Measures of postural stability are not predictors of recovery from large postural disturbances in healthy older adults

Changes in balance and strength parameters induced by training on a motorised rotating platform: a study on healthy subjects

AIM

The aim of the present study was to analyse the effects of training performed on a rotating, motorised platform (the Huber/SpineForce device from LPG Systems, Valence, France) intended to improve, postural control and muscle function.

SUBJECTS

Twelve healthy adults (divided into a sedentary group and an active group) took part in a two-month training programme (involving three sessions a week) on the SpineForce whole body rehabilitation device.

METHOD

Instrumental assessment of postural control (on a Satel platform) and muscle function (on a Cybex Norm) was performed before and after training. Postural control in various conditions was measured using a position parameter (the mean anteroposterior position of the centre of foot pressure [CoP]) and two stability parameters (maximum CoP displacement and CoP sway area). Assessment of the muscle function was performed during knee and spine extension and featured maximum voluntary isometric contraction (MVIC), root mean square (RMS) and neuromuscular efficiency (MVIC/RMS) measurements.

RESULTS

For static postural control, we observed a more forward CoP position in the maximum backward inclination condition (p<0.01) and a decrease in maximum CoP displacement in the "eyes closed on foam" and "maximum anterior inclination" conditions. In this latter condition, a lower CoP sway area was also noted (p<0.01). In terms of muscle function, a greater MVIC for knee extension was observed in the sedentary group only (p<0.05). These changes were not correlated with each another (p<0.05). However, the value of the pretraining maximum CoP displacement predicted its final value (p<0.05).

CONCLUSION

Our results suggest that static postural control responds to training on a Huber((R))/SpineForce rehabilitation device. It seems probable that a population with a low initial level of physical activity would benefit most from training on this type of device. This training could notably be applied to elderly or disabled people and especially those with sensorimotor disabilities.

Validity of functional stability limits as a measure of balance in adults aged 23-73 years

Functional stability limits (FSLs) are the percentage of the base of support that individuals are willing to extend their centre of pressure. The objective of this study was to provide construct validation of FSLs as a measure of balance by comparing FSLs across ages and with clinical balance measures. A total of 52 participants volunteered. FSLs significantly decreased with age (p < 0.004). Correlations between FSLs and age (-0.56 < R < -0.73), multi-directional reach test (0.35 < R < 0.75) and anteroposterior and mediolateral centre of pressure excursions during static stance (-0.29 < R < -0.72) were generally moderate to good. Cronbach's alpha (0.75) indicated that these measures were internally consistent, i.e. measuring similar aspects of the balance construct. FSLs appear to be valid indicators of balance ability. They may be used in posture prediction models to determine when a step is required when reaching or lifting objects and in biomechanical models as a means of incorporating stability constraints.

Instructions limiting the number of steps do not affect the kinetics of the threshold of balance recovery in younger adults

To our knowledge, no one has explored the effect of modifications in balance recovery instructions on the kinetics of the threshold of balance recovery. In particular, the effect of instructions limiting the number of steps on joint torques at the maximum lean angle has not been quantified. We determined the joint torques at the ankle, knee and hip of 28 younger adults recovering balance at their maximum lean angle using: (i) only a single step, (ii) no more than two steps and (iii) no limit on the number of steps. Results showed that instructions limiting the number of steps did not affect peak normalized joint torques by more than 0.0083 or 10Nm except for knee and hip flexion torques from first to second heel strike for the first step leg as well as from second toe-off to heel strike for the second step leg. However, these large differences in peak normalized joint torques after the first step were simply caused by the additional steps used when participants could take more than one step compared to when participants were limited to only a single step. Between the three limits on the number of steps, the kinetics of both legs were nearly identical up to the end of the first step and the additional steps did not help to increase the maximum lean angle. Therefore, we have demonstrated that instructions limiting or not limiting the number of steps appear to be equally valid to study falls in younger adults.

Lower extremity strength plays only a small role in determining the maximum recoverable lean angle in older adults

BACKGROUND

The purpose of this study was to determine the extent to which measures of lower extremity strength and power contribute to the ability of older men and women to restore postural equilibrium using a single-step recovery following a large postural disturbance.

METHODS

The postural disturbance, which has been used as a surrogate for forward-directed falls, involved a sudden release from a forward-leaning angle. The ability to recover using a single step was evaluated as the maximum recoverable lean angle for 56 healthy older women and men. Maximum voluntary isometric and isokinetic strength was measured for ankle plantarflexion and dorsiflexion, knee flexion and extension, and hip flexion and extension. Discriminant analysis was used to determine the strength measures that best classified participants as members of the highest (n = 14) or lowest (n = 14) quartiles of maximum recoverable lean angle. Those variables were subsequently entered into a regression analysis to characterize the relationship between strength and maximum recoverable lean angle for the entire participant sample.

