Deficits in the way to achieve balance related to mechanisms of dynamic stability control in the elderly

Spatiotemporal modulation of a common set of muscle synergies during unpredictable and predictable gait perturbations in older adults

Muscle synergies as functional low-dimensional building blocks of the neuromotor system regulate the activation patterns of muscle groups in a modular structure during locomotion. The purpose of the current study was to explore how older adults organize locomotor muscle synergies to counteract unpredictable and predictable gait perturbations during the perturbed steps and the recovery steps. Sixty-three healthy older adults (71.2±5.2 years) participated in the study. Mediolateral and anteroposterior unpredictable and predictable perturbations during walking were introduced using a treadmill. Muscle synergies were extracted from the electromyographic activity of 13 lower limb muscles using Gaussian non-negative matrix factorization. The four basic synergies responsible for unperturbed walking (weight acceptance, propulsion, early swing and late swing) were preserved in all applied gait perturbations, yet their temporal recruitment and muscle contribution in each synergy were modified (P<0.05). These modifications were observed for up to four recovery steps and were more pronounced (P<0.05) following unpredictable perturbations. The recruitment of the four basic walking synergies in the perturbed and recovery gait cycles indicates a robust neuromotor control of locomotion by using activation patterns of a few and well-known muscle synergies with specific adjustments within the synergies. The selection of pre-existing muscle synergies while adjusting the time of their recruitment during challenging locomotor conditions may improve the effectiveness to deal with perturbations and promote the transfer of adaptation between different kinds of perturbations.

Notes on the margin of stability

The concept of the 'extrapolated center of mass (XcoM)', introduced by Hof et al., (2005, J. Biomechanics 38 (1), p. 1-8), extends the classical inverted pendulum model to dynamic situations. The vector quantity XcoM combines the center of mass position plus its velocity divided by the pendulum eigenfrequency. In this concept, the margin of stability (MoS), i.e., the minimum signed distance from the XcoM to the boundaries of the base of support was proposed as a measure of dynamic stability. Here we describe the conceptual evolution of the XcoM, discuss key considerations in the estimation of the XcoM and MoS, and provide a critical perspective on the interpretation of the MoS as a measure of instantaneous mechanical stability.

Stepping responses for reactive balance for individuals with incomplete spinal cord injury

Incomplete spinal cord injury (iSCI) causes impairment of reactive balance control, leading to higher fall risk. In our previous work, we found that individuals with iSCI were more likely to exhibit multiple-step response during the lean-and-release (LR) test, where the participant leaned forward while a tether supported 8-12% of the body weight and received a sudden release, inducing reactive steps. Here we investigated the foot placement of people with iSCI during the LR test using margin-of-stability (MOS). Twenty-one individuals with iSCI (age: 56.1 ± 16.1 years old; mass: 72.5 ± 19.0 kg; height: 166 ± 12 cm), and fifteen age- and sex-matched able-bodied (AB) individuals (age: 56.1 ± 12.9 years old; mass: 57.4 ± 10.9 kg; height: 164 ± 8 cm) participated in the study. The participants performed ten trials of the LR test and also completed clinical assessment of balance and strengths, including the Mini-Balance Evaluations Systems Test, the Community Balance and Mobility Scale, gait speed, and lower extremity manual muscle testing. MOS was significantly smaller during multiple-step responses than during single-step responses for both individuals with iSCI and AB counterparts. Using binary logistic regression and receiver operating characteristic analyses, we demonstrated that MOS can distinguish single- and multiple-step responses. In addition, individuals with iSCI demonstrated significantly larger intra-subject variability of MOS compared to AB individuals at first foot contact. Further, we found that MOS correlated with clinical measures of balance including one for reactive balance. We conclude that individuals with iSCI were less likely to demonstrate foot placement with sufficiently large MOS, which may increase the tendency to exhibit multiple-step responses.

Role of Knee and Ankle Extensors' Muscle-Tendon Properties in Dynamic Balance Recovery from a Simulated Slip

Participants exposed to a simulated slip with forward loss of balance (FLB) develop large lower limb joint moments which may be a limiting factor for those whose muscle-tendon units’ (MTUs) properties are deteriorated. Whether the age-related decline in these properties limits participants’ capacity to recover their balance following a slip with FLB remains unclear. We combined isokinetic dynamometry, ultrasound and EMG to understand how knee extensor and ankle plantarflexor muscle strength and power, rate of moment development, electromechanical delay, and tendon stiffness affected the balance of young (25.3 ± 3.9 years) and older adults (62.8 ± 7.1 years) when recovering from a single slip with FLB triggered whilst walking on a split-belt instrumented treadmill. Except for the patellar tendon’s stiffness, knee extensor and ankle plantarflexor electromechanical delays, older adults’ MTUs properties were deteriorated compared to those of young participants (p < 0.05). We found no significant relationship between age or the MTUs properties of participants and balance recovery. These findings provide additional support that neither maximal nor explosive strength training are likely to be successful in preventing a fall for healthy older adults, and that other type of interventions, such as task-specific training that has already proved efficacious in reducing the risk of falling, should be developed.

Differences in muscle synergies among recovery responses limit inter-task generalisation of stability performance

Generalisation of adaptations is key to effective stability control facing variety of postural threats during daily life activity. However, in a previous study we could demonstrate that adaptations to stability control do not necessarily transfer to an untrained motor task. Here, we examined the dynamic stability and modular organisation of motor responses to different perturbations (i.e. unpredictable gait-trip perturbations and subsequent loss of anterior stability in a lean-and-release protocol) in a group of young and middle-aged adults (n = 57; age range 19-53 years) to detect potential neuromotor factors limiting transfer of adaptations within the stability control system. We hypothesized that the motor system uses different modular organisation in recovery responses to tripping and lean-and-release, which may explain lack in positive transfer of adaptations in stability control. After eight trip-perturbations participants increased their dynamic stability during the first recovery step (p < 0.001), yet they showed no significant improvement to the untrained lean-and-release transfer task compared to controls who did not undergo the perturbation exposure (p = 0.44). Regarding the neuromuscular control of responses, lower number of synergies (3 vs. 4) was found for the lean-and-release compared to the gait-trip perturbation task, revealing profound differences in both the timing and function of the recruited muscles to match the biomechanical specificity of different perturbations. Our results provide indirect evidence that the motor system uses different modular organisation in diverse perturbation responses, what possibly inhibits inter-task generalisation of adaptations in stability control.

