthosis effects on postural stability in hemiplegic patients.

OBJECTIVES: To evaluate the effects of an anterior ankle-foot orthosis (AFO) on static and dynamic postural stability in hemiplegic patients. DESIGN: A cross-sectional assessment of hemiplegic subjects with and without an AFO. SETTING: Outpatient department of a rehabilitation hospital. PATIENTS: A convenience sample of 24 subjects who had been prescribed an anterior AFO. OUTCOME MEASURES: Postural sway index and postural symmetry (body weight distribution through the affected leg) when standing were measured as static postural stability. Maximal balance range in anterior-posterior and lateral directions and the affected leg's weight bearing after weight shift to affected side were measured as dynamic postural stability. RESULTS: When wearing the anterior AFO, there was no significant difference and small effect size (r<0.3) in postural sway index (p = .35), postural symmetry (p = .21), and maximal balance range in anterior-posterior direction (p = .46). There was a significant improvement and large effect size (r>0.5) in lateral weight shifting (p<.01) and weight bearing through the affected leg after weight shifted to the affected side (p<.01). CONCLUSIONS: The significant effects of the anterior AFO in long-term hemiplegic patients were on lateral weight shifting and weight bearing through affected leg after weight shifted to the affected side. Postural sway, postural symmetry, and anterior-posterior weight shifting were not significantly affected.

Adult↗
PubMed2001

Prism adaptation to rightward optical deviation improves postural imbalance in left-hemiparetic patients.

Left-hemiparetic patients show predominant postural imbalance as compared to right-hemiparetic patients. The right hemisphere is crucial for generating internal maps used for perceptual and premotor processing of spatial information. Predominant postural imbalance with right-brain damage could thus result from a distortion of an internal postural map. Well-known manifestations of distorted internal maps due to right-hemisphere lesions, such as hemineglect, may show improvement following prism adaptation shifting the visual field to the right. We therefore investigated the effect of prism adaptation on postural imbalance in left-hemiparetic patients. Three groups of five patients were either adapted to prisms deviating the visual field to the right or left or exposed to neutral prisms while performing reaching movements of the right arm. Postural imbalance was reduced only following prism adaptation to the right. Thus, brief adaptation (i.e., 3 min) to rightward-shifting prisms can dramatically improve postural imbalance. This result shows that the effect of exposure to prisms that horizontally shift the visual field to the right in a reaching task generalizes to the postural system, and it suggests an interaction between horizontal and vertical reference frames. This also supports the theory that predominant postural imbalance in patients with right-brain damage may be partly related to a distortion of an internal postural map.

Adult↗
PubMed2001

Transcutaneous electric nerve stimulation reduces neglect-related postural instability after stroke.

OBJECTIVE: To test the existence of a neglect-related component of postural imbalance in some stroke patients to determine whether neglect patients (1) show worse postural control compared with nonneglect patients and healthy subjects and (2) have latent postural capacities that could be unmasked by an appropriate somatosensory manipulation. DESIGN: Intervention study with and without transcutaneous electric nerve stimulation (TENS). SETTING: Rehabilitation center research laboratory. PARTICIPANTS: Twenty-two stroke patients (mean age, 58.3 +/- 2.5yr; average days since stroke, 83.2d) and 14 age-matched healthy subjects. Stroke patients were subdivided into 3 groups: 6 with spatial neglect and 16 without (8 with left lesion, 8 with right lesion). INTERVENTIONS: All participants were subjected to a dynamic balance task, performed while sitting for 8 seconds on a laterally rocking platform. Seated on this mobile support, they were asked to maintain actively an erect posture, sitting as still as possible. In patients, TENS was applied on the contralesional side of the neck during the postural task. An effective stimulation (intensity corresponding to the threshold of perception, TENS+) was compared with a placebo stimulation (.01 x threshold of perception, TENS-). MAIN OUTCOME MEASURES: Postural performance in each trial was monitored by using 2 criteria: the number of aborted trials caused by loss of balance, and the angular dispersion of the support oscillations in roll. The latter criterion, which increased with body instability, was defined as 2 standard deviations of the angular distribution. RESULTS: Patients showing neglect displayed pronounced postural instability compared with other patients and controls. Although dramatic postural instability in the neglect patients was spectacularly and systematically reduced with TENS, no effect was observed in patients without neglect. CONCLUSION: This is among the first studies to provide clinical evidence supporting the "postural body scheme" concept.