RESULTS

Maximum isokinetic dorsiflexion strength at 90 degrees /s satisfied the criteria of the stepwise discriminant analysis, and correctly classified 82.1% of the participants in the highest or lowest quartiles of maximum recoverable lean angle. The multiple regression procedure, performed on all participants (n = 56) revealed a significant quadratic relationship between maximum isokinetic dorsiflexion strength at 90 degrees /s and maximum recoverable lean angle (R2 = 0.295; p <.001).

CONCLUSIONS

Lower extremity strength makes a small, but significant contribution to maximum recoverable lean angle. However, because 70% of the shared variability remained unaccounted for, it is suggested that other performance factors, such as coordination, may be of greater importance to performance of this time-critical motor task.

A direct comparison of local dynamic stability during unperturbed standing and walking

Standing and walking are very different tasks. It might be reasonable, therefore, to assume that the mechanisms used to maintain the stability of standing and walking should be quite different. However, many studies have shown that postural stability measures can generally predict risk of falls, even though most falls occur during locomotor tasks and not during postural tasks. This suggests that there is at least some commonality among the mechanisms governing the control of both standing and walking. The present study was conducted to determine whether the postural stability either is or is not directly related to locomotor stability. Twenty healthy adults, age 18-73 years, walked on a motorized treadmill at their preferred walking speed for three trials of 5 min. They also stood on a force plate for three trials of 5 min. Both tasks were performed without imposing any additional external perturbations. The motion of each subject's trunk segment was recorded and described using a multi-dimensional state space defined in the same manner for both tasks. Local dynamic stability was quantified from the mean divergence over time of locally perturbed trajectories in state space, which was parameterized as a double exponential process. Divergence parameters were compared to determine the relationship between local dynamic stability during standing and walking. Standing and walking exhibited local dynamic stability properties that were significantly different (P<0.001) and not correlated (P>0.1). Divergence parameters were also compared to traditional center of pressure (COP) measures obtained from standing trials. COP measures were significantly correlated to local divergence parameters for standing, but not to those for walking. This study provides direct evidence that the mechanisms governing standing and walking stability are significantly different.

Postural steadiness during quiet stance does not associate with ability to recover balance in older women

BACKGROUND

Fall risk depends on ability to maintain balance during daily activities, and on ability to recover balance following a perturbation such as a slip or trip. We examined whether similar neuromuscular variables govern these two domains of postural stability.

METHODS

We conducted experiments with 25 older women (mean age=78 yrs, SD=7 yrs). We acquired measures of postural steadiness during quiet stance (mean amplitude, velocity, and frequency of centre-of-pressure movement when standing with eyes open or closed, on a rigid or compliant surface). We also measured ability to recover balance using the ankle strategy after release from a forward leaning position (based on the maximum release angle where recovery was possible, and corresponding values of reaction time, rate of ankle torque generation, and peak ankle torque).

FINDINGS

We found that balance recovery variables were not strongly or consistently correlated with postural steadiness variables. The maximum release angle associated with only three of the sixteen postural steadiness variables (mean frequency in rigid, eyes open condition (r=0.36, P=.041), and mean amplitude (r=0.41, P=.038) and velocity (r=0.49, P=.015) in compliant, eyes closed condition). Reaction time and peak torque did not correlate with any steadiness variables, and rate of torque generation correlated moderately with the mean amplitude and velocity of the centre-of-pressure in the compliant, eyes closed condition (r=0.48-0.60).

INTERPRETATION

Our results indicate that postural steadiness during quiet stance is not predictive of ability to recover balance with the ankle strategy. Accordingly, balance assessment and fall prevention programs should individually target these two components of postural stability.

Development of a Lateral Mobility Task to Identify Individuals at Risk for Mobility Disability and Functional Decline

Lateral mobility is integral to many activities of daily living involving transfer from one position to another. The objective of this study was to develop and evaluate the validity and test–retest reliability of a lateral-mobility (LATMOB) task for older adults. Measurements of lateral mobility, balance, and strength and self-reported and performance-based physical functioning were obtained in 63 women and 77 men ≥50 years of age. The LATMOB task was significantly correlated with age, knee-extensor strength, grip strength, functional reach, and one-leg-stance time. Test–retest reliability of the task was excellent. The LATMOB task was highly correlated with the car task. Balance was significantly correlated with time to get into and out of a car and performance on the LATMOB task. The LATMOB task was significantly correlated with the Short Physical Performance Battery score. The LATMOB task is valid and reliable, but additional work is needed to assess its sensitivity to change and predictive validity.