The Stepping Threshold Test for Reactive Balance: Validation of Two Observer-Based Evaluation Strategies to Assess Stepping Behavior in Fall-Prone Older Adults

Introduction: Measurement of reactive balance is critical for fall prevention but is severely underrepresented in the clinical setting due to the lack of valid assessments. The Stepping Threshold Test (STT) is a newly developed instrumented test for reactive balance on a movable platform, however, it has not yet been validated for fall-prone older adults. Furthermore, different schemes of observer-based evaluation seem possible. The aim of this study was to investigate validity with respect to fall risk, interpretability, and feasibility of the STT using two different evaluation strategies. Methods: This study involved 71 fall-prone older adults (aged ≥ 65) who underwent progressively increasing perturbations in four directions for the STT. Single and multiple-step thresholds for each perturbation direction were determined via two observer-based evaluation schemes, which are the 1) consideration of all steps (all-step-count evaluation, ACE) and 2) consideration of those steps that extend the base of support in the direction of perturbation (direction-sensitive evaluation, DSE). Established balance measures including global (Brief Balance Evaluations Systems Test, BriefBEST), proactive (Timed Up and Go, TUG), and static balance (8-level balance scale, 8LBS), as well as fear of falling (Short Falls Efficacy Scale-International, FES-I) and fall occurrence in the past year, served as reference measurements. Results: The sum scores of STT correlated moderately with the BriefBEST (ACE: r = 0.413; DSE: r = 0.388) and TUG (ACE: r = -0.379; DSE: r = -0.435) and low with the 8LBS (ACE: r = 0.173; DSE: r = 0.246) and Short FES-I (ACE: r = -0.108; DSE: r = -0.104). The sum scores did not distinguish between fallers and non-fallers. No floor/ceiling effects occurred for the STT sum score, but these effects occurred for specific STT thresholds for both ACE (mean floor effect = 13.04%, SD = 19.35%; mean ceiling effect = 4.29%, SD = 7.75%) and DSE (mean floor effect = 7.86%, SD = 15.23%; mean ceiling effect = 21.07%, SD = 26.08). No severe adverse events occurred. Discussion: Correlations between the STT and other balance tests were in the expected magnitude, indicating convergent validity. However, the STT could not distinguish between fallers and non-fallers, referring to a need for further studies and prospective surveys of falls to validate the STT. Current results did not allow a definitive judgment on the advantage of using ACE or DSE. Study results represented a step toward a reactive balance assessment application in a clinical setting.

Stability recovery performance in adults over a wide age range: A multicentre reliability analysis using different lean-and-release test protocols

The ability to effectively increase the base of support is crucial to prevent from falling due to stability disturbances and has been commonly assessed using the forward-directed lean-and-release test. With this multicentre study we examined whether the assessment of stability recovery performance using two different forward lean-and-release test protocols is reliable in adults over a wide age range. Ninety-seven healthy adults (age from 21 to 80 years) were randomly assigned to one out of two lean angle protocols: gradual increase to maximal forward-lean angle (maximal lean angle; n = 43; seven participants were excluded due to marker artefacts) or predefined lean angle (single lean angle; n = 26; 21 participants needed to be excluded due to multiple stepping after release or marker artefacts). Both protocols were repeated after 0.5 h and 48 h to investigate intra- and inter-session reliability. Stability recovery performance was examined using the margin of stability at release (MoS_RL) and touchdown (MoS_TD) and increase in base of support (BoS_TD). Intraclass correlation coefficients (confidence intervals at 95%) for the maximal lean angle and for the single lean angle were respectively 0.93 (0.89-0.96) and 0.94 (0.89-0.97) in MoS_RL, 0.85 (0.77-0.91) and 0.67 (0.48-0.82) in MoS_TD and 0.88 (0.81-0.93) and 0.80 (0.66-0.90) in BoS_TD, with equivalence being revealed for each parameter between all three measurements (p < 0.01). We concluded that the assessment of stability recovery performance parameters in adults over a wide age range with the means of the forward lean-and-release test is reliable, independent of the used lean angle protocol.

Margin of Stability May Be Larger and Less Variable during Treadmill Walking Versus Overground

Margin of stability (MOS) is considered a measure of mechanical gait stability. Due to broad application of treadmills in gait assessment experiments, we aimed to determine if walking on a treadmill vs. overground would affect MOS during three speed-matched conditions. Eight healthy young participants walked on a treadmill and overground at Slow, Preferred, and Fast speed-matched conditions. The mean and variability (standard deviation) of the MOS in anterior-posterior and mediolateral directions at heel contact were calculated. Anterior-posterior and mediolateral mean MOS values decreased with increased speed for both overground and treadmill; although mediolateral mean MOS was always wider on the treadmill compared to overground. Due to lack of optic flow and different proprioceptive inputs during treadmill walking, subjects may employ strategies to increase their lateral stability on treadmill compared to overground. Anterior-posterior MOS variability increased with speed overground, while it did not change on treadmill, which might be due to the fixed speed of treadmill. Whereas, lateral variability on both treadmill and overground was U-shaped. Walking at preferred speed was less variable (may be interpreted as more stable) laterally, compared to fast and slow speeds. Caution should be given when interpreting MOS between modes and speeds of walking. As sagittal plane walking is functionally unstable, this raises the consideration as to the meaningfulness of using MOS as a global measure of gait stability in this direction.

Stepping impairment and falls in older adults: A systematic review and meta-analysis of volitional and reactive step tests

OBJECTIVE

To systematically examine stepping performance as a risk factor for falls. More specifically, we examined (i) if step tests can distinguish fallers from non-fallers and (ii) the type of step test (e.g. volitional vs reactive stepping) that is required to distinguish fallers from non-fallers.

DATA SOURCE

PubMed, EMBASE, CINAHL, Cochrane Database of Systematic Reviews and reference lists of included articles.

STUDY SELECTION

Cross-sectional and cohort studies that assessed the association between at least one step test and falls in older people (age ≥ 60 and/or mean age of 65).

RESULTS

A meta-analysis of 61 studies (n = 9536) showed stepping performance was significantly worse in fallers compared to non-fallers (Cohen'sd 0.56, 95 % CI 0.48 to 0.64, p < 0.001, I² 66 %). This was the case for both volitional and reactive step tests. Twenty-three studies (n = 3615) were included in a diagnostic meta-analysis that showed that step tests have moderate sensitivity (0.70, 95 % CI 0.62 to 0.77), specificity (0.68, 95 % CI 0.58 to 0.77) and area under the receiver operating characteristics curve (AUC) (0.75, 95 % CI 0.59 to 0.86) in discriminating fallers from non-fallers.

CONCLUSIONS

This large systematic review demonstrated that both volitional and reactive stepping impairments are significant fall risk factors among older adults. Step tests can identify fallers from non-fallers with moderate accuracy.

Volitional step execution is an ineffective predictor of recovery performance after sudden balance loss across the age range

Rapid stepping to preserve stability is a crucial action in avoiding a fall. It is also an important measure in the assessment of fall-resisting skills. We examined whether volitional step execution correlates with recovery stepping performance after sudden balance loss for adults of different ages. In addition, we investigated whether volitional step performance can discriminate between individuals with high and low balance recovery capabilities, i.e. between those making single versus multiple steps after balance perturbation. Healthy adults (28 young, 43 middle-aged and 26 older; 24 ± 4, 52 ± 5 and 72 ± 5 years respectively) performed a single step in the anterior direction volitionally in response to a mechanical stimulus to the heel. In a second stepping task, participants experienced sudden anterior balance loss in a lean-and-release protocol. For both tasks, an optical motion capture system was used to assess stepping kinematics. We found on average 28% shorter reaction times, 46% faster maximal step velocities and 48% higher rates of increase in base of support across all participants after sudden balance loss compared to volitional stepping (p < 0.001). There was a significant age-related decline in recovery stepping performance after sudden balance loss: 24/26 older, 15/43 middle-aged and none of the younger adults required two or more steps to regain balance (p < 0.001). Multiple- compared to single-steppers had on average 23% shorter step lengths and 12% lower maximal step velocities for the lean-and-release task (p < 0.01). Multiple-steppers also had reduced rates of increase in base of support for both stepping tasks (14% for balance recovery and 11% for volitional stepping). Furthermore, in examining the relationship between the results of the two tasks, only weak to moderate correlations were observed for step velocity and rate of increase in base of support (0.36 ≤ r ≤ 0.52; p < 0.001). Thus, performance in volitional step execution has a low potential to explain variability in recovery response after sudden balance loss in adults across the lifespan and hence seems less suitable to be used to identify deficiencies in reactive stepping responses necessary to cope with sudden balance disturbances.