Adult↗
PubMed2000

Postural stability using different neck positions in normal subjects and patients with neck trauma.

Subjects with neck problems, such as whiplash injuries, often complain of disturbed equilibrium and, in some instances, provocation of the neck position can elicit such problems. The importance of neck proprioceptors for maintaining balance is gaining increased interest, moreover the function or malfunction of the otoliths may disturb equilibrium in certain head positions. The aim of the study was to create a reference material for postural control and its dependence on head position in healthy subjects and to compare this with a set of patients with known neck problems and associated vertiginous problems. A total of 32 healthy subjects (16 men, 16 women, age range 21-58 years) as well as 10 patients age range 27-62 years (mean 44 years) with neck problems and associated balance problems since a whiplash injury were tested for postural control using the EquiTest dynamic posturographic model. The normal subjects were initially split into four age groups in order to estimate the effects of age on performance. The postural stability was evaluated for dependence of support surface conditions (stable or sway-referenced), visual input (eyes open or closed) and head position (neutral, left rotated, right rotated, extended backwards or flexed forward) using analysis of variance (ANOVA) with Tukey's post hoc test in case of a significant factor effect. As expected, visual cues as well as stable support surface improve postural stability (p < 0.001). Postural stability is statistically different in the head extended backwards condition compared with the other four head positions (p < 0.001 in all cases) in both patients and controls. Eliminating this test condition from the analysis, only a slight (p < 0.05) difference between head forwards and head turned left remained. This pattern of results remained if the normal subjects were only split into two age groups instead of four. Finally, the patient group exhibited significantly lower postural performance than all the groups of normal subjects (p <0.01), but none of the normal groups differed significantly from each other. It is concluded that the postural control system is significantly challenged in the head extended backwards condition in both normal subjects and patients with previous whiplash injury and persistent neck problems. The patient group differed statistically from all groups of normal subjects. This suggests that neck problems impair postural control, and that the head extended position is a more challenging task for the postural system to adapt to. Whether this is due to utricular malpositioning, central integrative functions or cervical proprioceptive afferents is not within the scope of this study to answer.

Adult↗
PubMed1998

Postural motor programming in paraplegic patients during rehabilitation.

One of the basic aims in the rehabilitation of thoracic spinal cord injured (SCI) patients concerns the regaining of sitting posture control. This implies the development of new postural strategies requiring the adjustment of motor programming processes. The aim of this study was to investigate the time course of postural reorganization during active, clinical rehabilitation of thoracic SCI patients with different SCI levels. Thus changes in motor programming in sitting balance control were investigated in two groups of complete low or high thoracic SCI patients. At several stages during the rehabilitation process an experiment was held in which sitting posture was perturbed systematically using submaximal reaching movements over four reaching distances. This bimanual reaching task was presented as a visual precue choice reaction time (RT) task in which reaching distance (i.e. grade of postural perturbation) was precued. Results indicated that in both high and low thoracic SCI patients RTs in movements involving postural perturbation became shorter during the course of the rehabilitation period. However, low thoracic SCI patients were generally slower in the programming of balance perturbing movements than high thoracic SCI patients, a phenomenon that did not change over time. Furthermore, initial differences in RTs as a function of grade of postural perturbation disappeared in both groups in the course of the rehabilitation phase. Precue benefit, equally large for both groups, did not change as a function of rehabilitation time. It is concluded that the observed phenomena signify the gradual development of new central postural control processes in both SCI groups during rehabilitation. Low thoracic SCI patients, having more residual sensorimotor functions, seem to adopt more complex strategies in maintaining and restoring sitting balance that take longer to specify and to programme. High thoracic SCI patients seem to rely on simpler strategies using more passive postural support.

Adolescent↗
PubMed2006

Gender and limb differences in dynamic postural stability during landing.