Age and Stepping Limb Performance Differences During a Single-Step Recovery From a Forward Fall

BACKGROUND

The purpose of this study was two-fold: 1) to evaluate any age-related differences in peak joint velocities of the stepping limb during single-step recovery from a forward fall, and 2) to determine if the ability to recover from a forward fall with a single step differs when stepping with the dominant or nondominant lower limb (LL).

METHODS

Ten young (19-23 years old) and ten older (65-83 years old) men were released from forward-leaning positions and attempted to recover their balance with a single step. Lean magnitude was increased until the men failed to recover their balance with a single step. The men performed the experiment twice, once while stepping with the dominant LL and once while stepping with the nondominant LL, to determine if the ability to recover from a forward fall is limb dependent. Peak joint velocities during single-step recoveries were determined.

RESULTS

No age-related differences in peak joint velocities were found during recovery from small lean magnitudes, but older men exhibited slower velocities during recovery from maximum lean magnitudes. There was no difference in the maximum lean magnitude achieved by the men when stepping with the dominant or nondominant LL.

CONCLUSIONS

The previously reported age-related reduction in stepping speed seems to be due to localized reductions in maximum hip flexion velocity, knee flexion and extension velocity, and ankle plantar flexion velocity. Also, the ability of young and older men to recover from a forward fall with a single step does not seem to be limb dependent.

The presence of an obstacle influences the stepping response during induced trips and surrogate tasks

Falling is a frequent cause of serious injury in older adults and trips are a dominant cause of falls in this rapidly growing population. Although there are few laboratory protocols that induce actual trips, there are many protocols that utilize surrogate tasks. These surrogate tasks, which are time-critical but do not involve an obstacle, appear to share a number of biomechanical characteristics with stepping responses following a trip. However, although rapid and safe negotiation of the obstacle and restoration of dynamic equilibrium are common requisites for success, we expected that stepping response kinematics during a successful recovery from a trip over a previously unseen obstacle would be substantially different than those of surrogate tasks without an obstacle. Unexpected trips were induced in 13 older men and women by an obstacle, the presence of which the subjects were previously unaware. Selected kinematics of the leading and trailing limb stepping responses during recovery from the induced trip were compared to those of two surrogate tasks that did not involve an obstacle. Multivariate analysis of variance (MANOVA) revealed that step height, step length, peak horizontal velocity, and peak vertical velocity of the leading and trailing limbs were significantly different during recovery from the induced trip compared to the surrogate tasks. These between-task performance differences may limit the extent to which performance of the surrogate tasks accurately and precisely reflect the potential to recover dynamic equilibrium following a trip. Therefore, these findings may be applicable in the design of new or modification of existing interventions to reduce falls in older adults.

Effects of a physical activity program on postural stability in older people

BACKGROUND AND AIMS

The objective of this non-randomized study was to determine the influence of a specific physical activity program on the postural stability of older people.

METHODS

Seventy-four subjects (72.4 +/- 0.7 yrs) participated in an individualized three-month physical activity program designed to improve posture, balance and mobility--the PBM program. Sessions were held twice weekly. Postural stability was assessed using a force platform, subjects being in static and dynamic conditions, and with open and closed eyes. Changes in stabilometric parameters (Sway area, ML mean, AP mean, Total length, ML length and AP length) of the intervention group were compared to those of 14 control subjects (71.8 +/- 1.5 years).

RESULTS

A two-way analysis of variance with repeated measures did not show any significant post-program change in postural stability in the hard floor condition. In contrast, Sway area (p < 0.0005), Total length (p < 0.001) and AP length (p < 0.01) were significantly reduced after the training program in the foam floor condition, with open and closed eyes. In addition, in the medio-lateral axis condition and with closed eyes, AP length in the intervention group was significantly reduced (p < 0.01), and in the antero-posterior axis condition with both open and closed eyes, Sway area (p < 0.0005), Total length (p < 0.0005) and AP length (p < 0.05) decreased significantly.

CONCLUSIONS

As shown by the results in the foam floor and dynamic conditions, our individualized physical activity program improved the postural stability of older people when the standing position was challenged. However, the lack of significant results for the hard floor condition suggests that three months is not sufficient to improve static balance. The PBM physical activity program can be used for balance training in older people, but further studies are required to determine the time needed to effect improvements in static balance in this population.

The effect of cane use on the compensatory step following posterior perturbations

OBJECTIVE

The compensatory step is a critical component of the balance response and is impaired in older fallers. The purpose of this research was to examine whether utilization of a cane modified the compensatory step response following external posterior perturbations.

