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PubMed1992

Postural disturbances due to homonymous hemianopic visual ataxia.

Patients with visual defects often complain of disturbances of equilibrium. In order to measure the influence of visual stimulation on posture, a study was made comparing the posture of hemianopic patients with that of healthy volunteers before and after occlusion of half of the monocular visual field. Hemianopia increases lateral oscillations in patients in the standing position and the projection of the body's centre of gravity shifts towards the hemianopia: in the volunteers, the same shift is noted after masking half the visual field. A tonic effect of visual information is discussed.

Adult↗
PubMed1993

Vestibular induced postural responses in Parkinson's disease.

We have tested the hypothesis that dysfunction of the vestibular control of posture is a principal cause of parkinsonian instability by measuring the body sway response induced by galvanic vestibular stimulation (0.5 mA for 2 s) in a group of patients with Parkinson's disease (n = 15). Responses were compared with those obtained from a group of age-matched control subjects (n = 10). Subjects were stimulated (randomized polarity) whilst standing with feet together, eyes closed and maintaining one of five head yaw angles. The motion of the body and the ground reaction forces were measured in three dimensions. There was no significant difference between patients and controls in the speed or direction of the induced body sway response. When the patients were subdivided into two groups according to a clinical assessment of postural deficit, the more disabled group was found to respond with significantly greater body speed than either the control group or the mildly affected patient group. However, the baseline speed of spontaneous body sway was also greater in these patients and it was found that response speed was linearly related to baseline body sway even for the control group. There were no significant differences between any of these groups in the latency to onset, latency to peak or peak amplitude of the initial horizontal ground reaction force response to the stimulus. We conclude that vestibular dysfunction does not explain the postural deficits of patients who are mildly or moderately affected by Parkinson's disease.

Biomechanical Phenomena↗
PubMed1999

Brain activation during maintenance of standing postures in humans.

The regulatory mechanism of bipedal standing in humans remains to be elucidated. We investigated neural substrates for maintaining standing postures in humans using PET with our mobile gantry PET system. Normal volunteers were instructed to adopt several postures: supine with eyes open toward a target; standing with feet together and eyes open or eyes closed; and standing on one foot or with two feet in a tandem relationship with eyes open toward the target. Compared with the supine posture, standing with feet together activated the cerebellar anterior lobe and the right visual cortex (Brodmann area 18/19), and standing on one foot increased cerebral blood flow in the cerebellar anterior vermis and the posterior lobe lateral cortex ipsilateral to the weight-bearing side. Standing in tandem was accompanied by activation within the visual association cortex, the anterior and posterior vermis as well as within the midbrain. Standing with eyes closed activated the prefrontal cortex (Brodmann area 8/9). Our findings confirmed that the cerebellar vermis efferent system plays an important role in maintenance of standing posture and suggested that the visual association cortex may subserve regulating postural equilibrium while standing.

Adult↗
PubMed2002

Deficits and recovery of head and trunk orientation and stabilization after unilateral vestibular loss.

The aim of the study was to analyse changes in the orientation and stabilization of the head and trunk and their recovery after complete unilateral loss of vestibular information in humans. The ability of nine Ménière's patients to orient and stabilize their heads and trunks in space was investigated during a simple dynamic task of knee-bends and compared with the performance of 10 healthy subjects. Patients' performance was recorded before unilateral vestibular neurotomy (UVN) and during the time-course of recovery (1 week, 1 month, 3 months). Experiments were performed both in eyes open (EO) and eyes closed (EC) conditions to evaluate the role of visual cues in the recovery process. Head and trunk mean angular position (orientation) and mean maximal angular rotation (stabilization) in the roll plane and the yaw plane were recorded using a video motion analysis system. The results indicate that, in the acute stage after UVN (1 week), patients exhibit marked impairments in head and trunk orientation in both visual conditions. In the EC condition, head and trunk were deviated towards the operated side in the roll plane and the yaw plane. Head and trunk stabilization in space was impaired in the roll plane and associated with increased stabilization of the head on the shoulders. Interestingly, vision caused a complete inversion of the orientation pattern, with head and trunk rotations towards the intact side in the roll plane and the yaw plane. Relative to darkness, vision also reduced head and trunk oscillations. Recovery from abnormal head orientation in the light and impaired head stability in both visual conditions was achieved within 1 month and 3 months after UVN, respectively. However, head and trunk orientation in the dark and trunk stabilization in the roll plane remained uncompensated 3 months post-lesion. These results suggest that unilateral vestibular loss leads to a postural syndrome similar to that described previously for various animal species. They confirm the necessity of vestibular inputs for properly stabilizing head and trunk during self-generated displacements in healthy subjects. They also support the notion that vestibular compensation relies on visual cues whose substitution role gradually decreases after UVN.

