Extending the center of pressure to incorporate handhold forces: Derivation and sample application.

Measures describing movement of the center of pressure (CoP) are frequently used to characterize postural control. Estimates of CoP often focus on forces that individuals exert in a single plane through the feet (standing on force plates). However, balance may also be supported by forces other than those developed at the feet, especially when walkers, handrails, and other aids are used. In these cases, it is common to neglect the contributions of handheld supports. Here, we derive and apply equations for an extended CoP that incorporates handhold forces. We then examine the influence of CoP definition (i.e., including or ignoring handhold forces) on common metrics (path length; RMS and maximum excursion; average and maximum velocity) for 12 younger adults with a handrail located lateral to the participants' dominant hand. Participants attempted balance recovery in response to a range of small, medium and large forward and backward platform translations. Significant interactions between perturbation magnitude and CoP definition were found for most metrics. Notably, the interaction of CoP definition and perturbation magnitude significantly affected path length (p-values < 0.001). Post-hoc analyses revealed larger CoP path length when handrail forces were incorporated in CoP estimates compared to ignoring handrail forces at medium (backward: 59.9 vs. 19.0% height; forward: 70.5 vs 22.4% height) and large perturbation magnitudes (backward: 69.9 vs 22.4% height; forward: 103.5 vs 24.6% height). Incorporation of hand forces in CoP calculations can present a different view of postural balance control than relying on a feet-only CoP. This measure could be useful in assessing balance control tasks that involve the use of handrails or hand-held mobility devices.

Age-related differences in dynamic balance control during stair descent and effect of varying step geometry.

The incidence of stairway falls and related injuries remains persistently high; however, the risk of stair injuries could be reduced through improved stairway design. The current study investigated dynamic balance control during stair descent and the effects of varying the step geometry. Data were collected from 20 healthy young and 20 older adults as they descended three staircases (riser heights of 7, 7.5 and 8 inches (178, 190 and 203 mm, respectively)). At each riser height, the tread run length was varied between 8 and 14 inches (203 mm and 356 mm) in one-inch (25 mm) increments. Kinematic data provided measures of segmental and whole-body dynamic control. Results demonstrated that older adults had greater lateral tilt of the upper body than young adults, but actually had larger margins of stability than the young in the antero-posterior direction as a result of their slower cadence. Nonetheless, for both age groups, the longer run lengths were found to provide the largest margins of stability. In addition, increase in run length and decrease in riser height tended to reduce forward upper body tilt. These results help to explain the underlying biomechanical factors associated with increased risk of falls and the relationship with step geometry. Considering the importance of stair ambulation in maintaining independence and activity in the community, this study highlights the definite need for safer stair design standards to minimize the risk of falls and increase stair safety across the lifespan.

Cognitive demands and cortical control of human balance-recovery reactions.

A traditional view has been that balance control occurs at a very automatic level, primarily involving the spinal cord and brainstem; however, there is growing evidence that the cerebral cortex and cognitive processing are involved in controlling specific aspects of balance. The purpose of this review is to summarize recent literature pertaining to the cognitive demands and cortical control of balance-recovery reactions, focussing on five emerging sources of evidence: 1) dual-task studies demonstrating that concurrent performance of cognitive and balance-recovery tasks leads to interference effects; 2) dual-task studies that have examined the temporal dynamics associated with the reallocation of cognitive resources to the balance-recovery task; 3) visual attention studies that have inferred contributions of visual attention based on gaze measurements and/or manipulations to occlude vision; 4) measurements of brain potentials evoked by postural perturbation; and 5) use of transcranial magnetic stimulation to alter contributions from specific cortical areas.

Cognitive demands of executing postural reactions: does aging impede attention switching?

A new dual-task paradigm was used to investigate age-related differences in attentional dynamics during rapid balancing reactions evoked by small, unpredictable antero-posterior platform movements. The perturbations were delivered while subjects performed a continuous visuo-motor pursuit-tracking task. Onset of significant deviation in tracking was inferred to indicate switching of attentional resources between tracking and balancing tasks. Although tracking deviation was equally likely to occur subsequent to postural perturbation in healthy young and older adults, deviation onset was delayed, on average, by 67% (123 ms) in the older subjects. Delay in onset of tracking deviation correlated with subsequent delay in generating the peak stabilizing postural response at the ankle. These results suggest that impaired attentional dynamics may exacerbate postural instability in older adults.