OBJECTIVE: To determine if gender and limb dominance affect dynamic postural stability and vertical ground reaction force data during jump landings. Secondary objective was to assess the reliability of the dynamic postural stability index (DPSI). DESIGN: A mixed model (2 genderx2 limb) repeated measures design was used to determine the effects of gender and limb dominance on dynamic postural stability. Subjects were required to perform a two-legged jump to a height equivalent of 50% of their maximum vertical leap, land on a single-leg and balance for three seconds. SETTING: Sports Medicine Research Laboratory. PARTICIPANTS: Forty healthy subjects (20 men, 20 women) participated in this investigation. MAIN OUTCOME MEASURES: The DPSI and its directional components quantified dynamic postural stability during a single-leg jump landing. Normalized vertical ground reaction force data quantified energy absorption. RESULTS: DPSI values revealed that females had significantly different dynamic postural stability as compared to males in the vertical plane [T (78)=-4.2, P<0.01], and in the composite score (dynamic postural stability index) [T (78)=-6.3, P<0.01]. In addition, females had significantly higher peak vertical ground reaction forces [T (78)=-13, P=0.01] than males. The DPSI also showed excellent reliability (ICC=0.96), with a 95% confidence interval ranging from 0.94 to 0.97. CONCLUSIONS: The results indicate that females have higher dynamic postural stability scores in the vertical direction as well as the composite score. This suggests that females used different dynamic postural stability strategies than males. There were no side-to-side dynamic postural stability differences between healthy contralateral limbs.

Adult↗
PubMed2003

Postural stability changes in the elderly with cataract simulation and refractive blur.

PURPOSE: To determine the influence of cataractous and refractive blur on postural stability and limb-load asymmetry (LLA) and to establish how postural stability changes with the spatial frequency and contrast of the visual stimulus. METHODS: Thirteen elderly subjects (mean age, 70.76 +/- 4.14 [SD] years) with no history of falls and normal vision were recruited. Postural stability was determined as the root mean square [RMS] of the center of pressure (COP) signal in the anterior-posterior (A-P) and medial-lateral directions and LLA was determined as the ratio of the average body weight placed on the more-loaded limb to the less-loaded limb, recorded during a 30-second period. Data were collected under normal standing conditions and with somatosensory system input disrupted. Measurements were repeated with four visual targets with high (8 cyc/deg) or low (2 cyc/deg) spatial frequency and high (Weber contrast, approximately 95%) or low (Weber contrast, approximately 25%) contrast. Postural stability was measured under conditions of binocular refractive blur of 0, 1, 2, 4, and 8 D and with cataract simulation. The data were analyzed in a population-averaged linear model. RESULTS: The cataract simulation caused significant increases in postural instability equivalent to that caused by 8-D blur conditions, and its effect was greater when the input from the somatosensory system was disrupted. High spatial frequency targets increased postural instability. Refractive blur, cataract simulation, or eye closure had no effect on LLA. CONCLUSIONS: Findings indicate that cataractous and refractive blur increase postural instability, and show why the elderly, many of whom have poor vision along with musculoskeletal and central nervous system degeneration, are at greater risk of falling. Findings also highlight that changes in contrast sensitivity rather than resolution changes are responsible for increasing postural instability. Providing low spatial frequency information in certain environments may be useful in maintaining postural stability. Correcting visual impairment caused by uncorrected refractive error and cataracts could be a useful intervention strategy to help prevent falls and fall-related injuries in the elderly.

Aged↗
PubMed2001

[Gait disorders in Parkinson disease. Clinical description, analysis of posture, initiation of stabilized gait].

A WELL INFORMED DESCRIPTION: The parkinsonian posture is generally described as a stooped one. At the beginning of the disease, the gait troubles remain moderate; gradually the gait is composed of small steps without a wide base; the patient tends to run after his centre of gravity by accelerating the step (festination phenomenon). Difficulties occurs for starting up (delay of gait initiation), for about-turn or for clearing obstacles. Kinetic jammings and standing around (freezing) can last several seconds and be responsible for falls. POSTURAL INSTABILITY, A MAJOR SYMPTOM IN PARKINSON'S DISEASE: This symptom is little improved by therapies and is responsible for serious disability. Postural instability induces a disequilibrium and is partially due to a simultaneous antagonist muscles contraction and to the impossibility of modifying postural responses to changing support conditions. The passive viscoelastic properties of muscles and tendons constitute a first line of defence against the disequilibrium and contribute to postural stability in the case of medium disturbances. Automatic and voluntary postural responses which come into play in the case of major disturbances can also be impaired (delay or defect of the responses). GAIT INITIATION FAILURE ARE FREQUENT: They result from an increase of the postural phase and a decrease of the propulsion forces, depending on a deficit of the postural anticipation mechanisms and also the sequential organization and the integration of two different motor programs, postural and locomotor. They can be controlled partially with sensory stimuli, notably visual inputs. DATA CONCERNING STABILIZED WALKING AND ITS PATHOPHYSIOLOGY REMAINS TO BE CLARIFIED: Spatial and temporal parameters are impaired: speed, step length and swing phase are reduced, while cadence increases to compensate these troubles. These modifications are the consequence of an incapacity to produce internal marks to generate regular steps. When the parkinsonian patient is supplied with external marks, these parameters can be normalized. From a pathophysiological point of view, gait disorders could result from defective central integration of proprioceptive information during movement within the basal ganglia, associated with a visual perceptive deficit linked with a retinal dopaminergic cells dysfunction and finally from an impairment of the proprioceptive feedback of the load receptors in the leg extensor muscles.