Adaptation, Physiological↗
PubMed2002

Postural instability in idiopathic Parkinson's disease: the role of medication and unilateral pallidotomy.

Postural instability (PI) is common in idiopathic Parkinson's disease (IPD). We measured sensory and motor contributions to PI in 50 patients with advanced IPD, off and on medication and in a subset pre- and 3, 6 and 12 months post-unilateral pallidotomy, using computerized dynamic posturography [specifically, the Sensory Organization Test (SOT) and the Unified Parkinson's Disease Rating Scale (UPDRS) subscale PIGD (Postural Instability and Gait Disorder)]. Off medication, all patients had abnormal PIGD scores. The group could be separated into those with normal SOT equilibrium scores (SOTN) and those, the majority, with abnormal postural control when sensory feedback was limited (SOTABN). Medication improved the PIGD scores but worsened the SOT scores in the majority of patients. Increases in spontaneous sway in some patients contributed to the negative effect of medication on SOT scores. However, this could not explain the detrimental effect of medication on SOT scores in at least 40% of patients. On the other hand, pallidotomy improved both PIGD and SOT scores in both groups. A predictor of good outcome from pallidotomy concerning PI was the degree of worsening of the effect that medication had on SOT5 scores. PI in IPD appears to be multifactorial. We propose that the PIGD score reflects sensory and motor aspects of postural control, with normal sensory feedback, while the SOT equilibrium scores measure the sensory organizational process of postural control in the presence of altered sensory inputs. There is a dissociation between the effects of medication and pallidotomy on motor and sensory components of postural control, which may reflect the underlying pathophysiological mechanism responsible for these different components of PI. We suggest that patients with advanced IPD and PI on medication should consider adjuvant surgical treatment for better postural control.

Adult↗
PubMed2003

Effect of chronic bilateral subthalamic nucleus (STN) stimulation on postural control in Parkinson's disease.

Postural instability is one of the most incapacitating factors in Parkinson's disease (PD). The underlying deficits and the effects of treatment are still not well understood. The aims of the present study were: (i) to identify abnormalities of postural control in PD patients during unperturbed stance and externally perturbed stance (anterior-posterior tilts of the support surface and of the visual scene); (ii) to assess the effects of L-dopa medication and subthalamic nucleus (STN) stimulation on posture control; and (iii) to characterize potential differential or additive effects of both treatments. Eight PD patients under chronic STN stimulation were investigated and compared with 10 normal controls. The assessment was performed in a crossover design (+/- STN stimulation, +/- L-dopa). During unperturbed stance, we recorded measures of spontaneous sway in terms of displacement, velocity and frequency of the centre of pressure (COP), lower body (LB) and upper body (UB) excursions. In addition, inter-segmental UB-LB coupling was investigated as a measure of axial stiffness. All these measures were abnormally large in patients OFF treatment. Under L-dopa treatment, the velocity, frequency and coupling measures were reduced, whereas sway amplitude increased. Very similar effects were obtained under STN stimulation, and these effects became more pronounced in the combined treatment condition. In these data, reduction of inter-segmental coupling correlated with increase in sway amplitude. The finding suggests that axial stiffness reduction under treatment revealed a treatment- resistant deficit in the sensorimotor postural control loop. However, these two effects did not correlate with the motor subscores of the unified Parkinson's disease rating scale (UPDRS), which indicates that they are of minor functional relevance for posture control. A frequency peak in the COP excursions at 0.7-1.1 Hz, which we take to indicate a resonance behaviour of the postural control loop, became reduced under therapy. The reduction of this peak did correlate with most improvements in the UPDRS under therapy. Support surface tilt revealed that an UB righting on the LB segment, which is present in normal controls, is missing in the patients. The postural responses to visual tilt were abnormally large in patients, independent of whether the support was stable or slightly moving, while the control subjects clearly profited from a stable support. This finding suggests that PD patients lack the ability of normal subjects to use sensory or cognitive information when suppressing the destabilizing effect of visual tilt. These abnormal tilt reactions of the patients were resistant to treatment with L-dopa, STN stimulation and a combination of the two. Overall, the effects of STN stimulation on posture control essentially paralleled those of L-dopa during both unperturbed and externally perturbed stance.