Passive and active lower-limb movements delay upper-limb balance reactions.

This study investigated the influence of rhythmic lower-limb activity on the timing of upper-limb balance reactions. Compensatory grasping reactions were evoked in healthy subjects by rapid sagittal tilts of a chair under three conditions: (1) active leg pedaling, (2) passive (motor-driven) leg pedaling, and (3) no lower-limb movement (control task). Compared with control trials, both active and passive pedaling resulted in similar delays in the initiation (43-47 ms) and execution (12-17 ms) of grasping reactions. The similarity between effects due to active and passive movement suggests that the conditioning arose predominantly from sensory discharge associated with lower-limb movement. These results may have important implications for understanding the influence of locomotion or other ongoing movement on the control of stability.

The role of plantar cutaneous mechanoreceptors in the control of compensatory stepping reactions evoked by unpredictable, multi-directional perturbation.

The role of plantar pressure sensation in controlling compensatory stepping was explored via hypothermic anesthesia of the foot soles, in 10 healthy young adults. Stepping reactions were evoked by unpredictable platform translation in forward, backward and lateral directions. The findings suggest three specific direction- and phase-dependent roles for the plantar cutaneous afferents: (1) sensing posterior stability limits during initiation of backward steps, (2) sensing and controlling heel-contact and subsequent weight transfer during termination of forward steps, and (3) maintaining stability during the prolonged swing phase of lateral crossover steps.

Age-related differences in laterally directed compensatory stepping behavior.

BACKGROUND: Lateral falls are common in older adults and are associated with an elevated risk of hip fracture, compared with falls in other directions. Although rapid stepping movements can play an important functional role in maintaining balance, control of lateral stepping is a complex and demanding motor task. This study examined whether there are age-related differences in the stepping behavior used to recover from lateral loss of balance. METHODS: Rapid stepping reactions were evoked in healthy, active young (aged 20-30 years; N = 10) and older (aged 65-73 years; N = 10) volunteers by means of a sudden unpredictable motion of a platform on which the subject either stood quietly or walked in place. Subjects were instructed to respond naturally. Video analysis was performed to characterize the patterns of limb movement evoked by lateral platform motion. RESULTS: In responding to lateral perturbation of stance, the older adults were much more likely than the young adults to take multiple steps or use arm reactions to regain equilibrium, particularly when attempting crossover steps. During walk-in-place trials, both young and older subjects more frequently used a sequence of side steps rather than crossovers; however, older adults were still more likely to take extra steps or use arm reactions. Collisions between swing foot and stance limb occurred in 55% of walk-in-place trials in older adults versus only 8% in young adults. CONCLUSIONS: Control of lateral-stepping reactions appears to create difficulties for active and healthy older adults above and beyond previously reported problems in controlling forward and backward stepping. Impaired control of lateral-stepping reactions may be an early indicator of increased risk for lateral falls and hip fracture and should be an important consideration in the development of clinical approaches to predicting and preventing falls and related injuries.

Thresholds for step initiation induced by support-surface translation: a dynamic center-of-mass model provides much better prediction than a static model.

The need to initiate a step in order to recover balance could, in theory, be predicted by a static model based solely on displacement of the center of mass (COM) with respect to the base of support (BOS), or by a dynamic model based on the interaction between COM displacement and velocity. The purpose of this study was to determine whether the dynamic model provides better prediction than the static model regarding the need to step in response to moving-platform perturbation. The COM phase plane trajectories were determined for 10 healthy young adults for trials where the supporting platform was translated at three different acceleration levels in anterior and posterior directions. These trajectories were compared with the thresholds for step initiation predicted by the static and dynamic COM models. A single-link-plus-foot biomechanical model was employed to mathematically simulate termination of the COM movement, without stepping, using the measured platform acceleration as the input. An optimization routine was used to determine the stability boundaries in COM state space so as to establish the dynamic thresholds where a compensatory step must be initiated in order to recover balance. In the static model, the threshold for step initiation was reached if the COM was displaced beyond the BOS limits. The dynamic model showed substantially better accuracy than the static model in predicting the need to step in order to recover balance: 71% of all stepping responses predicted correctly by the dynamic model versus only 11% by the static model. These results support the proposition that the central nervous system must react to and control dynamic effects, i.e. COM velocity, as well as COM displacement in order to maintain stability with respect to the existing BOS without stepping.