Antiparkinson Agents↗
PubMed2003

Postural control after a 2-mile run.

Postural control is essential for normal daily activities. It is also a crucial requirement for athletic events and activities. Deterioration in this postural control mechanism is termed postural instability and results in increased postural sway. Decrements in postural sway have been documented following a 15.5-mile run. It remains to be seen whether a shorter distance run will cause a similar decrease in postural control. Therefore, the purpose of this study was to assess the effects of a 2-mile run on postural sway in healthy subjects. Forty-four healthy subjects were assigned to an experimental group (2-mile run) or a control group. Postural sway was measured before and after the run with the Biodex Stability System Overall Stability Index and a clinical balance test called the Military Academy Stance Test. There was a significant before-after run Biodex Stability System Overall Stability Index (p < 0.05) and Military Academy Stance Test index (p < 0.05) increase in the experimental group. There was no significant change for the control group. It was concluded that a 2-mile run does produce a significant increase in postural sway as measured using the Biodex Stability System Overall Stability Index and Military Academy Stance Test.

Adult↗
PubMed1988

Effect of posture and locomotion on energy expenditure.

Energy expenditure for human adults and infants and for dogs was measured in resting (supine or lateral) posture, in bipedal posture and locomotion, and in quadrupedal posture and locomotion. Variations in respiratory and heart rate and in body temperature were utilized in this comparative study. Oxygen consumption was also measured in human adults. In human adults, bipedal posture and locomotion were shown to be much less energy-consuming than corresponding quadrupedal posture and locomotion. The opposite was observed in adult dogs, where bipedalism was shown to be much more energy-consuming than quadrupedalism. In addition, this study demonstrated, for human adults in their natural erect posture, an energy expenditure barely higher than in supine or lateral resting posture, while the dogs in their natural quadrupedal stance, the energy expenditure is much higher than in their resting posture. With respect to energy, therefore, humans are more adapted to bipedalism than dogs to quadrupedalism. Human children, at the transitional stage between quadrupedalism and bipedalism, have high and almost equal requirements for all postures and locomotions. This demonstrates, in term of energy, their incomplete adaptation to erect behavior.

Adolescent↗
PubMed2006

Corticomuscular coherence is increased in the small postural tremor of Parkinson's disease.

The mechanisms and electrophysiological characteristics of the postural tremor in Parkinson's disease (PD) have not been defined. We hypothesized that PD subjects with small amplitude postural tremor would show increased corticomuscular coherence at certain frequencies compared to PD subjects without visible tremor. Four groups of participants were studied: (1) Control without postural tremor, (2) Control with small amplitude postural tremor, (3) PD without postural tremor, and (4) PD with small amplitude postural tremor. Accelerometry and electroencephalography-electromyography fast-fourier transform and corticomuscular coherence spectra were generated. Findings showed (1) elevated corticomuscular coherence centered at 12-18 Hz in PD with small amplitude postural tremor; (2) 5-12 Hz accelerometer frequency peaks that did not shift with increasing weight loads in some individuals; and (3) 5-8 Hz accelerometer peaks that shifted frequency with increasing weight loads, consistent with a peripheral-mechanical oscillator in all groups. The small amplitude postural tremor in PD arises from heterogeneous oscillator mechanisms. The discovery of increased corticomuscular coupling shows cortical involvement in the small amplitude postural tremor of PD.

Aged↗
PubMed1989

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.

Adult↗
PubMed1990

Postural strategies associated with somatosensory and vestibular loss.