DESIGN

Single subject withdrawal design was employed. Single subject statistical analysis--the standard deviation bandwidth-method--supplemented visual analysis of the data.

METHODS

Four older adults (range: 73-83 years) with balance impairment who habitually use a cane completed this study. Subjects received a series of sudden backward pulls that were large enough to elicit compensatory stepping. We examined the following variables both with and without cane use: timing of cane loading relative to step initiation and center of mass acceleration, stability margin, center of mass excursion and velocity, step length and width.

RESULTS

No participant loaded the cane prior to initiation of the first compensatory step. There was no effect of cane use on the stability margin, nor was there an effect of cane use on center of mass excursion or velocity, or step length or width.

CONCLUSIONS

These data suggest that cane use does not necessarily improve balance recovery following an external posterior perturbation when the individual is forced to rely on compensatory stepping. Instead these data suggest that the strongest factor in modifying step characteristics is experience with the perturbation.

New Intervention Program for Preventing Falls Among Frail Elderly People

OBJECTIVE

To determine the effects of a perturbed walking exercise using a bilateral separated treadmill in physically disabled elderly.

DESIGN

Participants of the study were 32 long-term care facility residents and outpatients aged 66-98 yrs. Participants were randomly assigned to a usual exercise group or to a treadmill exercise group. Perturbed gait exercise on a treadmill continued for 6 mos. Number of falls and time to first fall during a 6-mo period, balance and gait functions, and reaction time were evaluated before and after intervention.

RESULTS

The treadmill exercise group showed significant improvement in balance and reaction time when compared with the usual exercise group. Number of falls in the treadmill exercise group was 21% lower than that in the usual exercise group. However, this difference was not significant. No significant differences were seen in time to first fall.

CONCLUSIONS

Gait training with unexpected perturbation seems to have a beneficial impact on physical function in disabled elderly individuals. The results suggest that this program may be used as an exercise intervention to reduce falls in institutional settings.

Contribution of genetic and environmental effects to postural balance in older female twins

The aim of the present study was to determine the relative roles of genetic and environmental influences on postural balance in older women. The participants were 97 monozygotic (MZ) and 102 dizygotic (DZ) female twins, aged 64-76 yr. Postural sway was measured during side-by-side stance with eyes open and eyes closed, and during semitandem stance with eyes open on a force platform. Sway data were condensed into four first-order and one second-order latent factors. The second-order factor, named balance, incorporates sway data from multiple tests and thus best describes the phenotype of postural balance. The contribution of genetic and environmental influences on the variability of the latent factors was assessed by using structural equation modeling. Additive genetic influences accounted for 35% and shared environmental influences accounted for 24% of the total variance in the balance factor. In the present study, postural balance in older women had a moderate genetic component. Genetic influences on postural balance may be mediated through gene variation in the systems that control posture. The finding that individual environmental influences accounted for almost one-half of the variance in postural balance points to the potential of targeted interventions to maintain and improve balance control in older persons.

Relationship with dynamic balance function during standing and walking

OBJECTIVE

To investigate the relationship between dynamic balance functions in young adults and elderly adults while standing and walking.

DESIGN

In standing balance tests, the Sensory Organization Test (SOT) of six combinations of three visual and two support-surface conditions was used to measure standing balance, and the Motor Coordination Test (MCT) was used to provoke automatic postural reactions through a series of sudden translations of support surface. The gait test measured maximum anterior acceleration (MAA) and maximum posterior acceleration (MPA) of the trunk during perturbed walking using a bilaterally separated treadmill, and calculated latency until MAA and latency until MPA.

RESULTS

The elderly adults showed more significant functional decline than young adults in SOT1, SOT4, SOT6, medium intensity MCT, large intensity MCT, and MPA. In the correlation analysis of the outcome from the standing examinations, close correlations among SOT4, SOT5, and SOT6 conditions were observed in both groups of young adults and elderly adults. In the MCT, there was very close correlation between varied translation intensity in two groups. On the other hand, the only weak correlation between SOT and MCT findings was between SOT4 and large intensity MCT in elderly adults (r = -0.471, P = 0.049). In the gait test, although correlation was not significant in young adults, the significant correlations between MAA and latency until MAA (r = 0.705, P = 0.001) and latency until MAA and latency until MPA (r = 0.497, P = 0.036) were recognized in elderly adults. In the balance function findings of the standing examinations and the gait examinations, there was significant correlation between medium intensity MCT and latency until MAA (r = -0.552, P = 0.018) in young adults, and SOT6 and latency until MPA (r = -0.473, P = 0.047) in elderly adults. However, no relationship was observed in most of other factors.