Temporal properties of attention sharing consequent to disturbed balance.

The time course and extent of attentional shifts associated with compensatory balancing reactions were explored using a novel dual-task paradigm. Seated subjects performed a continuous visuomotor tracking task with the hand while the feet simultaneously balanced an inverted pendulum. The pendulum was randomly perturbed, evoking compensatory balance reactions. Changes in tracking performance were held to reflect attentional shifts. Discrete deviation in visuomotor tracking, typically a pause in tracking, began on average 235 ms after the onset of the balance reaction (TA EMG; average latency 90 ms). Such pauses lasted on average 600 ms, although additional errors in tracking lasted up to 9 s following the perturbation. The findings reveal evidence of dynamic shifts in attention associated with distinct phases of compensatory balance control. The initial phase appears to be triggered automatically, whereas later phases involve varying degrees of attentional resources.

The control of lateral stability during rapid stepping reactions evoked by antero-posterior perturbation: does anticipatory control play a role?

Volitional step initiation invariably includes a medio-lateral anticipatory postural adjustment (ML APA), which causes the center of mass (COM) to be propelled toward the stance-limb side prior to the lifting of the swing foot. The present study examined whether this type of anticipatory control plays a functional role in maintaining lateral stability during the rapid compensatory stepping reactions that are evoked when whole-body stability is challenged by unpredictable perturbation. Forward and backward stepping reactions were evaluated in five healthy young adults (ages 22-28) under three task conditions: (1) unconstrained compensatory stepping evoked by platform translation (no specific instructions), (2) constrained compensatory stepping cued by platform translation (prior instruction to step rapidly), and (3) rapid voluntary stepping to a light cue. ML APAs occurred during 70% of perturbation reactions but were too small and brief to have a substantive influence on the lateral movement of the COM occurring during leg lift or swing phase. In contrast, during the light-cued stepping, the ML APA propelled the COM toward the stance-limb side prior to the lifting of the swing limb, and effectively reduced the tendency of the COM to fall toward the swing-limb side during the execution of the step. It is proposed that the presence of an ML APA during compensatory stepping may represent an attempt to preplan a stereotypical stepping response, but that the ability to fully express the anticipatory phase is disrupted by the need to react rapidly to the unpredictable antero-posterior instability imposed by the perturbation. The results suggest that anticipatory control is not the primary mechanism by which the central nervous system deals with the lateral instability arising during rapid compensatory stepping reactions evoked by large, unpredictable antero-posterior perturbation.

The efficacy of head immobilization techniques during simulated vehicle motion.

STUDY DESIGN: Laboratory experiment. OBJECTIVE: To compare the efficacy of different head immobilization techniques during motion simulating ambulance transport. BACKGROUND: A significant number of neurologic injuries associated with cervical spine fractures arise or are aggravated during emergency extrication or patient transport. Previous studies have not addressed the effect of head immobilization on the passive motion that could occur across the neck during transport. METHODS: Three different head-immobilization methods were compared in six healthy young adults by using a computer-controlled moving platform to simulate the swaying and jarring movements that can occur during ambulance transport. In all tests, the trunk was secured by means of a commonly used "criss-cross" strapping technique. Efficacy of head immobilization was evaluated using measures of head motion and neck rotation. RESULTS: None of the three immobilization techniques was successful in eliminating head motion or neck rotation. Movement of the trunk contributed substantially to the lateral bending that occurred across the neck. A new product involving the placement of wedges underneath the head provided some small, but statistically significant improvements in fixation of the head to the fracture board; however, there was no improvement in terms of the relative motion occurring across the neck. CONCLUSIONS: Somewhat improved fixation of the head to the fracture board can be achieved by placing wedges under the head; however, the benefits of any fixation method, in terms of cervical spine immobilization, are likely to be limited unless the motion of the trunk is also controlled effectively. Future research and development should address techniques to better control head and trunk motion.

Effect of facilitation of sensation from plantar foot-surface boundaries on postural stabilization in young and older adults.