This study examines the roles of somatosensory and vestibular information in the coordination of postural responses. The role of somatosensory information was examined by comparing postural responses of healthy control subjects prior to and following somatosensory loss due to hypoxic anesthesia of the feet and ankles. The role of vestibular information was evaluated by comparing the postural responses of control subjects and patients with bilateral vestibular loss. Postural responses were quantified by measuring 1) spatial and temporal characteristics of leg and trunk EMG activation; 2) ankle, knee, and hip joint kinematics, and 3) surface forces in response to anterior and posterior surface translations under different visual and surface conditions. Results showed that neither vestibular nor somatosensory loss resulted in delayed or disorganized postural responses. However, both types of sensory deficits altered the type of postural response selected under a given set of conditions. Somatosensory loss resulted in an increased hip strategy for postural correction, similar to the movement strategy used by control subjects while standing across a shortened surface. Vestibular loss resulted in a normal ankle strategy but lack of a hip strategy, even when required for the task of maintaining equilibrium on a shortened surface. Neither somatosensory nor vestibular loss resulted in difficulty in utilizing remaining sensory information for orientation during quiet stance. These results support the hypothesis that cutaneous and joint somatosensory information from the feet and ankles may play an important role in assuring that the form of postural movements are appropriate for the current biomechanical constraints of the surface and/or foot. The results also suggest that vestibular information is necessary in controlling equilibrium in a task requiring use of the hip strategy. Thus, both somatosensory and vestibular sensory information play important roles in the selection of postural movement strategies appropriate for their environmental contexts.

Adult↗
PubMed1986

Dynamic posture analysis of Spacelab-1 crew members.

Dynamic posture testing was conducted on the science crew of the Spacelab-1 mission on a single axis linear motion platform. Tests took place in pre- and post-flight sessions lasting approximately 20 min each. The pre-flight tests were widely spaced over the several months prior to the mission while the post-flight tests were conducted over the first, second, fourth, and sixth days after landing. Two of the crew members were also tested on the day of landing. Consistent with previous postural testing conducted on flight crews, these crew members were able to complete simple postural tasks to an acceptable level even in the first few hours after landing. Our tests were designed to induce dynamic postural responses using a variety of stimuli and from these responses, evaluate subtle changes in the postural control system which had occurred over the duration of the flight. Periodic sampling post-flight allowed us to observe the time course of readaptation to terrestrial life. Our observations of hip and shoulder position, when subjected to careful analysis, indicated modification of the postural response from pre- to post-flight and that demonstrable adjustments in the dynamic control of their postural systems were taking place in the first few days after flight. For transient stimuli where the platform on which they were asked to stand quickly moved a few centimeters fore or aft then stopped, ballistic or open loop 'programs' would closely characterize the response. During these responses the desired target position was not always achieved and of equal importance not always properly corrected some 15 seconds after the platform ceased to move. The persistent observation was that the subjects had a much stronger dependence on visual stabilization post-flight than pre-flight. This was best illustrated by a slow or only partial recovery to an upward posture after a transient base-of-support movement with eyes open. Postural responses to persistent wideband pseudorandom base-of-support translation were modeled as time invarient linear systems arrived at by Kalman adaptive filter techniques. Derived model parameters such as damping factor and fundamental frequency of the closed loop system showed significant modification between pre- and post-flight. This phenomenon is best characterized by movement of the poles toward increasing stability. While pre-flight data tended to show shoulders and hips moving in phase with each other, post-flight data showed a more disjoint behavior.(ABSTRACT TRUNCATED AT 400 WORDS)

Adaptation, Physiological↗
PubMed1995

Effects of the spatio-temporal structure of optical flow on postural readjustments in man.

How does the spatio-temporal structure of an oscillating radial optical flow affect postural stability? In order to investigate this problem, two different types of stimulus pattern were presented to human subjects. These stimuli were generated either with a constant spatial frequency or with a spatial frequency gradient providing monocular depth cues. When the stimulation was set in motion, the gain response of the antero-posterior postural changes depended upon the oscillation frequency of the visual scene. The amplitude of the postural response did not change with the amplitude of the visual scene motion. The spatial orientation of the postural sway (major axis of sway) depended strictly and solely on the structure of the visual scene. In static conditions, depth information resulting from the presence of a spatial frequency gradient enhanced postural stability. When set in motion, a visual scene with a spatial frequency gradient induced an organization of postural sway in the direction of the visual motion. Considering visual dynamic cues, postural instability depended linearly both on the logarithm of the velocity and on the logarithm of the temporal frequency. A nonlinear relationship existed between the amplitude of the fore-aft postural sway at the driving frequency and the temporal frequency, with a peak around 2-4 Hz. These results are discussed in terms of their implications for the separation of visual and biomechanical factors influencing visuo-postural control.