Neuromechanics of Dynamic Balance Tasks in the Presence of Perturbations

Understanding the neuromechanical responses to perturbations in humans may help to explain the reported improvements in stability performance and muscle strength after perturbation-based training. In this study, we investigated the effects of perturbations, induced by unstable surfaces, on the mechanical loading and the modular organization of motor control in the lower limb muscles during lunging forward and backward. Fifteen healthy adults performed 50 forward and 50 backward lunges on stable and unstable ground. Ground reaction forces, joint kinematics, and the electromyogram (EMG) of 13 lower limb muscles were recorded. We calculated the resultant joint moments and extracted muscle synergies from the stepping limb. We found sparse alterations in the resultant joint moments and EMG activity, indicating a little if any effect of perturbations on muscle mechanical loading. The time-dependent structure of the muscle synergy responsible for the stabilization of the body was modified in the perturbed lunges by a shift in the center of activity (later in the forward and earlier in the backward lunge) and a widening (in the backward lunge). Moreover, in the perturbed backward lunge, the synergy related to the body weight acceptance was not present. The found modulation of the modular organization of motor control in the unstable condition and related minor alteration in joint kinetics indicates increased control robustness that allowed the participants to maintain functionality in postural challenging settings. Triggering specific modulations in motor control to regulate robustness in the presence of perturbations may be associated with the reported benefits of perturbation-based training.

A comparison of balance-correcting responses induced with platform-translation and shoulder-pull perturbation methods

The understanding of reactive balance control mechanisms in humans emanates from studies utilizing a variety of perturbation methods, instructions, and sensory conditions. The use of different perturbation methods may produce method-specific balance-correcting responses. This study evaluated balance-correcting responses induced with platform-translation and shoulder-pull methods with equilibrated perturbation intensities, and whether the absence of vision affects balance-correcting responses differently between perturbation methods. Fifteen healthy young males participated. Unexpected forward and backward platform-translation and shoulder-pull perturbations were induced with eyes-open and eyes-closed. Participants were asked to behave naturally. Forward stepping trials were analyzed. Margin of stability (MOS) was calculated from the position data of reflective markers placed strategically around the body. MOS was reported at step onset and at foot contact. MOS was smaller at step onset (0.01 ± 0.01 m) and at foot contact (0.09 ± 0.01 m) during platform-translation trials and at both time points during shoulder-pull trials (0.04 ± 0.01 m and 0.17 ± 0.01 m, respectively). The absence of vision did not affect MOS at step onset. At foot contact during platform-translation with eyes-closed MOS was larger (0.11 ± 0.01 m) than with eyes-open (0.08 ± 0.01 m), but not different between eyes-open (0.17 ± 0.01 m) and eyes-closed (0.18 ± 0.01 m) during shoulder-pull. Participants required a second step to recover balance in 14% of the platform-translation and 3% of the shoulder-pull trials. During platform-translation trials participants demonstrated smaller MOS which placed them in a less favorable circumstance for balance recovery. Platform-translation appears to be more challenging than shoulder-pull perturbation in terms of balance recovery. This study underscores that caution is required when interpreting results of studies utilizing different perturbation paradigms.

Reactive gait and postural adjustments following the first exposures to (un)expected stepdown

This study evaluated the reactive biomechanical strategies associated with both upper- and lower-body (lead and trail limbs) following the first exposures to (un)expected stepdown at comfortable (1.22 ± 0.08 m/s) and fast (1.71 ± 0.11 m/s) walking velocities. Eleven healthy adults completed 34 trails per walking velocity over an 8-m, custom-built track with two forceplates embedded in its center. For the expected stepdown, the track was lowered by 0-, -10- and -20-cm from the site of the second forceplate, whereas the unexpected stepdown was created by camouflaging the second forceplate (-10-cm). Two-way repeated-measurement ANOVAs detected no velocity-related effects of stepdown on kinematic and kinetic parameters during lead limb stance-phase, and on the trail limb stepping kinematics. However, analyses of significant interactions revealed greater peak flexion angles across the trunk and the trail limb joints (hip, knee and ankle) in unexpected versus expected stepdown conditions at a faster walking velocity. The -10-cm unexpected stepdown (main effect) had a greater influence on locomotor behavior compared to expected conditions due mainly to the absence of predictive adjustments, reflected by a significant decrease in peak knee flexion, contact time and vertical impulse during stance-phase. Walking faster (main effect) was associated with an increase in hip peak flexion and net anteroposterior impulse, and a decrease in contact time and vertical impulse during stepdown. The trail limb, in response, swung forward faster, generating a larger and faster recovery step. However, such reactive stepping following unexpected stepdown was yet a sparse compensation for an unstable body configuration, assessed by significantly smaller step width and anteroposterior margin-of-stability at foot-contact in the first-recovery-step compared with expected conditions. These findings depict the impact of the expectedness of stepdown onset on modulation of global dynamic postural control for a successful accommodation of (un)expected surface elevation changes in young, healthy adults.

The cross-sectional relationships between age, standing static balance, and standing dynamic balance reactions in typically developing children

BACKGROUND

Static balance performance is a common metric for evaluating the development of postural control in children. Less is known about the potentially independent development of dynamic balance performance.

RESEARCH QUESTION

How does age relate to static (i.e. postural sway) and dynamic (i.e. stepping thresholds) standing balance performance, and what is the relationship between postural sway and stepping thresholds?

METHODS

Twenty-six typically developing children (12 males, 14 females; 5-12 years of age) were recruited for this cross-sectional study. Static balance performance was quantified as the total path length during a postural sway assessment using a force platform with conditions of eyes open and eyes closed. Dynamic balance performance was quantified using a single-stepping threshold assessment, whereby participants attempted to prevent a step in response to treadmill-induced perturbations in the anterior and posterior directions. Relationships between age and body-size scaled measures of static and dynamic balance performance were assessed using Spearman rank correlations.

RESULTS

There was a weak correlation between age and postural sway (|r_s| < 0.10, p >  0.68), but a moderate-to-strong correlation between age and single-stepping thresholds (r_s > 0.68, p < 0.001). A weak correlation was found between postural sway and single-stepping thresholds (|r_s| < 0.20, p >  0.39).

SIGNIFICANCE

Dynamic, but not static standing balance performance, may improve with typical development between the ages of 5 and 12 years. Static and dynamic balance should be considered as unique constructs when assessed in children.

Reliability of measures of dynamic stability for the assessment of balance recovery after a forward loss of balance

BACKGROUND

Falls are common and serious events, which mostly occur during locomotion, that are associated with deficient dynamic balance. An experimental approach that simulates falling forward has become increasing popular to investigate dynamic balance. However, research has not been conducted to examine the test-retest reliability of this experimental approach.

RESEARCH QUESTION

What is the reliability of dynamic stability measures that are used for the assessment of balance recovery after forward loss of balance?

METHODS

Nineteen healthy young adults (24.3 ± 2.8 yrs; nine females) volunteered for this study. They reported twice to the laboratory to perform two tests: (i) a stepping task, in which they were instructed to recover balance by taking a step after being suddenly released from an inclined forward position; and (ii) a standing task, in which we aimed to identify the maximum forward leaning angle they were able to compensate for without taking a step. Intra-class correlation coefficients (ICC) were calculated for the margin of stability (MoS) and spatiotemporal parameters for both tests.