BACKGROUND: One of the more pervasive effects of aging is loss of cutaneous sensation, which appears to correlate with impaired postural control and increased risk of falling. This study examined the potential for compensating for the destabilizing effects of reduced cutaneous sensitivity by placing a raised edge underneath the perimeter of the plantar foot surface, so as to facilitate sensation from the stability boundaries of the base of support. METHODS: The main experiment involved 14 healthy older adults (aged 65-73) selected because they were known, from a previous study, to have moderate plantar cutaneous insensitivity. We also report results of an initial experiment involving 7 healthy young adults (aged 23-31). In both experiments, we studied effects of the plantar facilitation on control of rapid stepping reactions evoked by unpredictable postural perturbation, applied via sudden platform movement in forward, backward, and lateral directions. We also studied effects on "feet-in-place" responses evoked by continuous pseudorandom platform motion in mediolateral and anteroposterior directions. Subjects were blindfolded in all tests. RESULTS: Plantar facilitation reduced the incidence of "extra" limb movements, beyond the initial step, during forward-step reactions in the older adults. There also appeared to be an improved ability to control feet-in-place reactions: young subjects were better able to recover balance without stepping when falling backward (given instructions to "try not to step"), and both young and older subjects reduced the extent to which the center of foot pressure approached the posterior foot boundary during continuous anteroposterior platform motion. CONCLUSIONS: This study provides evidence that mechanical facilitation of sensation from the boundaries of the plantar surface of the foot can improve the efficacy of certain types of stabilizing reactions evoked by unpredictable postural perturbation. The results may be directly transferable to the design of special footwear insoles to reduce instability and risk of falling in older adults.

Preparatory balance adjustments precede withdrawal response to noxious stimulation in standing humans.

Self-initiated leg movement in standing humans is preceded by a medio-lateral preparatory balance adjustment (PBA); however, such preparatory balance control is often absent in reflex-like stepping responses evoked by whole-body instability. The presence or absence of the PBA may reflect a task-dependent modulation of the response serving to preserve lateral stability (PBA present) or avoid delay in the lifting of the foot (PBA absent). To examine whether such task-dependent modulation can occur during more stereotypical limb movements, we examined spinally-mediated withdrawal responses evoked by noxious stimulation of the foot. Results showed that rapid limb withdrawal was preceded by a large PBA when subjects were standing but not when they were supine. The PBA caused limb withdrawal to the noxious stimulation to be delayed. However, the onset of the PBA in the standing trials was equivalent in timing to the onset latency of the classic withdrawal responses recorded during the supine trials. Evidence of a preparatory balance adjustment evoked, in advance of a delayed withdrawal response, at very rapid latencies (underlying muscle activation at 70-120 ms) may raise new questions about the neural mechanisms underlying the co-ordination of balance and movement.

The control of foot placement during compensatory stepping reactions: does speed of response take precedence over stability?

Rapid, reflex-like stepping movements are a prevalent and functional compensatory reaction to destabilization, however, little is known about the underlying control. In this paper, a model is developed to examine how speed and stability demands affect control of foot placement during forward and backward compensatory stepping reactions. The concept of the velocity stability margin (VSM) is introduced to characterize the degree to which the horizontal velocity of the falling body approaches biomechanical limits on the capacity to decelerate the center of mass; analogous limits on center-of-mass displacement are quantified in terms of the displacement stability margin (DSM). The model is used to predict, for any initial step characteristics, the variation in DSM and VSM that would occur as a function of changes in timing of foot placement. The VSM was found to prevail over the DSM in establishing limits of stability. Model simulations demonstrated that there typically exists a minimum swing duration that maximizes speed of response while meeting minimum requirements for stability (VSM > or = 0), as well as a slower speed of response (longer swing duration) at which stability (VSM) is maximized. Experimental data from platform-perturbation tests in 20 healthy young (22-28) and older (65-81) adults were used, in conjunction with the model, to investigate whether speed or stability takes precedence during natural behavior. Control of single-step reactions appeared to favor stability; although the model predicted that a minimally stable step (VSM = 0) could be attained by swing durations as short as 30 ms, the observed swing durations were, on average, 135 ms longer than this, and the average VSM was nearly as large (80%) as the optimally stable value predicted by the model. Control of the initial step of multiple-step reactions was distinctly different. The average swing duration was only 55 ms greater than the minimally stable value and the average VSM was 81% smaller than in the single-step reactions. This reduction in VSM is consistent with a need to execute additional steps and appears to support the validity of the model. This model may help to provide insight into the biomechanical factors that govern the neural control of compensatory stepping reactions.