Adult↗
PubMed1989

Acquisition of co-ordination between posture and movement in a bimanual task.

The acquisition of co-ordination between posture and movement was investigated in human subjects performing a load lifting task. Sitting subjects held their left (postural) forearm in a horizontal position while supporting a 1 kg load via an electromagnet. Perturbation of the postural forearm position consisted of the load release triggered either by the experimenter (control) or by the subject voluntarily moving the other arm. In the latter case, the movement involved the elbow joint (load lifting (A), isometric force change at the wrist level (B), elbow rotation (C) and pressing a button with the wrist (D] or the fingers (grip isometric force change). We recorded the maximal amplitude and maximal velocity of the rotation of the postural forearm, the EMG of the forearm flexors on both sides and the force exerted either by the load on the postural arm or by the isometric contraction of the moving arm. The maximal forearm angular velocity after unloading was known to be related to the level of muscle contraction before unloading. 1. In the control situation, repetition of the imposed unloading test resulted in a progressive reduction in the maximal forearm rotation without any decrease in the maximal velocity. The amplitude and duration of the unloading reflex were found to increase in parallel. These results suggest that an adaptive mechanism took place which increased the gain of the unloading reflex loop and reduced the mechanical effect of the perturbation. This mechanism was found to come into play not only in the control situation but also in other paradigms where the perturbation was expected by the subjects. 2. A decrease in both maximal amplitude and velocity of forearm rotation together with a weak "anticipatory" deactivation of the forearm postural flexors was observed when the unloading was caused by an elbow movement (situations A, B, C) which indicates that a feedforward postural control took place. An interlimb coordination was built up and stabilized after 40-60 trials. Pressing a button with the wrist (weak force and displacement) was a less effective means of inducing an anticipatory control of the flexors of the postural forearm, which indicates that the intensity of the central control plays a role in the building up of the coordination. 3. A distal grip action exerting either weak (100 g) or a high (1 kg) force was able to reduce the maximal amplitude of the forearm rotation, but not the maximal velocity, which indicates that an improved reflex action takes place, but not a feedforward anticipatory postural control.(ABSTRACT TRUNCATED AT 400 WORDS)

Elbow Joint↗
PubMed1986

Postural interneurons in the abdominal nervous system of lobster. III. Pathways mediating intersegmental spread of excitation.

The multisegmented abdomen of crayfish and lobster assumes a variety of postures as components of different behavioral acts. Experimentally these postures can be maintained by activating any of a number of premotor positioning interneurons. The pathways by which the motor output in two or more segments is coordinated were here investigated for a small group of identified postural interneurons whose somata lie in the 2nd abdominal ganglion (A2). Stimulation of all postural interneurons examined evokes a motor output in other abdominal ganglia through which the axon of the neuron passes as well as in the ganglion of origin (ganglion containing the neuron's cell body). The spread of motor excitation away from the originating ganglion occurs via two general pathways. In the first pathway connections to postural motoneurons are made directly by processes of the postural interneuron which pass into ganglia distal to the originating ganglion. Examples of this are shown for two flexion producing interneurons (FPIs) 201 and 301. Each of these FPIs makes monosynaptic connections with motoneurons in A2 and with a homologous set of motoneurons in A3. All postural interneurons fired a set of corollary discharge interneurons (CDIs) whose activities were recorded from the abdominal connectives. Two FPIs, 202 and 301, and a third interneuron, 503, produced motor outputs in ganglia to which they did not project. The motor specificity established in A2 by stimulation of FPIs 202 and 301 (whose axons pass caudally) was preserved in more rostral ganglia, such as A1. Therefore, different sets of CDIs can be specifically recruited to spread the same motor program that is initiated in the originating ganglion to ganglia that do not receive projections from the stimulated postural interneuron. CDIs, in addition, have the capacity to elicit motor programs in distal ganglia that are markedly different from that expressed in the ganglion of origin. For example, although 503 produced an inhibitory output in the abdominal ganglia that it innervated (A1 and A2), a flexion response was generated by it in more caudal ganglia. The caudal flexion response was mediated in part through a monosynaptic activation of FPI 201 and through other unidentified CDIs. Thus, the interneuronal circuitry for postural control is composed of numerous components, some of which have regional control over different portions of the abdominal nerve cord. Depending upon the required movement, select components are coactivated, either serially or in parallel, to effect a variety of spatially distinct positions.

Abdomen↗