RESULTS

The reliability of the stepping task variables ranged from poor to excellent, with ICCs tending to increase with the number of trials included in the analysis. Intra-session analysis (one-way rm ANOVA) revealed a significant trial effect for the MoS, indicating that stepping responses changed across repeated trials. With respect to the standing task, test-retest reliability was only fair for the maximal initial leaning angle.

SIGNIFICANCE

In essence, these results indicate that the inter-session reliability of the stepping task is acceptable, depending on the measures used and the number of trials conducted. However, one must be aware that behavioral adaptations arise with repeated exposure to simulated forward falls. Finally, this study's results suggest that the reproducibility of the standing task is limited.

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.

Retention, savings and interlimb transfer of reactive gait adaptations in humans following unexpected perturbations

Reactive locomotor adaptations are crucial for safe mobility, but remain relatively unexplored. Here we assess reactive gait adaptations, and their retention, savings and interlimb transfer. Using new methods to normalise walking speed and perturbation magnitude, we expose eighteen healthy adults to ten unexpected treadmill belt accelerations during walking (the first and last perturbing the right leg, the others perturbing the left leg) on two days, one month apart. Analysis of the margins of stability using kinematic data reveals that humans reactively adapt gait, improving stability and taking fewer recovery steps, and fully retain these adaptations over time. On re-exposure, retention and savings lead to further improvements in stability. Currently, the role of interlimb transfer is unclear. Our findings show that humans utilise retention and savings in reactive gait adaptations to benefit stability, but that interlimb transfer may not be exclusively responsible for improvements following perturbations to the untrained limb.

Follow-up efficacy of physical exercise interventions on fall incidence and fall risk in healthy older adults: a systematic review and meta-analysis

Background

The risk of falling and associated injuries increases with age. Therefore, the prevention of falls is a key priority in geriatrics and is particularly based on physical exercising, aiming to improve the age-related decline in motor performance, which is crucial in response to postural threats. Although the benefits and specifications of effective exercise programs have been well documented in pre-post design studies, that is during the treatment, the definitive retention and transfer of these fall-related exercise benefits to the daily life fall risk during follow-up periods remains largely unclear. Accordingly, this meta-analysis investigates the efficacy of exercise interventions on the follow-up risk of falling.

Methods

A systematic database search was conducted. A study was considered eligible if it examined the number of falls (fall rate) and fallers (fall risk) of healthy older adults (≥ 65 years) during a follow-up period after participating in a randomized controlled physical exercise intervention. The pooled estimates of the fall rate and fall risk ratios were calculated using a random-effects meta-analysis. Furthermore, the methodological quality and the risk of bias were assessed.

Results

Twenty-six studies with 31 different intervention groups were included (4739 participants). The number of falls was significantly (p <0.001) reduced by 32% (rate ratio 0.68, 95% confidence interval 0.58 to 0.80) and the number of fallers by 22% (risk ratio 0.78, 95% confidence interval 0.68 to 0.89) following exercising when compared with controls. Interventions that applied posture-challenging exercises showed the highest effects. The methodological quality score was acceptable (73 ± 11%) and risk of bias low.

Conclusions

The present review and meta-analysis provide evidence that physical exercise interventions have the potential to significantly reduce fall rate and risk in healthy older adults. Posture-challenging exercises might be particularly considered when designing fall prevention interventions.

Electronic supplementary material

The online version of this article (10.1186/s40798-018-0170-z) contains supplementary material, which is available to authorized users.

Age-Related Changes in Achilles Tendon Stiffness and Impact on Functional Activities: A Systematic Review and Meta-Analysis

Achilles tendon stiffness determines calf muscle functioning during functional activities. However, contrasting evidence was found in studies comparing Achilles tendon stiffness in older and younger adults. Therefore, this systematic review compares Achilles tendon stiffness and elastic modulus in older and younger adults and reviews functional implications. Studies revealed by systematic bibliographic searches were included if healthy older adults were investigated, and if Achilles tendon stiffness was measured using ultrasound and dynamometry. Meta-analyses were performed to compare Achilles tendon stiffness and elastic modulus in older and younger adults. Achilles tendon stiffness (weighted standardized mean difference = 1.40, 95% confidence intervals [0.42-2.38]) and elastic modulus (weighted standardized mean difference = 1.74, 95% confidence intervals [0.99-2.49]) were decreased in older compared with younger adults. Decreased Achilles tendon stiffness was related to walking performance and balance. Possibly, decreased Achilles tendon stiffness is caused by altered elastic modulus in older adults. Training interventions increasing Achilles tendon stiffness could improve functional capacity.

Neuromuscular and Kinematic Adaptation in Response to Reactive Balance Training - a Randomized Controlled Study Regarding Fall Prevention

Slips and stumbles are main causes of falls and result in serious injuries. Balance training is widely applied for preventing falls across the lifespan. Subdivided into two main intervention types, biomechanical characteristics differ amongst balance interventions tailored to counteract falls: conventional balance training (CBT) referring to a balance task with a static ledger pivoting around the ankle joint versus reactive balance training (RBT) using externally applied perturbations to deteriorate body equilibrium. This study aimed to evaluate the efficacy of reactive, slip-simulating RBT compared to CBT in regard to fall prevention and to detect neuromuscular and kinematic dependencies. In a randomized controlled trial, 38 participants were randomly allocated either to CBT or RBT. To simulate stumbling scenarios, postural responses were assessed to posterior translations in gait and stance perturbation before and after 4 weeks of training. Surface electromyography during short- (SLR), medium- (MLR), and long-latency response of shank and thigh muscles as well as ankle, knee, and hip joint kinematics (amplitudes and velocities) were recorded. Both training modalities revealed reduced angular velocity in the ankle joint (P < 0.05) accompanied by increased shank muscle activity in SLR (P < 0.05) during marching in place perturbation. During stance perturbation and marching in place perturbation, hip angular velocity was decreased after RBT (P from TTEST, P_t < 0.05) accompanied by enhanced thigh muscle activity (SLR, MLR) after both trainings (P < 0.05). Effect sizes were larger for the RBT-group during stance perturbation. Thus, both interventions revealed modified stabilization strategies for reactive balance recovery after surface translations. Characterized by enhanced reflex activity in the leg muscles antagonizing the surface translations, balance training is associated with improved neuromuscular timing and accuracy being relevant for postural control. This may result in more efficient segmental stabilization during fall risk situations, independent of the intervention modality. More pronounced modulations and higher effect sizes after RBT in stance perturbation point toward specificity of training adaptations, with an emphasis on the proximal body segment for RBT. Outcomes underline the benefits of balance training with a clear distinction between RBT and CBT being relevant for training application over the lifespan.

