Untangling biomechanical differences in perturbation-induced stepping strategies for lateral balance stability in older individuals.

When recovering balance from a lateral perturbation, younger adults tend to stabilize balance with a single lateral sidestep while older adults often take multistep responses. Using multiple steps to recover balance is consistently associated with increased fall risk, altered body center of mass (CoM) control and instability. The aim of this study was to compare the spatio-temporal stepping characteristics and the margin of stability (MoS) of single lateral sidesteps (LSS1) with the first and second steps of a two-step protective step sequence. Two-step sequences begin with either a cross-over step to the front or back, or a medial step followed by a lateral sidestep. Seventy-one older adults received random lateral waist-pull perturbations to either side. We hypothesized that LSS1 would be more stable (larger MoS) than either step in a two-step sequence. With some exceptions, utilizing a two-step sequence was associated with a reduced CoM velocity and distance between the base of support and CoM and decreased stability in the frontal plane following limb loading of the first and second step. There were no differences in the time available to arrest the extrapolated CoM at the end of a single lateral sidestep or the final step of a two-step sequence. Two-step sequences involving a cross-over step include more complex stepping trajectories and also challeng stability in the sagittal plane requiring a multidimensional balance correction. These results indicate important step type differences in center of mass control in recovering balance with a single lateral sidestep as opposed to a two-step sequence among older adults.

Perturbation-evoked lateral steps in older adults: Why take two steps when one will do?

BACKGROUND: Hip fractures in older adults often result from a fall in the lateral direction. While younger adults tend to recover balance from a lateral perturbation with a single lateral sidestep, older adults are prone to multistep responses which are associated with an increased fall risk. This study compared the stepping characteristics and stability of single and multistep responses to lateral perturbation in healthy older adults. METHODS: Eighty-four older adults received lateral waist-pull perturbations to either side. Spatio-temporal stepping characteristics and balance stability were quantified. FINDINGS: Fewer steps were taken to recover balance when the first step was a lateral sidestep. The stability margin of single lateral sidesteps was greater than medial sidesteps and cross-over steps to the back but not significantly different from single cross-over steps to the front at step termination. Single step responses were more stable than multistep responses at step termination and at step initiation for lateral sidesteps and cross-over steps to the front. The decreased stability of multistep responses was attributed to an increased center of mass velocity and a smaller distance between the center of mass and base-of-support at step termination. INTERPRETATION: Although lateral sidesteps result in fewer steps than cross-over steps to the front, the stability margin was not significantly different at step termination. These results suggest difficulty terminating center of mass motion and/or inefficient center of mass control differentiates single and multistep responses. Future studies should investigate perturbation training and/or hip abductor muscle conditioning as a means of improving compensatory stepping reactions.

The impact of hemolysis on Ortho-Clinical Diagnostic's ECi and Roche's elecsys immunoassay systems.

BACKGROUND: Hemolysis is regularly encountered in clinical specimens and often interferes with a variety of laboratory test methods. Although not widely recognized, immunoassays based on nonisotopic detection systems can also be affected by hemolysis. For this reason, we investigated the effect of differing amounts of hemolysis across a range of values for several immunoassays on the Ortho-Clinical Diagnostics ECi and Roche Elecsys platforms. METHODS: Hemolysate was prepared from whole blood and spiked at varying concentrations into pooled patient serum samples for different analytes. RESULTS: Out of the 21 analytes tested, six (28.6%) exhibited significant increases or decreases in measured concentrations with increasing amounts of hemolysis. CONCLUSIONS: Although immunoassays are generally thought to be impervious to hemolysis interference, hemolysis can interfere in immunoassay testing platforms. For these reasons, we recommend that laboratories conduct hemolysis interference studies for all laboratory test protocols.

Thresholds for inducing protective stepping responses to external perturbations of human standing.