Activities of Daily Living↗
PubMed1998

Aging and time to instability in posture.

Two experiments are reported that were set up to examine the spatial-temporal boundaries of postural instability in upright stance as a function of age (60-96 years) and postural conditions. Subjects stood on a force platform under different experimental conditions (vision/no vision and arms up/down) so that the effect of age on key dynamic properties of postural stability could be determined. The findings showed that the ratio of the area of the motion of the center of pressure to the area within the stability boundary increased with age. Also, the virtual time-to-contact with the postural stability boundary decreased with age. Collectively, the findings show that the margins to the spatial-temporal boundaries of postural stability decrease with advancing age in the elderly. These reduced margins of dynamic stability may be a factor contributing to the progressive instability of posture with aging in the elderly.

Aged↗
PubMed1999

Risk factors for falls in hospitalized older medical patients.

BACKGROUND: The incidence of falls among older hospitalized patients is higher than that of community-dwelling older persons. Prevention is important, but factors associated with these falls are less well studied than falls occurring in the community or nursing homes. METHODS: This study was conducted in an acute-care general hospital in Hong Kong. During November 1995 to March 1997, all older inpatients who fell during hospitalization were assessed by a geriatrician, a physiotherapist, and an occupational therapist. A standardized protocol to study the clinical and performance-oriented functional factors was employed. A sex- and age-matched hospital control was recruited for each case. In total, 51 cases and controls (mean ages 77.8 +/- 7.3 years and 77.5 +/- 7.0 years, respectively) were studied. RESULTS: Among the multiple clinical and functional risk factors for falls identified, lower limb weakness (i.e., power less than MRC grade 4 in one or both lower limbs) and poor tandem walk ability emerged as two significant predictive factors for falls in the hospital. The overall classification accuracy of fallers and nonfallers was 79%. The sensitivity was 84% and specificity was 75%. Clinical factors were the underlying causes for the lower limb weakness and poor tandem walk performance. CONCLUSIONS: Among clinical and functional risk factors for falls in the older medical patient, lower limb weakness and poor tandem walk ability were most predictive. Falls prevention programs in hospitals should employ these two tests as screening instruments.

Accidental Falls↗
PubMed1999

The sex and age of older adults influence the outcome of induced trips.

BACKGROUND: Falls are a significant source of morbidity and mortality in older adults, with up to 53% of these falls due to tripping. To aid in fall prevention, there is a need to identify the factors that determine whether a trip is recoverable and those factors that increase an older adult's risk of falling. METHODS: Trips were induced during gait in 79 healthy, community-dwelling, safety-harnessed older adults (50 women) using a concealed, mechanical obstacle. Trip outcomes were graded as recoveries, falls, rope-assists, or misses. Kinematics were recorded during normal gait, without and with the safety harness. Selected gait parameters were compared to determine whether the experimental conditions affected gait at the time of the trip. RESULTS: Thirty-nine trip outcomes were classified as recoveries, 10 as falls, 12 as rope-assists, and 18 as misses. Women fell more than four times as frequently as men. Women younger than 70 years fell more than three times as frequently as those older. Trip outcomes in the men were essentially unaffected by age. The foot obstructed to induce the trip did not affect the trip outcome. The presence of the safety harness had almost no effect on gait. The length of the stride preceding the trip did not differ from normal. CONCLUSIONS: The majority of trips in healthy older adults did not result in falls. Older women were more likely than men to fall following a trip. The likelihood of falling from a trip was greatest in the youngest older women.

Accidental Falls↗
PubMed1999

Phase-dependent modulation of proximal and distal postural responses to slips in young and older adults.

