The cost of walking downhill: Is the preferred gait energetically optimal?

Humans tend to prefer walking patterns that minimize energetic cost, but must also maintain stability to avoid falling over. The relative importance of these two goals in determining the preferred gait pattern is not currently clear. We investigated the relationship between energetic cost and stability during downhill walking, a context in which gravitational energy will assist propulsion but may also reduce stability. We hypothesized that humans will not minimize energetic cost when walking downhill, but will instead prefer a gait pattern that increases stability. Simulations of a dynamic walking model were used to determine whether stable downhill gaits could be achieved using a simple control strategy. Experimentally, twelve healthy subjects walked downhill at 1.25 m/s (0, 0.05, 0.10, and 0.15 gradients). For each slope, subjects performed normal and relaxed trials, in which they were instructed to reduce muscle activity and allow gravity to maximally assist their gait. We quantified energetic cost, stride timing, and leg muscle activity. In our model simulations, increase in slope reduced the required actuation but also decreased stability. Experimental subjects behaved more like the model when using the relaxed rather than the normal walking strategy; the relaxed strategy decreased energetic cost at the steeper slopes but increased stride period variability, an indicator of instability. These results indicate that subjects do not take optimal advantage of the propulsion provided by gravity to decrease energetic cost, but instead prefer a more stable and more costly gait pattern.     

Effects of walking speed, strength and range of motion on gait stability in healthy older adults

Falls pose a tremendous risk to those over 65 and most falls occur during locomotion. Older adults commonly walk slower, which many believe helps improve walking stability. While increased gait variability predicts future fall risk, increased variability is also caused by walking slower. Thus, we need to better understand how differences in age and walking speed independently affect dynamic stability during walking. We investigated if older adults improved their dynamic stability by walking slower, and how leg strength and flexibility (passive range of motion (ROM)) affected this relationship. Eighteen active healthy older and 17 healthy younger adults walked on a treadmill for 5min each at each of 5 speeds (80-120% of preferred). Local divergence exponents and maximum Floquet multipliers (FM) were calculated to quantify each subject's inherent local dynamic stability. The older subjects walked with the same preferred walking speeds as the younger subjects (p=0.860). However, these older adults still exhibited greater local divergence exponents (p<0.0001) and higher maximum FM (p<0.007) than the younger adults at all walking speeds. These older adults remained more locally unstable (p<0.04) even after adjusting for declines in both strength and ROM. In both age groups, local divergence exponents decreased at slower speeds and increased at faster speeds (p<0.0001). Maximum FM showed similar changes with speed (p<0.02). Both younger and older adults exhibited decreased instability by walking slower, in spite of increased variability. These increases in dynamic instability might be more sensitive indicators of future fall risk than changes in gait variability.     

Kinematic variability and local dynamic stability of upper body motions when walking at different speeds

A ubiquitous characteristic of elderly and patients with gait disabilities is that they walk slower than healthy controls. Many clinicians assume these patients walk slower to improve their stability, just as healthy people slow down when walking across ice. However, walking slower also leads to greater variability, which is often assumed to imply deteriorated stability. If this were true, then slowing down would be completely antithetical to the goal of maintaining stability. This study sought to resolve this paradox by directly quantifying the sensitivity of the locomotor system to local perturbations that are manifested as natural kinematic variability. Eleven young healthy subjects walked on a motorized treadmill at five different speeds. Three-dimensional movements of a single marker placed over the first thoracic vertebra were recorded during continuous walking. Mean stride-to-stride standard deviations and maximum finite-time Lyapunov exponents were computed for each time series to quantify the variability and local dynamic stability, respectively, of these movements. Quadratic regression analyses of the dependent measures vs. walking speed revealed highly significant U shaped trends for all three mean standard deviations, but highly significant linear trends, with significant or nearly significant quadratic terms, for five of the six finite-time Lyapunov exponents. Subjects exhibited consistently better local dynamic stability at slower speeds for these five measures. These results support the clinically based intuition that people who are at increased risk of falling walk slower to improve their stability, even at the cost of increased variability.     