Standing subjects were unexpectedly pulled forward to identify a threshold boundary that evokes stepping in terms of the size of the pull relative to the base of support (BoS). Performances in a range of sensorimotor tests were correlated with the threshold boundary parameters. Younger and older subjects were studied to identify age-related changes in stepping and the threshold boundaries. The threshold boundary had a forward limit (T(L)) that, when crossed, always made subjects step no matter how slowly they were pulled. As velocity increased, the threshold position that produced a step shifted nearer to the ankles. Eventually a pull velocity was reached above which velocity had no further effect and a position threshold (T(H)) was identified behind which subjects never stepped. Thus the position threshold boundary for stepping is a posterior-going sigmoidal function of perturbation velocity. Older subjects stepped more than the young (69% vs. 40% of trials). For the older subjects, T(L) (91% vs. 107% BoS) and T(H) (59% vs. 72% BoS) were closer to the ankles, and the transition between T(L) and T(H) occurred at lower velocities (96% vs. 121% BoS.s(-1)). Across the entire study population many sensorimotor factors were associated with T(L) and T(H). However, these associations were not present when age was removed as a factor. Thus, although the older subjects use protective stepping more often, this cannot be attributed directly to the sensorimotor factors tested here. It can be explained by stepping as a triggered response to the perturbation event rather than later sensory input about body movement.

Lateral stability during forward-induced stepping for dynamic balance recovery in young and older adults.

BACKGROUND: Balance dysfunction related to lateral instability has been associated with falls and fall-related injuries among older individuals. Protective stepping for dynamic balance recovery requires the effective control of lateral body motion. This study investigated the relationship between aging, falls, and lateral stability during forward-induced stepping for dynamic balance recovery. METHODS: Forward steps were induced by a motor-driven waist-pull system in 12 younger adults, 20 healthy community-dwelling older adult nonfallers, and 18 older adults who had reported falls. Group differences in kinetic and kinematic stepping characteristics for a range of postural disturbance magnitudes were evaluated. RESULTS: Despite group similarities in anticipatory postural adjustments for minimizing lateral instability, the older fallers demonstrated significantly greater sideways body motion toward the stepping side at first-step foot contact and a more laterally directed foot placement. During the first step, forward-stepping characteristics were generally comparable between the groups, but the older fallers had an earlier liftoff time and longer step duration. CONCLUSIONS: During forward-induced protective stepping, otherwise healthy older adults who had experienced falls showed particular differences in their control of lateral body motion that were not attributable to changes in anticipatory postural mechanisms. Aging changes in controlling lateral body motion during protective stepping appear to involve factors that intervene between the first-step liftoff and foot contact and/or adaptations in stepping patterns related to prior planning.

The influence of stimulus cue on the initiation of stepping in young and older adults.

OBJECTIVES: To investigate the influence of different reaction stimulus cues on the characteristics of ground reaction forces during the initiation of rapid forward stepping, and to determine whether age-related differences in step initiation are influenced by the type of stimulus cue used to trigger stepping. DESIGN: Case-control study. SETTING: University-based research laboratory. PARTICIPANTS: Fifteen healthy younger adults and 35 community-dwelling older adult volunteers. MAIN OUTCOME MEASURES: Subjects performed forward step initiation in response to 3 different reaction stimulus cues (light, sound, electrocutaneous) during simple reaction-time conditions. Ground reaction force data were collected and used to characterize the timing and magnitude features of the postural (weight-transfer) and step components. RESULTS: In comparison with the light and sound stimuli, the nonnoxious electrocutaneous trigger cue was associated with an increase in the magnitude of the initial displacement of the center of pressure (COP) in the mediolateral direction. Overall, older subjects were slower to initiate the postural and step components, and displayed a reduction in the initial posterior displacement of the COP. CONCLUSIONS: The postural component for lateral weight transfer was augmented by the electrocutaneous trigger cue, suggesting its potential to enhance step initiation among clinical populations. Age-related changes in stepping were unaffected by the type of stimulus cue, but the older group had a slower response initiation time and less forward propulsion. The abilities of many older persons may be compromised in situations where rapid adjustments in the base of support through stepping are triggered by environmental stimuli.

Passive tactile sensory input improves stability during standing.