BACKGROUND: Phase-dependent modulation of postural responses plays an important functional role in integrating reflexes into ongoing locomotion behaviors. This study tested the hypotheses that proximal and distal postural responses are modulated differently according to the phases of the gait cycle in young adults and that there is a decline in this modulatory ability with normal aging. METHODS: Thirty-three healthy young adults (age = 25 +/- 4 years) and 32 healthy older adults (mean age 74 +/- 14 years) participated. Subjects walked across a movable force plate with its movement timed to heel strike or midstance to simulate a forward slip occurring at different times during the gait cycle. Surface electromyography was recorded from bilateral leg, thigh, hip, and trunk muscles. Kinematic data were collected from the right (perturbed) side of the body. RESULTS: Postural responses to the heel strike slips occurred more frequently, and were of shorter latency, longer burst duration, and greater burst magnitude, than those in response to the midstance slips. Whereas the early and predominant postural responses came from bilateral tibialis anterior, rectus femoris, and biceps femoris muscles in heel strike slips, early postural responses were observed in bilateral erector spinae muscles in midstance slips. The late postural responses in midstance slips (from bilateral biceps femoris muscles and medial gastrocnemius and tibialis anterior of the nonperturbed leg) assisted in foot liftoff of the perturbed leg and earlier and safe landing with the nonperturbed leg. In response to the heel strike versus midstance slips, older adults preserved the phase-dependent modulatory abilities of the occurrence, onset latency, and burst duration of their postural responses, but not the ability to modulate burst magnitude, as compared to young adults. CONCLUSION: Postural responses from the proximal and distal muscles in reaction to different temporal phasing of slips during the step cycle showed differential modulation to meet the different task requirements. Older adults preserve this modulatory ability but with limited capacity. Physiological or psychological factors may influence older adults' phase-dependent modulatory capacity.

Accidental Falls↗
PubMed2002

Attentional dynamics in postural control during perturbations in young and older adults.

Temporal dynamics of attention during postural perturbations in young and older adults were investigated. Nineteen young and older subjects performed simple reaction time tasks during translational platform perturbations. Auditory or visual stimuli were presented randomly at delays from the onset of the platform perturbation. Reaction time was slowed before and during the platform movement, particularly in older subjects. Reaction times to stimuli presented at a 250-millisecond delay or later were not influenced by perturbation. The reaction times to the auditory stimuli were influenced more by perturbation than were those to visual targets. The postural response was unaltered by the presence of the reaction time task, but it differed between groups. Attention is engaged in response to a perturbation during the preperturbation time and during the initiation of the postural response in young and older adults. Sensory selection occurs in young and older adults, but to a greater degree in older subjects.

Accidental Falls↗
PubMed2002

Age influences the outcome of a slipping perturbation during initial but not repeated exposures.

BACKGROUND: Fall incidence in older adults might be reduced through learning to better recover from or adjust to perturbations. Extents of age-related declines and limitations in the ability to recover are not well established, however. METHODS: Slips were induced, using bilateral low-friction platforms, during a sit-to-stand task in 60 young and 41 older, healthy, safety-harnessed adults. Subjects underwent five slips, a block of nonslipping trials, then two reexposures to the slip. The first slip was novel and unexpected. Age-group and trial effects on fall incidence (evidenced by excessive hip descent) and on the direction of the initial protective step were examined. RESULTS: More older than young adults fell upon the first slip (73% vs 28%; p <.001). With repeated exposure, fall incidence decreased at similar exponential rates in both age groups. All but one subject eventually learned to slip without falling, and two-thirds of fallers fell only once. Repeat fallers fell without stepping in 63% of falls. Upon later slip reexposure, more older than young adults fell (20% vs 2%), but fewer falls occurred than did originally (p =.001). Likelihoods of forward and backward stepping during successful recovery changed with repeated slip exposure and upon reexposure, but did not differ between age groups. CONCLUSIONS: Older adults are more likely to fall upon initial, unexpected perturbation exposure, but, upon repeated exposure, healthy young and older adults rapidly learn to avoid falling at a similar rate. Healthy older adults appear fully capable of learning to better recover from or adjust to a perturbation through repeated exposure.

Accidental Falls↗
PubMed2005

Upper and lower limb muscle power relationships in mobility-limited older adults.