The role of limb movements in maintaining upright stance: the "change-in-support" strategy.

Change-in-support strategies, involving stepping or grasping movements of the limbs, are prevalent reactions to instability and appear to play a more important functional role in maintaining upright stance than has generally been appreciated. Contrary to traditional views, change-in-support reactions are not just strategies of last resort, but are often initiated well before the center of mass is near the stability limits of the base of support. Furthermore, it appears that subjects, when given the option, will select these reactions in preference to the fixed-support "hip strategy" that has been purported to be of functional importance. The rapid speed of compensatory change-in-support reactions distinguishes them from "volitional" arm and leg movements. In addition, compensatory stepping reactions often lack the anticipatory control elements that are invariably present in non-compensatory stepping, such as gait initiation. Even when present, these anticipatory adjustments appear to have little functional value during rapid compensatory movements. Lateral destabilization complicates the control of compensatory stepping, a finding that may be particularly relevant to the problem of falls and hip fractures in elderly people. Older adults appear to have problems in controlling lateral stability when stepping to recover balance, even when responding to anteroposterior perturbation. Increased understanding and awareness of change-in-support reactions should lead to development of new diagnostic and therapeutic approaches for detecting and treating specific causes of imbalance and falling in elderly people and in patients with balance impairments.

Gait changes in older adults: predictors of falls or indicators of fear.

OBJECTIVE: To determine, in a cohort of ambulatory older adults, whether spatial-temporal measures of foot placement during gait can predict the likelihood of future falls or whether these measures are more likely to be indicative of adaptations associated with pre-existing fear of falling. DESIGN: Prospective cohort study. SETTING: Baseline gait measurements were performed in a gait and balance laboratory; subsequent history of falling was monitored prospectively for 1 year in two self-care facilities. PARTICIPANTS: Fourteen male and 61 female consecutive volunteers (mean age = 82, SD = 6) who were independent in activities of daily living and able to walk 10 m unaided. MEASUREMENTS: Spatial gait parameters were derived from digitized "footprints"; temporal parameters were derived using footswitches. A clinical activity-based gait assessment was also performed. The dependent variables were pre-existing fear of falling (reported at baseline) and future falling (experiencing one or more falls during the 1-year follow-up). MAIN RESULTS: Reduced stride length, reduced speed, increased double-support time, and poorer clinical gait scores were associated with fear but showed little evidence of an independent association with falling. Conversely, increased stride-to-stride variability in stride length, speed, and double-support was associated independently with falling but showed little evidence of relationship to fear. Increased stride width showed some evidence of association with both falling and fear. Stride-to-stride variability in speed was the single best independent predictor of falling. CONCLUSIONS: Changes in gait cited previously as risk factors for falling, i.e., decreased stride length and speed and prolonged double support, may in fact be stabilizing adaptations related to fear of falling. Stride-to-stride variability in the control of gait is an independent predictor of falling and may be a useful measure for identifying high-risk individuals and evaluating preventive interventions. Stride width may also be a useful outcome measure. Contrary to common expectation, a wider stride does not necessarily increase stability but instead seems to predict an increased likelihood of experiencing falls.

Preferred placement of the feet during quiet stance: development of a standardized foot placement for balance testing.

OBJECTIVE: To establish a standardized stance position for balance testing based on average preferred foot placement, and to compare this to existing standards. DESIGN: Cross-sectional study. BACKGROUND: It has been shown that the orientation of the feet can have a marked influence on the results obtained during balance testing, prompting the need for standardized foot positioning. Unfortunately, current recommendations do not appear to address concerns about the potential effects of 'uncomfortable' or 'unnatural' foot positions on the control of stabilizing reactions. METHODS: The present study identifies the central tendency and variance of the preferred stance width and foot angle, measured from foot tracings in 262 subjects (89 male, 173 female) ranging in age from 19 to 97 years. RESULTS: Results revealed a great degree of variability in preferred stance width and angle across subjects, although mean differences due to gender or age ( 60) were small. The average preferred foot position was 0.17 m between heel centres, with an angle of 14 degrees between the long axes of the feet. Existing standards for stance position lie well outside the range of preferred foot placement. CONCLUSIONS: The wide range of preferred foot placements clearly highlights the need for standardization during balance testing. A standard based on average preference would reduce potential effects of 'uncomfortable' or 'unnatural' foot positions, in comparison to existing standards. RELEVANCE: The development of a standardized stance position for balance testing is necessary since foot placement can influence stabilizing reactions. The present results provide a standardization based on average preferred foot placement, which will minimize between-subject variability and reduce the abnormal constraints that are placed on subjects by existing standards.