The effects of passive tactile cues about body sway on stability during standing were evaluated in subjects with a wide range of sensorimotor and balance performance. Healthy young adults, diabetic subjects with varying degrees of peripheral sensory neuropathy and older subjects aged 70-80 years were studied. Body sway was measured when subjects stood on the floor and on a foam rubber mat, with or without an applied stimulus that rubbed on the skin at the leg or shoulder as the body swayed. The results show that this stimulus reduced body sway (mean reduction 24.8%+/-1.5) and thus had a stabilizing effect as big as vision or sensory information from the feet. The reduction in sway was not based on active touch. The stimulus was not restricted to a particular region of the body, but was more effective on the shoulder than the leg, and was more effective when standing with eyes shut or when standing on the foam mat. It was also most effective in those subjects who had the greatest sway during normal standing. Thus, the response appears to be graded with the amplitude of the stimulus. We concluded that, if passive sensory input about posture is available, the postural control process adapts to this input, modulating postural stabilizing reactions.

Vestibulopathy and the age effects on protective stepping during unperturbed standing.

This study examined the prevalence of protective stepping and accompanying preparatory postural responses associated with lateral weight transfer (WT) while subjects attempted to sustain stationary standing. The subjects were 92 healthy young and older adults and persons with vestibular hypofunction. Force platform and whole-body-motion recordings were used to evaluate the prevalence of stepping and WT responses during stationary standing (eyes open or closed) using a semi-tandem foot position. WT components were also evaluated for volitionally requested step initiation, and as a function of support base configuration and direction of stepping among younger subjects. Only 10% of trials by subjects with bilateral vestibular hypofunction (BVH) during semi-tandem standing with eyes closed were completed without a step, while 31% of subjects with UVH, 69% of healthy elders, and all young healthy subjects were able to stand for the entire 7-sec trials. WT responses always preceded volitional steps from a standard feet-parallel orientation but occurred in only 13% of the spontaneous steps. The prevalence of WT was influenced by the direction of volitional stepping from semi-tandem standing, but not by the initial standing width. Spontaneous stepping to maintain standing balance is a naturally occurring and prevalent behavior among older adults and persons with vestibular hypofunction during tests of quasi-static standing. Differences between volitional and spontaneous step initiation involving the prevalence of preparatory lateral WT are a complex function of motor planning, mechanical constraints, and functional context.

Lateral stability, sensorimotor function and falls in older people.

AIMS: To design simple tests of lateral stability for assessing balance in older people and to determine whether poor performances in these tests are associated with impaired vision, lower limb sensation, quadriceps strength, simple reaction time, and falling in this group. DESIGN: A cross-sectional and retrospective study. SETTING: Falls and Balance Laboratory, Prince of Wales Medical Research Institute. PARTICIPANTS: One hundred fifty-six community-dwelling men and women aged 63-90 years (mean age 76.5, SD = 5.1). OUTCOME MEASURES: The maximal lateral sway in a near-tandem stability test with eyes open and closed and the necessity of taking a protective step in the near-tandem stability test with eyes closed. RESULTS: All 156 subjects could complete the near-tandem stability test with eyes open, but only 99 subjects (63.5%) could undertake the test with eyes closed without taking a protective step. Subjects with a history of falls had increased lateral sway both with eyes open and eyes closed as well as poorer visual acuity, proprioception, and quadriceps strength. Fallers were also significantly more likely to take a protective step when undertaking the near-tandem stability test with eyes closed. Multiple regression analysis revealed that impaired lower limb proprioception, quadriceps strength, and reaction time were the best predictors of increased maximal sway in the near-tandem stability test with eyes open. Reduced proprioception and quadriceps strength, in addition to age, were also found to be the best determinants of the necessity of taking a protective step in the near-tandem stability test with eyes closed. CONCLUSIONS: The study findings identify simple new tests that are associated with falling in older people and elucidate the relative importance of specific physiological systems in the maintenance of lateral stability.

A closed-loop stepper motor waist-pull system for inducing protective stepping in humans.

Past approaches for inducing protective stepping to preserve standing balance in humans have either lacked the flexibility of control over the initial conditions of falling, or involved considerable mass which limits mobility of the system. This report describes the design and function of a stepper motor closed-loop waist-pull system for evoking protective stepping responses. Bench testing with applied load-motion profile combinations indicated performance degradation for force levels greater than 204 N which was well within the levels encountered in human experiments. An optical encoder feedback design allowed a positional accuracy of 0.00225 mm per step. A regression analysis of specified versus recorded velocities resulted in an acceptable fit (r2 = 0.99). The mean rise time was 63.0+/-18.0 (SD) ms and was consistent up to the load limits of the device. In human experiments repeated perturbations were consistently realized. Applied motion profiles were generally comparable at the pelvic level for subjects of different geometry, weight, and inertia despite a tendency for slight positional lag. The method allows flexible and accurate control of the initial conditions of a perturbation-induced fall to elicit steps. The system dimensions and moveability allow it to be implemented into clinical environments.