BACKGROUND: Lower limb muscle power impairments are modifiable factors underlying mobility limitations in older adults. This study examined relationships between upper and lower limb muscle power and their role in predicting mobility performance among community-dwelling older adults. METHODS: A cross-sectional analysis was conducted. Participants included 37 mobility-limited adults (24 women, 13 men), aged 65 to 93 years. Measures included upper (elbow extension) and lower limb (double leg press) one repetition maximum (1RM), and muscle power at both 40% and 70% one repetition maximum. Physical performance measures included stair climb time, the Short Physical Performance Battery, and 4-meter walk time. Factors commonly mediating the relationship between impairments and physical performance were analyzed as covariates. RESULTS: Participants had a mean age of 76 years, had five chronic medical conditions, and manifested moderate mobility limitations. Although the associations between the upper and lower limbs were strong (p <.001), the magnitude of association was greater for power (r =.88-.89) as compared to strength (r =.69). Multivariate regression analyses revealed consistently strong relationships between limb muscle power and mobility performance measures. Substituting upper for lower limb power within these models did not materially weaken the relationships. CONCLUSION: Muscle power appears to be a more generalized attribute between the upper and lower limbs than is muscle strength, suggesting that mechanisms underlying velocity of movement, as opposed to force production, may be important factors underlying muscle power in elderly persons. Additionally, upper limb muscle power measures may serve as a useful surrogate measure of limb power having implications for clinicians and researchers.

Aged↗
PubMed2006

Age-related differences in the effect of a perceived threat to stability on postural control.

BACKGROUND: Studies indicate that the strategy for postural control may be affected by psychological factors, and that young adults respond to perceived threat to stability by increasing their ankle stiffness. The objective of this study was to compare the postural control strategy adopted by young and old adults when faced with postural threat induced by manipulating surface height. METHODS: Sixty elderly (mean age 77.5+/-4.4 years) and 20 young volunteers (mean age 21.5+/-3.7 years) participated in the study. Movement of the center of pressure (COP) was recorded for 60 seconds with a portable force-plate, as participants stood with eyes open and closed, both at ground level and 85 cm above ground level. Analysis of variance and Tukey-Kramer tests were conducted to determine the effects of age, postural threat, and vision on mean power frequency (MPF), on amplitude variability both in the anterior-posterior (AP) and medio-lateral (ML) directions, and on mean COP sway velocity, with the significance level set at p=.05. RESULTS: Postural threat did not affect COP measures for the young adults, whereas it induced greater changes in MPF in the AP and ML directions in the elderly participants. A similar trend was observed in the elderly group for amplitude variability in the ML direction and for mean COP sway velocity. CONCLUSIONS: In comparison with young adults, elderly adults tend to have an exaggerated postural response to conditions that are perceived as threatening to stability. This response involves primarily increases in MPF, with limited compensatory decreases in amplitude variability observed only in the ML direction.

Accidental Falls↗
PubMed1975

Aging and postural sway in women.

The effects of aging on postural sway in upright and forward lean stance were investigated. Postural sway was measured on a center of gravity apparatus using two age groups of female subjects 20 to 30 years old and 70 to 80 years old. The older adults demonstrated significantly larger sway areas than the young adults in both stance positions. The patterning of the center of gravity projections on the base of support tended to be similar under all conditions except in the young adults/upright position where the antero-postero excursion was larger than the medial-lateral. The mean locations of the center of gravity projections were often posterior and to the left of the geometric center of the base of support. The distance between the two points was least in the older adults/forward lean position, i.e., the experimental unit which demonstrated the largest area of sway.

Adult↗
PubMed1989

Age, functional postural reflexes, and voluntary sway.

This experiment considered age-related changes in functional relationships between postural reflexes and voluntary movement. Young and older adults received horizontal perturbations during normal stance and when engaged in voluntary sway. Electromyographic activity showed that (a) older adults had poorer coordination between postural reflexes and voluntary movement, and (b) their stabilizing responses to postural disturbances during voluntary sway were slower. In addition, the onsets of activity of functionally important muscles were less tightly bilaterally coupled, and patterns of muscle onsets were less stereotypically organized in older adults. The results suggest that older adults experience some breakdown in the timing and sequencing of muscle activity and in the functional coordination of their postural reflexes with voluntary sway.

Adolescent↗