Effect of forward lean on postural ankle dynamics.

Previous studies investigating postural control using platform perturbations demonstrated that forward leaning occurs under certain experimental conditions. This study examined a potential benefit of forward leaning, investigating the hypothesis that forward lean acts to increase the effective stiffness of the postural ankle dynamics. A systems modeling approach was used to evaluate the effect of forward lean (4 degrees ankle flexion beyond normal stance) on dynamic postural responses to continuous random antero-posterior platform acceleration in four healthy young adults. Motion was limited to the ankle joint using a restraint device and responses were characterized in terms of center-of-pressure displacement. Also, EMG and kinematic data were used to partition measured ankle torque into impedance and activation (postural "reflex") components. The results failed to show a consistent effect, due to lean, on the ankle dynamics. A significant increase in phasic ankle torque due to increased muscle impedance during forward lean was counterbalanced by a comparable decrease in net "reflex" ankle torque. The change in "reflex" torque was apparently due to decreased phasic dorsiflexor activity in forward lean, since phasic plantarflexor activity did not change significantly. Functionally, the results suggest that forward lean occurs for reasons other than stiffening of the overall postural ankle dynamics.

Age-related changes in compensatory stepping in response to unpredictable perturbations.

BACKGROUND: Recent studies highlight the importance of compensatory stepping to preserve stability, and the spatial and temporal demands placed on the control of this reaction. Age-related changes in the control of stepping could greatly influence the risk of falling. The present study compares, in healthy elderly and young adults, the characteristics of compensatory stepping responses to unpredictable postural perturbations. METHODS: A moving platform was used to unpredictably perturb the upright stance of 14 naive, active and mobile subjects (5 aged 22 to 28 and 9 aged 65 to 81). The first 10 randomized trials (5 forward and 5 backward) were evaluated to allow a focus on reactions to relatively novel perturbations. The behavior of the subjects was not constrained. Forceplate and kinematic measures were used to evaluate the responses evoked by the brief (600 msec) platform translation. RESULTS: Subjects stepped in 98% of the trials. Although the elderly were less likely to execute a lateral anticipatory postural adjustment prior to foot-lift, the onset of swing-leg unloading tended to begin at the same time in the two age groups. There was remarkable similarity between the young and elderly in many other characteristics of the first step of the response. In spite of this similarity, the elderly subjects were twice as likely to take additional steps to regain stability (63% of trials for elderly). Moreover, in elderly subjects, the additional steps were often directed so as to preserve lateral stability, whereas the young rarely showed this tendency. CONCLUSIONS: Given the functional significance of base-of-support changes as a strategy for preserving stability and the age-related differences presently revealed, assessment of the capacity to preserve stability against unpredictable perturbation, and specific measures such as the occurrence or placement of multiple steps, may prove to be a significant predictor of falling risk and an important outcome in evaluating or developing intervention strategies to prevent falls.

Postural control in the older adult.

Age-related changes in the neural, sensory, and musculoskeletal systems can lead to balance impairments that have a tremendous impact on the ability to move about safely. The many complex substrates of the posture control system subserve a common functional goal: regulation of the relationship between the center of mass and the base of support. Traditional approaches, which have focused on the control of the center-of-mass displacement, have documented age-related changes in "feet-in-place" responses: during quiet standing, during volitional movement, or in response to applied perturbation. Recently, increasing attention has been directed toward the control of the base of support, that is, compensatory leg and arm movement, as an important element of the postural repertoire, and early results suggest profound age-related impairment in the control of compensatory stepping movements. For both feet-in-place and stepping responses, control of lateral stability appears to be a major problem associated with increased risk of falling. In view of age-related differences in ability to adapt postural responses under predictable task conditions, future work will likely benefit by mimicking, as much as possible, the varied and unpredictable nature of the events that often precipitate falls in daily life, in order to draw connections between the laboratory or clinic and "real-life" stability.