Static versus dynamic predictions of protective stepping following waist-pull perturbations in young and older adults.

The purposes of this study were: (1) to determine the frequency of protective stepping for balance recovery in subjects of different ages and fall-status, and (2) to compare predicted stepping based on a dynamic model (Pai and Patton, 1997. Journal of Biomechanics 30, 347 354) involving displacement and velocity combinations of the center of mass (COM) versus a static model based on displacement alone against experimentally induced stepping. Responses to three different magnitudes of forward waist pulls were recorded for 13 young, 18 older-non-fallers and 18 older-fallers. The COM phase plane trajectories derived from motion analysis were compared with the model-predicted threshold values for stepping. We found that the older fallers had the highest percentage of stepping trials (52%), followed by older-non-fallers (17.3%), and young (2.7%) at the lowest perturbation level. Younger subjects stepped less often than the elderly at the middle level. Everyone consistently stepped at the highest level of perturbation. Overall, the dynamic model showed better predictive capacity (65%) than the static model (5%) for estimating the initiation of stepping. Furthermore, the threshold for step initiation derived from the dynamic model could consistently predict when a step must occur. However, it was limited, especially among older fallers at the low perturbation level, in that it considered some steps 'unnecessary' that were presumably triggered by fear of falling or other factors.

Electromyographic analysis of postural responses during standing leg flexion in adults with hemiparesis.

The purpose of this study was to examine muscle activation patterns during standing leg single leg flexion in adults with hemiparesis. Specifically, the electromyographic activation patterns of the flexing limb biceps femoris and gluteus medius, and the stance limb gluteus medius muscles were analyzed as a function of whether the muscles were paretic or not. Delayed activation of the affected flexing side gluteus medius, as compared with unaffected flexing side gluteus medius, resulted in it being activated simultaneous with the flexing biceps femoris rather than preceding it as was previously found in healthy subjects. This suggests a temporal change in the sequential mode of coordination of the postural and intended components of the task. In addition, the magnitude of the electromyographic integrals of both the affected and unaffected flexing side gluteus medius in the early propulsive phase of the task was significantly reduced in comparison with healthy subjects. These alterations can be attributed to spatial alterations in the sequential form of organization or to a shift to a different mode of neural control in order to perform a relatively novel task. These results suggest a potential adaptive capacity in these individuals.

Disorders of posture, balance, and gait in Parkinson's disease.

Disorders of movement function related to posture, balance, and gait are common occurrences for many persons with Parkinson's disease. Numerous studies have identified a broad variety and heterogeneous distribution of postural and locomotor changes. These alterations are often associated with diminished functional ability, poor prognosis, and frequent falls. Present pharmacologic management appears to have a limited influence on postural instability and associated falls, and, despite anecdotal information, there is scant information on the therapeutic effectiveness of physical interventions aimed at enhancing these functions. Future advances would likely be facilitated by: (1) determining the nature and extent of the specific involvement of postural and locomotor control mechanisms; (2) coalescing findings on motor control processes with clinical information to develop rehabilitation approaches that target specific motor manifestations; and (3) conducting long-term clinical trials to demonstrate the efficacy of interventions.

Differential effects of exercise training intensity on blood pressure and cardiovascular responses to stress in borderline hypertensive humans.