Analysis of biases in dynamic margins of stability introduced by the use of simplified center of mass estimates during walking and turning

The ability to control the body's center of mass (CoM) is critical for preventing falls, which are a major health concern in aging populations. Control of the CoM has been assessed by characterizing dynamic margins of stability (MoS) which capture the dynamic relationship between the CoM and the base of support. Accurate estimation of CoM dynamics is best accomplished using a full-body marker set. However, a number of simplified estimates have been used throughout literature. Here, we determined the biases and sources of bias when computing MoS using four simplified CoM models, and we characterized how these biases varied in straight walking versus turning. CoM kinematics were characterized using a full-body marker set, the lower extremities and trunk, lower extremities only, an average of four pelvic markers, and one pelvic marker alone. Significant bias was demonstrated for most methods and was larger during turning tasks compared to straight walking. In the fore-aft direction, only overestimates in the MoS were observed, and these ranged from 15 to 110% larger than the true MoS value. In the mediolateral direction, both under- and over-estimates were observed and ranged from -175 to 225%. Across tasks, bias was smallest when using the lower extremity plus trunk (-23 to 62%) and pelvis average methods (-71 to 43%). Sources of bias were attributed to misestimates of CoM height, velocity, and position. Together, our findings suggest that the 1) lower extremity and trunk model and 2) pelvis average model should be considered in future studies to minimize bias when simplified models of CoM dynamics are desired.

Muscle contributions to the acceleration of the whole body centre of mass during recovery from forward loss of balance by stepping in young and older adults

The purpose of this study was to determine the muscular contributions to the acceleration of the whole body centre of mass (COM) of older compared to younger adults that were able to recover from forward loss of balance with a single step. Forward loss of balance was achieved by releasing participants (14 older adults and 6 younger adults) from a static whole-body forward lean angle of approximately 18 degrees. 10 older adults and 6 younger adults were able to recover with a single step and included in subsequent analysis. A scalable anatomical model consisting of 36 degrees-of-freedom was used to compute kinematics and joint moments from motion capture and force plate data. Forces for 92 muscle actuators were computed using Static Optimisation and Induced Acceleration Analysis was used to compute individual muscle contributions to the three-dimensional acceleration of the whole body COM. There were no significant differences between older and younger adults in step length, step time, 3D COM accelerations or muscle contributions to 3D COM accelerations. The stance and stepping leg Gastrocnemius and Soleus muscles were primarily responsible for the vertical acceleration experienced by the COM. The Gastrocnemius and Soleus from the stance side leg together with bilateral Hamstrings accelerated the COM forwards throughout balance recovery while the Vasti and Soleus of the stepping side leg provided the majority of braking accelerations following foot contact. The Hip Abductor muscles provided the greatest contribution to medial-lateral accelerations of the COM. Deficits in the neuromuscular control of the Gastrocnemius, Soleus, Vasti and Hip Abductors in particular could adversely influence balance recovery and may be important targets in interventions to improve balance recovery performance.

After total knee replacement younger patients demonstrate superior balance control compared to older patients when recovering from a forward fall

BACKGROUND

National joint replacement registries have reported a substantial growth in younger knee osteoarthritic patients (<55years old) undergoing total knee replacement, however this younger population is generally understudied. Importantly, studies examining experimentally controlled perturbation have shown age-related differences between younger and older healthy adults, whether similar age-related differences exist among total knee replacement patients is unknown.

METHODS

A total of 59 participants, including 29 unilateral total knee replacement patients (six-months post-surgery) made up the four experimental groups: 1) younger patient (54.3 (SD 7.9) years), 2) younger control (55.2 (SD 4.0) years), 3) older patient (76.9 (SD 4.7) years), and 4) older control (77.7 (SD 4.1) years). Using a tether-release method to perturb balance and simulate a forward fall, center of mass and stepping characteristics were analyzed.

FINDINGS

Younger patients recovered following the perturbation with a significantly smaller center of mass displacement compared to the older patients (14.85 (SD 0.01) v. 18.13 (SD 0.02) %ht, p=0.02); utilizing a longer (0.43 (SD 0.02) v. 0.39 (SD 0.03) m, p<0.001) and higher velocity (2.01 (SD 0.2) v. 1.59 (SD 0.2) m/s, p=0.001) recovery step. Importantly, younger patients did not differ significantly from the younger controls in center of mass displacement or recovery step characteristics (p>0.05).

INTERPRETATION

The younger patients demonstrated superior center of mass control in response to a forward perturbation, suggesting that younger patients would be at a reduced risk of falling when recovering from a forward-directed postural perturbation compared to older patients.

Predictive and Reactive Locomotor Adaptability in Healthy Elderly: A Systematic Review and Meta-Analysis

Background

Locomotor adaptability is based on the implementation of error-feedback information from previous perturbations to predictively adapt to expected perturbations (feedforward) and to facilitate reactive responses in recurring unexpected perturbations (‘savings’). The effect of aging on predictive and reactive adaptability is yet unclear. However, such understanding is fundamental for the design and application of effective interventions targeting fall prevention.

Methods

We systematically searched the Web of Science, MEDLINE, Embase and Science Direct databases as well as the reference lists of the eligible articles. A study was included if it addressed an investigation of the locomotor adaptability in response to repeated mechanical movement perturbations of healthy older adults (≥60 years). The weighted average effect size (WAES) of the general adaptability (adaptive motor responses to repeated perturbations) as well as predictive (after-effects) and reactive adaptation (feedback responses to a recurring unexpected perturbation) was calculated and tested for an overall effect. A subgroup analysis was performed regarding the factor age group [i.e., young (≤35 years) vs. older adults]. Furthermore, the methodological study quality was assessed.

Results

The review process yielded 18 studies [1009 participants, 613 older adults (70 ± 4 years)], which used various kinds of locomotor tasks and perturbations. The WAES for the general locomotor adaptability was 1.21 [95 % confidence interval (CI) 0.68–1.74, n = 11] for the older and 1.39 (95 % CI 0.90–1.89, n = 10) for the young adults with a significant (p < 0.05) overall effect for both age groups and no significant subgroup differences. Similar results were found for the predictive (older: WAES 1.10, 95 % CI 0.37–1.83, n = 8; young: WAES 1.54, 95 % CI 0.11–2.97, n = 7) and reactive (older: WAES 1.09, 95 % CI 0.22–1.96, n = 5; young: WAES 1.35, 95 % CI 0.60–2.09, n = 5) adaptation featuring significant (p < 0.05) overall effects without subgroup differences. The average score of the methodological quality was 67 ± 8 %.

Conclusions

The present meta-analysis provides elaborate statistical evidence that locomotor adaptability in general and predictive and reactive adaptation in particular remain highly effective in the elderly, showing only minor, not statistically significant age-related deficits. Consequently, interventions which use adaptation and learning paradigms including the application of the mechanisms responsible for an effective predictive and reactive dynamic stability control may progressively improve older adults’ recovery performance and, thus, reduce their risk of falling.

Electronic supplementary material

The online version of this article (doi:10.1007/s40279-015-0413-9) contains supplementary material, which is available to authorized users.

Possible recovery or unavoidable fall? A model to predict the one step balance recovery threshold and its stepping characteristics

In order to prevent fall related injuries and their consequences, one needs to be able to predict the outcome of a given balance perturbation: a possible Balance Recovery (BR) or an unavoidable fall? Given that results from the existing experimental studies are difficult to compare and to generalize, we propose to address this question with a numerical tool. Built on existing concepts from the biomechanics and robotics literature, it includes the optimal use of BR reactions and particularly the possibility to perform a recovery step. It allows estimating 1) the possibility to recover a steady balance from a given initial state or perturbation using at most one recovery step; 2) the set of recovery steps leading to a BR. Using standard sets of parameters for young and elderly population, we assessed this model's predictions against experimental data from the literature in the anterior direction. Two classical representations of the human body (inverted pendulum (IP) vs. linear inverted pendulum (LIP)) were also compared. The results showed that the model correctly predicted the possibility to recover using a single protective step (1-Step BR threshold) and the characteristics (step length and time) of the protective step for both the young and the elderly. This tool has a real potential in the field of fall prevention to detect risky situation. It could also be used to get insights into the neuromuscular mechanisms involved in the BR process.