CONCLUSIONS

Most falls experienced by elderly people are caused by tripping or slipping during walking. The fact that walking balance function did not correlate with standing balance function indicates that multifaceted evaluation is important to comprehend dynamic balance function while standing and walking.

Clinical prediction of falls in the elderly

OBJECTIVE

To assess the ability of physical and occupational therapists engaged in rehabilitation of the elderly to predict posttreatment falls.

DESIGN

Prospective cohort study of 15 mo in duration at an urban academic medical center rehabilitation unit. A total of 165 consecutively admitted geriatric individuals were rated for fall risk by 14 physical and seven occupational therapists. Measurements included the Mini-Mental State Examination, Geriatric Depression Scale, FIM, and therapists' ratings of fall likelihood.

RESULTS

Both disciplines evidenced an ability to predict who would fall in the 3 mo after discharge. Clinical judgment regarding fall risk, however, added little value over two major predictors of future falls, fall history and the presence of a neurologic condition.

CONCLUSION

Trying to predict an infrequent future event such as falls is inherently difficult. Education regarding known fall-risk factors and inclusion of standardized measurements of physical status are recommended to potentially improve rates of detection, along with adoption of a realistic attitude regarding our abilities to forecast infrequent events.

Logic modeling: a tool for improving educational programs

PURPOSE

Palliative care education programs must be systematically designed and accurately evaluated in order to account for their impact on learners and learning communities. Logic modeling is a framework for designing educational programs and monitoring their influence.

METHODS

We applied a logic modeling process to a Palliative Care Educational Program (PCEP), embedded within a required month-long geriatrics rotation for third-year general internal medicine residents at the VA Medical Center in Milwaukee, Wisconsin. Each of four main categories of a logic model and the process for applying the steps to the PCEP are described. The four logic model categories that form the basis of the analysis are: (1) inputs, the raw resources consumed by the program (human resources, money, and space); (2) outputs, program activities and the number and types of actual participants; (3) outcomes, what the program will achieve in the short term (knowledge, attitudes and behavior change); and (4) impact, the results that are of ultimate interest to program stakeholders, such as placement in medically underserved areas, improved health or more efficient care.

RESULTS

The application of logic modeling to the PCEP exposed achievements (e.g., resident knowledge gains) and gaps (no evidence of long-term impact) in each of the model categories and has resulted in the design team's reassessment of each program component.

DISCUSSION

Palliative care educators can improve their programs by using the logic model categories and process steps to explicitly define and assess the links between key program components.

Mechanisms of failed recovery following postural perturbations on a motorized treadmill mimic those associated with an actual forward trip

OBJECTIVE

To examine the recovery strategies employed during a treadmill acceleration task, to determine if mechanisms that contributed to failed recoveries on a motorized treadmill are the same general biomechanical mechanisms that contributed to falls from a trip, and to determine if failed recovery responses could be modified to allow for successful recoveries on subsequent trials.

DESIGN

A motorized treadmill was used to induce postural perturbations in healthy older adults.

BACKGROUND

Previously, we induced trips in older adults to identify the mechanisms of failed recovery. However, inducing trips is not a clinically practical test for identifying older adults who are predisposed to falling.

METHODS

Safety-harnessed older adults stood on a treadmill that was accelerated from 0 to 0.89 m/s to impose a postural perturbation. Recoveries were classified as successful (n=42) or failed (n=23). Selected biomechanical variables were calculated using motion analysis methods.

RESULTS

Initial failed recoveries had slower reaction times, shorter step lengths, and greater trunk flexion angles and velocities. Subjects who failed on the initial attempt modified their recovery strategy to successfully recover. The biomechanics of these recoveries resembled those used by subjects who successfully recovered on their initial attempt.

CONCLUSIONS

The biomechanical mechanisms involved with a failed treadmill recovery mimic those responsible for failed recoveries from an induced trip. Subjects who failed on their initial recovery response made modifications allowing successful recoveries on subsequent attempts.

RELEVANCE

This protocol may be useful as a testing and rehabilitation tool for fall recovery.

Changes in walking pattern caused by the possibility of a tripping reaction

This study investigated in 15 young adults whether their walking pattern was altered after forewarning for a possible trip. Such changes might affect tripping reactions and consequently the validity of experimental results. Kinematics and dynamics were measured during overground walking. No changes occurred in walking velocity, step frequency, duration of stride cycle, stance, swing and double support time, or step length. A small increase was found in step width and foot clearance due to ankle dorsiflexion, but these changes were not expected to alter the probability of tripping nor the recovery reactions after tripping in an experimental setup.