BACKGROUND: Psychologic stress has been associated with the development of hypertension. Aerobic exercise training appears to decrease cardiovascular responses to psychologic stress. OBJECTIVE: To determine the efficacy of low-intensity and moderate-intensity exercise training in reducing blood pressure and cardiovascular responses to stress. DESIGN: We sought to compare the cardiovascular responses to a psychologic stressor, the Stroop Color Word Task (Stroop), before and after 12 weeks of low-intensity (about 45% maximal oxygen uptake) and moderate-intensity (about 75% maximal oxygen uptake) aerobic exercise training. METHODS: Eighteen borderline hypertensive subjects (resting blood pressure 139 +/- 9/92 +/- 9 mmHg) were divided randomly into three groups: control (no exercise), low-intensity exercise (40-50% maximal oxygen uptake), and moderate-intensity exercise (70-80% maximal oxygen uptake). Training groups exercised three times per week at the prescribed exercise intensity. Heart rate and blood pressure were recorded during the Stroop before, and after 4 and 8 weeks of exercise training. RESULTS: In the low-intensity exercise group, exercise training attenuated mean blood pressure, systolic blood pressure, and diastolic blood pressure responses to the Stroop and decreased resting blood pressure. The moderate-intensity exercise group demonstrated a reduced diastolic blood pressure response to the Stroop. CONCLUSIONS: These results suggest that, in borderline hypertensive humans, 12 weeks of aerobic exercise training attenuates the cardiovascular responses to the Stroop. Furthermore, low-intensity exercise training appears to be a more effective stimulus than moderate-intensity exercise training in reducing resting blood pressure and blood pressure responses to stress.

Stimulus parameters and inertial load: effects on the incidence of protective stepping responses in healthy human subjects.

OBJECTIVE: To test the hypotheses that the incidence of protective stepping in response to sudden translations of the support would (1) increase as a function of both the magnitude of surface displacement and velocity of platform movement, and (2) decrease in association with an increase in external loading applied to the body. DESIGN: A log-linear approach was used to analyze the incidence of stepping by testing several models incorporating different platform stimulus parameters (direction, displacement, velocity) and external loading (0% and 20% body weight). SETTING: Institutional-based research laboratory. PARTICIPANTS: Eight healthy younger adult (21 to 28 years) volunteers. MAIN OUTCOME MEASURES: The incidence and number of protective steps served as the primary planned outcome variables. RESULTS: Steps occurred more frequently for anterior (83 steps) versus posterior (45 steps) translations. Step occurrence was generally proportional to platform velocity, and increased with displacements up to 15cm, but then plateaued. External loading was associated with a reduction in the number of steps for lower magnitudes of platform motion but had little effect at higher magnitudes. CONCLUSION: The tendency to step in response to externally applied disturbances to stance appears to be a complex function of direction, velocity, displacement, and inertial load.

Organization of preparatory postural responses for the initiation of lateral body motion during goal directed leg movements.

Postural responses preceding single leg flexion movements were examined in human subjects with respect to a hypothesized primary role of the hip joint abductor-adductor musculature in initiating lateral body motion during alterations in stance. The results indicated a speed sensitive interlimb response organization compatible with predictions based on a mechanical model. In conjunction with other recent studies, the findings may be indicative of a common organizational substrate for the initiation of laterally directed motion of the body.

Reliability of measurements of body center-of-mass momentum during sit-to-stand in healthy adults.

BACKGROUND AND PURPOSE: The purpose of this study was to determine the reliability of measurements of momentum of the body's center of mass (CM) during a sit-to-stand (STS) transfer in healthy adults. SUBJECTS: Nineteen healthy adults aged 25 to 38 years (mean = 31.7, SD = 4.2) participated. METHODS: Horizontal and vertical components of CM momentum were computed for STS transfers made at three movement speeds (fast, natural, and slow) with the aid of a motion analysis system. Two force platforms detected the time when the subject lost contact with the chair and the propulsive and braking impulses in the horizontal and vertical directions. Separate intraclass correlation coefficients (ICCs) were calculated for three temporal variables (time to peak horizontal and vertical momentum and time to when the subject lost contact with the chair) and two magnitude variables (peak horizontal and vertical momentum). RESULTS: The ICCs for magnitude variables were > or = .81 for all speeds of movement. The ICCs for temporal variables ranged from .28 for fast movements to .75 for slow movements. CONCLUSION AND DISCUSSION: Measurement of peak vertical and horizontal momentum magnitudes is highly reproducible during STS transfers. Measurement of temporal variables exhibits a range of reliability estimates. Implications include consideration of the speed at which STS transfer is performed and its effect on reliability estimates and the potential differences between reliability estimates for magnitude measurements versus temporal measurements.