Effect of Jaw Clenching on Balance Recovery: Dynamic Stability and Lower Extremity Joint Kinematics after Forward Loss of Balance

Postural control is crucial for most tasks of daily living, delineating postural orientation and balance, with its main goal of fall prevention. Nevertheless, falls are common events, and have been associated with deficits in muscle strength and dynamic postural stability. Recent studies reported on improvements in rate of force development and static postural control evoked by jaw clenching activities, potentially induced by facilitation of human motor system excitability. However, there are no studies describing the effects on dynamic stability. The present study, therefore, aimed to investigate the effects of submaximum jaw clenching on recovery behavior from forward loss of balance. Participants were 12 healthy young adults, who were instructed to recover balance from a simulated forward fall by taking a single step while either biting at a submaximum force or keeping the mandible at rest. Bite forces were measured by means of hydrostatic splints, whereas a 3D motion capture system was used to analyze spatiotemporal parameters and joint angles, respectively. Additionally, dynamic stability was quantified by the extrapolated CoM concept, designed to determine postural stability in dynamic situations. Paired t-tests revealed that submaximum biting did not significantly influence recovery behavior with respect to any variable under investigation. Therefore, reductions in postural sway evoked by submaximum biting are obviously not transferable to balance recovery as it was assessed in the present study. It is suggested that these contradictions are the result of different motor demands associated with the abovementioned tasks. Furthermore, floor effects and the sample size might be discussed as potential reasons for the absence of significances. Notwithstanding this, the present study also revealed that bite forces under both conditions significantly increased from subjects’ release to touchdown of the recovery limb. Clenching the jaw, hence, seems to be part of a common physiological repertoire used to improve motor performance.

Differences in Functional Fitness Among Older Adults With and Without Risk of Falling

PURPOSE

This study aimed to identify the differences in functional fitness between older adults who were at risk of falling and those who were not.

METHODS

A total of 104 older adults aged 65-74 years were recruited from a local community senior center. They were independent older adults without a history of falls in the preceding 12 months. Falling risk status was assessed using the Fall Risk Test. Five dimensions of functional fitness with seven testing parameters (i.e., 30-second chair stand test, 30-second arm curl test, 2-minute step test, chair sit and reach test, back scratch test, 8-foot up and go test, and body mass index) were evaluated by the Senior Fitness Test.

RESULTS

Only 78 participants completed all the tests, of which 48 participants were identified with risk of falling, and 30 participants were free from risk of falling. Results from multivariate analysis of variance found significant differences on the combined outcome variables, especially in the 8-foot up and go test, 2-minute step test, and 30-second arm curl test. Results from discriminant analysis found a significant discriminant function among all the seven testing parameters, where the 8-foot up and go test, and the 2-minute step test contributed most.

CONCLUSIONS

Older adults who are at the early stage of risk of falling tend to have lower functional fitness capacities, especially in agility and dynamic balance, aerobic endurance as well as in a combined relationship among all the testing parameters.

Relationships between performance and kinematic/kinetic variables of stair descent in patients with medial knee osteoarthritis: An evaluation of dynamic stability using an extrapolated center of mass

BACKGROUND

The ability to descend stairs independently is impaired from a relatively early stage in patients with knee osteoarthritis. The purpose of this study was to evaluate the performance in patients with knee osteoarthritis when stepping down a step by evaluating the dynamic stability using the extrapolated center of mass.

METHODS

Twenty-three individuals with medial knee osteoarthritis were evaluated during step descent without any assistance. Kinematic/kinetic data were collected using a three-dimensional motion analysis system and force platforms. The extrapolated center of mass and its deviation from the anterior boundary on the base of support (margin of stability) were calculated at the initiation of descent. Joint angles and internal joint moments were collected at the stance limb. The relationship between patients' dynamic stability control, which was measured by the timed up and go test, and the length of margin of stability was analyzed. Relationships between the length of the margin of stability and each kinematic/kinetic variable were also evaluated.

FINDINGS

The margin of stability positively correlated with the time taken for a timed up and go test. A positive correlation was additionally observed between the ankle dorsiflexion angle and the margin of stability. It was also found that a higher ratio of ankle plantar flexion moment by support moment was associated with a larger margin of stability.

INTERPRETATION

Patients with knee osteoarthritis who had high ability in dynamic stability control were observed to move their center of mass anteriorly at the initiation of stepping down. It was also suggested that these patients could dorsiflex their ankle joint and generate sufficient ankle plantar flexor torque.

Comparison between investigations of induced stepping postural responses and voluntary steps to better detect community-dwelling elderly fallers

In this paper, we review a physiological task that is predominant in preventing humans from falling, but that simultaneously also challenges balance: taking a step. In particular, two variants of this task are presented and compared: the voluntary step versus a step induced by an external and unpredictable perturbation. We show that, while these contribute different information, it is interesting to compare these. Indeed, they both are relevant in a global balance assessment and should be included within this, at the same level as tests usually dispensed in the clinical environment such as posturography. We choose to focus on the community-dwelling elderly population, to discuss means of early detection of risk of falls, in order to prescribe an appropriate prevention. An overview of posture-movement coordination and balance recovery strategies is also provided. Finally, a working hypothesis is suggested on how "compensatory protective" steps are controlled and how their evaluation could bring additional information to the global balance assessment of risk of fall.

Biomechanical predictors of maximal balance recovery performance amongst community-dwelling older adults

Falls are prevalent in older adults and are predicted by the maximum forward lean magnitude (MRLM) that can be recovered using a single step. The purpose of this study was to determine the relative contribution of selected neuromuscular and biomechanical variables associated with balance recovery to the MRLM. Forward loss of balance was induced by releasing participants (n=117 community-dwelling older adults) from a static forward lean angle. Participants were instructed to attempt to recover balance by taking a single step. A scalable anatomical model consisting of 36 degrees-of-freedom was used to compute kinematics and joint moments from motion capture and force plate data. Isometric muscle strength at the ankle, knee and hip joints was assessed using a dynamometer. A univariate analysis revealed that lower limb strength measures, step recovery kinematics, and stepping limb kinetics accounted for between 8 and 19%, 3 and 59%, and 3 and 61% of the variance in MRLM respectively. When all variables were entered into a stepwise multiple regression analysis, normalised step length, peak hip extension moment, trunk angle at foot contact, and peak hip flexion power during stepping together accounted for 69% of the variance in MRLM. These findings confirm that successful recovery from forward loss of balance is a whole body control task that requires adequate trunk control and generation of adequate lower limb moments and powers to generate a long and rapid step. Training programmes that specifically target these measures may be effective in improving balance recovery performance and thereby contribute to fall prevention amongst older adults.

Recovery performance and factors that classify young fallers and non-fallers in Parkinson's disease

Postural instability is a major problem for Parkinson's disease patients (PDs). Identifying the causes of postural instability at a young age would contribute to the development of adequate training interventions aiming to reduce falls. The purpose of this study was to investigate the effect of muscle strength and balance ability on dynamic stability control after simulated disturbances and to develop an applicable tool able to classify young PDs into fallers and non-fallers. Twenty-five young PDs (12 fallers, 13 non-fallers, 48±5 yrs.) and 14 healthy controls participated in the study. Dynamic stability was examined during simulated forward falls. Muscle strength was assessed by isometric maximal plantarflexion and knee extension contractions. Balance ability was evaluated by measuring the anterior and posterior limits of stability (LoS). The fallers showed lower recovery performance in forward falls and lower muscle strength compared to controls. Muscle strength and anterior LoS were significantly associated to stability performance. These two factors could correctly classify 90% of PD fallers, establishing an accurate assessment tool to predict the falling risk in young PDs. Furthermore, muscle strength partly explained recovery performance; therefore, we can argue that young PDs with an increased falling risk may benefit from leg-extensors strengthening and stability training.