Alterations in weight-transfer capabilities in adults with hemiparesis.

BACKGROUND AND PURPOSE: The purposes of this study were (1) to examine the position and displacement in the frontal plane of the body's center of mass (CM) with respect to the base of support during single-leg flexion movements in adults with hemiplegia and (2) to examine their relationship with other clinical scores. SUBJECTS: Fourteen ambulatory adult volunteers with hemiparesis of the right side of the body due to cerebrovascular accident participated in the study. METHODS: Subjects performed single-leg flexion movements with the paretic and nonparetic limbs while standing on two separate force platforms. Motion analysis and force platform data were used to determine the displacement of the CM. RESULTS: Successful performance of the transfer and holding single-limb stance occurred for 48% (to the nonparetic side) and 20% (to the paretic side) of the trials. Lack of success was due to insufficient displacement of the CM (26% of the trials to the nonparetic side and 17% of the trials to the paretic side) or a failure to maintain single-limb stance (26% of all trials to the nonparetic side and 63% of the trials to the paretic side). Overall, the final position of the CM with respect to the single-limb support region did not differ between sides. Successful performance was highly to moderately associated with clinical assessment scores for motor function and balance. Its association with gait velocity, however, was poor. CONCLUSION AND DISCUSSION: A classification scheme that can distinguish between four categories of bipedal to single-limb stance transitions has been established. Issues concerning clinical assumptions pertaining to the relationship between static and dynamic motor dysfunction in adults with hemiparesis are discussed.

Postexercise hypotension reduces cardiovascular responses to stress.

DESIGN: Psychological stress is associated with the development of hypertension. Exercise is purported to have a prophylactic effect on stress. Immediately after a single bout of aerobic exercise there is a transient decrease in blood pressure. OBJECTIVE: We sought to examine the cardiovascular responses to a psychological stressor, the Stroop color word task during the postexercise hypotensive period. METHODS: Eight borderline hypertensive subjects (resting blood pressure 137 +/- 1.9/85 +/- 1.8 mmHg) participated in three randomly assigned experimental trials: Stroop color word task without prior exercise (Stroop); Stroop color word task administered 10 min after 60 min exercise at 60% maximal oxygen uptake (E + Stroop); and 60 min exercise at 60% maximal oxygen uptake followed by 20 min seated recovery (Ex). Blood pressure and heart rate were monitored at the start and end of exercise and at every 2 min of recovery. RESULTS: During the Stroop trial there were significant increases in mean arterial (MAP), systolic (SBP) and diastolic blood pressure (DBP). During the Ex+Stroop trial the increases in MAP, SBP and DBP during the Stroop color word task were significantly less than the increases without prior exercise. During recovery in the Ex trial there were significant decreases in MAP and SBP. However, there were no significant changes in DBP during the Ex trial. CONCLUSIONS: These results suggest that following an acute bout of exercise there is a reduction in blood pressure, and during this postexercise hypotensive period the blood pressure response to a psychological stressor is attenuated.

Kinetic analysis of dynamic transitions in stance support accompanying voluntary leg flexion movements in hemiparetic adults.

The purpose of this study was to examine the extent to which adult hemiparesis due to cerebrovascular accident may alter the dynamic transitions from bipedal to single limb stance that accompany rapid voluntary single leg flexion movements while standing. Eight postacute hemiparetic adults performed rapid single-leg-flexion movements with the paretic (PL) and nonparetic (NL) limbs while standing on two separate force platforms that recorded the individual and resultant lateral horizontal (FY) ground reaction force (GRF) components acting on the body in the frontal plane. The results showed that for NL movements, the ipsilateral FY force-time integral contributed a significantly greater proportion to the resultant FY than that recorded beneath the upcoming stance PL, whereas PL movements showed a reverse trend indicating differences in the spatial distribution of GRFs. For some subjects, delays in the initial FY onset times between limbs and reversals in the normal direction of initial force application beneath the PL were observed. Such changes in the spatial and temporal aspects of GRF production may affect dynamic lateral weight transfer function regardless of the direction of total body motion. Implications for clinical practice pertaining to interventions that emphasize speed as well as magnitude of paretic muscle torque production, and factors related to the selection of movement activities for the retraining of dynamic weight transfer function are discussed.