Reactive stepping behaviour in response to forward loss of balance predicts future falls in community-dwelling older adults

BACKGROUND

a fall occurs when an individual experiences a loss of balance from which they are unable to recover. Assessment of balance recovery ability in older adults may therefore help to identify individuals at risk of falls. The purpose of this 12-month prospective study was to assess whether the ability to recover from a forward loss of balance with a single step across a range of lean magnitudes was predictive of falls.

METHODS

two hundred and one community-dwelling older adults, aged 65-90 years, underwent baseline testing of sensori-motor function and balance recovery ability followed by 12-month prospective falls evaluation. Balance recovery ability was defined by whether participants required either single or multiple steps to recover from forward loss of balance from three lean magnitudes, as well as the maximum lean magnitude participants could recover from with a single step.

RESULTS

forty-four (22%) participants experienced one or more falls during the follow-up period. Maximal recoverable lean magnitude and use of multiple steps to recover at the 15% body weight (BW) and 25%BW lean magnitudes significantly predicted a future fall (odds ratios 1.08-1.26). The Physiological Profile Assessment, an established tool that assesses variety of sensori-motor aspects of falls risk, was also predictive of falls (Odds ratios 1.22 and 1.27, respectively), whereas age, sex, postural sway and timed up and go were not predictive.

CONCLUSION

reactive stepping behaviour in response to forward loss of balance and physiological profile assessment are independent predictors of a future fall in community-dwelling older adults. Exercise interventions designed to improve reactive stepping behaviour may protect against future falls.

Shear modulus estimation on vastus intermedius of elderly and young females over the entire range of isometric contraction

Elderly people often suffer from sarcopenia in their lower extremities, which gives rise to the increased susceptibility of fall. Comparing the mechanical properties of the knee extensor/flexors on elderly and young subjects is helpful in understanding the underlying mechanisms of the muscle aging process. However, although the stiffness of skeletal muscle has been proved to be positively correlated to its non-fatiguing contraction intensity by some existing methods, this conclusion has not been verified above 50% maximum voluntary contraction (MVC) due to the limitation of their measurement range. In this study, a vibro-ultrasound system was set up to achieve a considerably larger measurement range on muscle stiffness estimation. Its feasibility was verified on self-made silicone phantoms by comparing with the mechanical indentation method. The system was then used to assess the stiffness of vastus intermedius (VI), one of the knee extensors, on 10 healthy elderly female subjects (56.7±4.9 yr) and 10 healthy young female subjects (27.6±5.0 yr). The VI stiffness in its action direction was confirmed to be positively correlated to the % MVC level (R² = 0.999) over the entire range of isometric contraction, i.e. from 0% MVC (relaxed state) to 100% MVC. Furthermore, it was shown that there was no significant difference between the mean VI shear modulus of the elderly and young subjects in a relaxed state (p>0.1). However, when performing step isometric contraction, the VI stiffness of young female subjects was found to be larger than that of elderly participants (p<0.001), especially at the relatively higher contraction levels. The results expanded our knowledge on the mechanical property of the elderly’s skeletal muscle and its relationship with intensity of active contraction. Furthermore, the vibro-ultrasound system has a potential to become a powerful tool for investigating the elderly’s muscle diseases.

Posture and cognition in the elderly: interaction and contribution to the rehabilitation strategies

In this paper we review the effects of aging on sensory systems and their impact on posture, balance and gait. We also address cognitive aging and attempt to specify which altered cognitive functions negatively impact balance and walking. The role of cognition in postural control is tested with dual-task experiments. This situation results in deleterious effects due to an attentional overload. Given the human cognitive system has limited capacities, we propose that simultaneously performing two tasks depends on the capacity of each individual to perform these tasks on a continuum between automatic execution to highly controlled performance. A level of maximum control exceeds the subject's attentional capacity, which makes it impossible to perform both tasks simultaneously. The subject therefore prioritizes one of the tasks. We use representative dual-task studies from the literature to illustrate the relationship between the different cognitive components and their impact on the control of posture and gait in elderly subjects with altered cognitive capacities and with elderly subjects who are fallers or who have altered sensory-motor capacities. Recently this postural-cognitive relationship was addressed with a new approach. We report how cognitive training can improve dual-task management and we attempt to define the cognitive mechanisms that may be responsible for better postural balance.

Exercise of mechanisms of dynamic stability improves the stability state after an unexpected gait perturbation in elderly

Unexpected changes during gait challenge elderly individuals to a greater degree than young adults. However, the adaptive potential of elderly seems to be retained, and therefore, the training of the mechanisms of dynamic stability as well as muscle strength training may improve the dynamic stability after unexpected perturbations. Thirty-eight subjects (65-75 years) participated in the study, divided into two experimental groups (stability training group, ST, n = 14 and mixed training group, MT, n = 14) and a control group (CG, n = 10). Both experimental groups performed exercises which focused on the mechanisms of dynamic stability. Additionally, the MT group executed a training to improve muscle strength. Session volume and duration were equal for both groups (14 weeks, twice a week, ~1.5 h per session). Pre- and post-intervention, subjects performed a gait protocol with an induced unexpected perturbation. Post-intervention, the margin of stability was significantly increased after the unexpected perturbation in the ST group, indicating an improvement in stability state (pre, -30.3 ± 5.9 cm; post, -24.1 ± 5.2 cm). Further, both intervention groups increased their base of support after the intervention to regain balance after gait perturbation, whereas only the ST group showed a statistically significant improvement (STpre, 90.9 ± 6.6 cm, STpost, 98.2 ± 8.5 cm; MTpre, 91.4 ± 6.2 cm; MTpost, 97.9 ± 12.7 cm). The CG showed no differences between pre- and post-measurements. The exercise of the mechanisms of dynamic stability led to a better application of these mechanisms after an unexpected perturbation during gait. We suggest that the repeated exercise of the mechanisms of dynamic stability contributes to significant improvements in postural stability. Additional strength training for healthy elderly individuals, however, shows no further effect on the ability to recover balance after unexpected perturbations during gait.

Assessing the stability of human locomotion: a review of current measures

Falling poses a major threat to the steadily growing population of the elderly in modern-day society. A major challenge in the prevention of falls is the identification of individuals who are at risk of falling owing to an unstable gait. At present, several methods are available for estimating gait stability, each with its own advantages and disadvantages. In this paper, we review the currently available measures: the maximum Lyapunov exponent (λS and λL), the maximum Floquet multiplier, variability measures, long-range correlations, extrapolated centre of mass, stabilizing and destabilizing forces, foot placement estimator, gait sensitivity norm and maximum allowable perturbation. We explain what these measures represent and how they are calculated, and we assess their validity, divided up into construct validity, predictive validity in simple models, convergent validity in experimental studies, and predictive validity in observational studies. We conclude that (i) the validity of variability measures and λS is best supported across all levels, (ii) the maximum Floquet multiplier and λL have good construct validity, but negative predictive validity in models, negative convergent validity and (for λL) negative predictive validity in observational studies, (iii) long-range correlations lack construct validity and predictive validity in models and have negative convergent validity, and (iv) measures derived from perturbation experiments have good construct validity, but data are lacking on convergent validity in experimental studies and predictive validity in observational studies. In closing, directions for future research on dynamic gait stability are discussed.

Young and old adults prioritize dynamic stability control following gait perturbations when performing a concurrent cognitive task

There are conflicting results regarding the effect of aging on postural prioritization. The present study investigated balance recovery performance of young and old adults following unexpected gait perturbations in a dual task condition. Thirty-two young and 30 elderly male subjects were assigned to either control or dual task group. After baseline assessment, an unexpected gait perturbation was induced by a sudden change of surface rigidity. The dual task groups performed a mental arithmetic task. Dynamic stability was quantified based on the 'extrapolated center of mass' concept. The margin of stability decreased significantly at touchdown of the recovery leg following the unexpected perturbation (P) compared with baseline (base), yet irrespective of cognitive load (base: -4.63cm; P: -13.32cm; p<0.05). The number of errors in the cognitive task increased significantly (base: 0.13; P: 0.48; p<0.05) in both age groups. Since the stability performance was unaffected by additional cognitive load, whereas the cognitive task performance declined following the perturbation in both groups, it is concluded that postural prioritization occurs independent of age in response to unexpected gait perturbations.

Lower limb muscle moments and power during recovery from forward loss of balance in male and female single and multiple steppers

BACKGROUND

Studying recovery responses to loss of balance may help to explain why older adults are susceptible to falls. The purpose of the present study was to assess whether male and female older adults, that use a single or multiple step recovery strategy, differ in the proportion of lower limb strength used and power produced during the stepping phase of balance recovery.

METHODS

Eighty-four community-dwelling older adults (47 men, 37 women) participated in the study. Isometric strength of the ankle, knee and hip joint flexors and extensors was assessed using a dynamometer. Loss of balance was induced by releasing participants from a static forward lean (4 trials at each of 3 forward lean angles). Participants were instructed to recover with a single step and were subsequently classified as using a single or multiple step recovery strategy for each trial.

FINDINGS

(1) Females were weaker than males and the proportion of females that were able to recover with a single step were lower than for males at each lean magnitude. (2) Multiple compared to single steppers used a significantly higher proportion of their hip extension strength and produced less knee and ankle joint peak power during stepping, at the intermediate lean angle.

INTERPRETATION

Strength deficits in female compared to male participants may explain why a lower proportion of female participants were able to recover with a single step. The inability to generate sufficient power in the stepping limb appears to be a limiting factor in single step recovery from forward loss of balance.

The effect of increasing external degrees of freedom on force production and neuromuscular stabilisation

Evidence suggests that during interaction with different environmental dynamics the necessity to stabilise the involved joints leads to altered efficiency in force transmission to the surroundings and a specific orchestration of motor control strategies. However, little is known about the modalities of the changes associated with altered environmental dynamics. In 29 healthy participants, electromyographic (EMG) signals from four muscles of the right leg (M. peroneus longus, M. tibialis anterior, M. vastus medialis, M. gastrocnemius medialis) and three dimensions of force (Fx, Fy, Fz) were recorded. The participants were to exert force against an external object by performing a unilateral leg extension task with the task being influenced by either 0, 1 or 3 mechanical degrees of freedom. We hypothesised that the ankle stabilising muscles would increase their activities with increasing degrees of freedom (DoF), and that increasing external degrees of freedom results in decreased muscle force exerted during the movement task. The progressive change in the type of mechanical interaction from stable to unstable caused a loss of the ability to apply force in movement direction (Fz) which was accompanied by a reduction of Fy and Fx force dimensions. These reductions corresponded to maximum losses of 23% for Fz, 33% for Fx and 41% for Fy in the three degrees of freedom condition (all P < 0.001). Next, the individual muscles showed specific tuning effects, depending on the type of mechanical interaction. Our results suggest that the loss of the ability to exert force effectively against the external object is due to the neuromuscular stabilisation process of the involved joints. The change of the degrees of freedom conditions allowed for assessment of movement- or stabilisation-related adjustments of the motor system.

Mechanisms of adaptation from a multiple to a single step recovery strategy following repeated exposure to forward loss of balance in older adults

When released from an initial, static, forward lean angle and instructed to recover with a single step, some older adults are able to meet the task requirements, whereas others either stumble or fall. The purpose of the present study was to use the concept of margin of stability (MoS) to investigate balance recovery responses in the anterior-posterior direction exhibited by older single steppers, multiple steppers and those that are able to adapt from multiple to single steps following exposure to repeated forward loss of balance. One hundred and fifty-one healthy, community dwelling, older adults, aged 65–80 years, participated in the study. Participants performed four trials of the balance recovery task from each of three initial lean angles. Balance recovery responses in the anterior-posterior direction were quantified at three events; cable release (CR), toe-off (TO) and foot contact (FC), for trials performed at the intermediate lean angle. MoS was computed as the anterior-posterior distance between the forward boundary of the Base of Support (BoS) and the vertical projection of the velocity adjusted centre of mass position (XCoM). Approximately one-third of participants adapted from a multiple to a single step recovery strategy following repeated exposure to the task. MoS at FC for the single and multiple step trials in the adaptation group were intermediate between the exclusively single step group and the exclusively multiple step group, with the single step trials having a significant, 3.7 times higher MoS at FC than the multiple step trials. Consistent with differences between single and multiple steppers, adaptation from multiple to single steps was attributed to an increased BoS at FC, a reduced XCoM at FC and an increased rate of BoS displacement from TO to FC. Adaptations occurred within a single test session and suggest older adults that are close to the threshold of successful recovery can rapidly improve dynamic stability following repeated exposure to a forward loss of balance.

The discriminant capabilities of stability measures, trunk kinematics, and step kinematics in classifying successful and failed compensatory stepping responses by young adults

This study evaluated the discriminant capability of stability measures, trunk kinematics, and step kinematics to classify successful and failed compensatory stepping responses. In addition, the shared variance between stability measures, step kinematics, and trunk kinematics is reported. The stability measures included the anteroposterior distance (d) between the body center of mass and the stepping limb toe, the margin of stability (MOS), as well as time-to-boundary considering velocity (TTB(v)), velocity and acceleration (TTB(a)), and MOS (TTB(MOS)). Kinematic measures included trunk flexion angle and angular velocity, step length, and the time after disturbance onset of recovery step completion. Fourteen young adults stood on a treadmill that delivered surface accelerations necessitating multiple forward compensatory steps. Thirteen subjects fell from an initial disturbance, but recovered from a second, identical disturbance. Trunk flexion velocity at completion of the first recovery step and trunk flexion angle at completion of the second step had the greatest overall classification of all measures (92.3%). TTB(v) and TTB(a) at completion of both steps had the greatest classification accuracy of all stability measures (80.8%). The length of the first recovery step (r ≤ 0.70) and trunk flexion angle at completion of the second recovery step (r ≤ -0.54) had the largest correlations with stability measures. Although TTB(v) and TTB(a) demonstrated somewhat smaller discriminant capabilities than trunk kinematics, the small correlations between these stability measures and trunk kinematics (|r| ≤ 0.52) suggest that they reflect two important, yet different, aspects of a compensatory stepping response.