Steps to Take to Enhance Gait Stability: The Effect of Stride Frequency,Stride Length,and Walking Speed on Local Dynamic Stability and Margins of Stability

The purpose of the current study was to investigate whether adaptations of stride length, stride frequency, and walking speed, independently influence local dynamic stability and the size of the medio-lateral and backward margins of stability during walking. Nine healthy subjects walked 25 trials on a treadmill at different combinations of stride frequency, stride length, and consequently at different walking speeds. Visual feedback about the required and the actual combination of stride frequency and stride length was given during the trials. Generalized Estimating Equations were used to investigate the independent contribution of stride length, stride frequency, and walking speed on the measures of gait stability. Increasing stride frequency was found to enhance medio-lateral margins of stability. Backward margins of stability became larger as stride length decreased or walking speed increased. For local dynamic stability no significant effects of stride frequency, stride length or walking speed were found. We conclude that adaptations in stride frequency, stride length and/or walking speed can result in an increase of the medio-lateral and backward margins of stability, while these adaptations do not seem to affect local dynamic stability. Gait training focusing on the observed stepping strategies to enhance margins of stability might be a useful contribution to programs aimed at fall prevention.     

Can stability really predict an impending slip-related fall among older adults?

The primary purpose of this study was to systematically evaluate and compare the predictive power of falls for a battery of stability indices, obtained during normal walking among community-dwelling older adults. One hundred and eighty seven community-dwelling older adults participated in the study. After walking regularly for 20 strides on a walkway, participants were subjected to an unannounced slip during gait under the protection of a safety harness. Full body kinematics and kinetics were monitored during walking using a motion capture system synchronized with force plates. Stability variables, including feasible-stability-region measurement, margin of stability, the maximum Floquet multiplier, the Lyapunov exponents (short- and long-term), and the variability of gait parameters (including the step length, step width, and step time), were calculated for each subject. Sensitivity of predicting slip outcome (fall vs. recovery) was examined for each stability variable using logistic regression. Results showed that the feasible-stability-region measurement predicted fall incidence among these subjects with the highest sensitivity (68.4%). Except for the step width (with an sensitivity of 60.2%), no other stability variables could differentiate fallers from those who did not fall for the sample included in this study. The findings from the present study could provide guidance to identify individuals at increased risk of falling using the feasible-stability-region measurement or variability of the step width.     

Walking at the preferred stride frequency maximizes local dynamic stability of knee motion

Healthy humans display a preference for walking at a stride frequency dependent on the inertial properties of their legs. Walking at preferred stride frequency (PSF) is predicted to maximize local dynamic stability, whereby sensitivity to intrinsic perturbations arising from natural variability inherent in biological motion is minimized. Previous studies testing this prediction have employed different variability measures, but none have directly quantified local dynamic stability by computing maximum finite-time Lyapunov exponent (λ^Max), which quantifies the rate of divergence of nearby trajectories in state space. Here, ten healthy adults walked 45 m overground while sagittal motion of both knees was recorded via electrogoniometers. An auditory metronome prescribed 7 different frequencies relative to each individual's PSF (PSF; ±5, ±10, ±15 strides/min). Stride frequencies were performed under both freely adopted speed (FS) and controlled speed (CS: set at the speed of PSF trials) conditions. Local dynamic stability was maximal (λ^Max was minimal) at the PSF, becoming less stable for higher and lower stride frequencies. This occurred under both FS and CS conditions, although controlling speed further reduced local dynamic stability at non-preferred stride frequencies. In contrast, measures of variability revealed effects of stride frequency and speed conditions that were distinct from λ^Max. In particular, movement regularity computed by approximate entropy (ApEn) increased for slower walking speeds, appearing to depend on speed rather than stride frequency. The cadence freely adopted by humans has the benefit of maximizing local dynamic stability, which can be interpreted as humans tuning to their resonant frequency of walking.     

Gait variability magnitude but not structure is altered in essential tremor

Essential tremor (ET) is a common tremor disorder affecting postural/action tremor of the upper extremities and midline. Recent research revealed a cerebellar-like deficit during tandem gait in persons with ET, though spatiotemporal variability during normal gait in ET has been relatively ignored. The first purpose of this study was to investigate gait variability magnitude and structure in ET as compared to healthy older adults (HOA). To address this issue, 11 ET and 11 age-matched HOAs walked on a treadmill for 5 min at preferred walking speeds. HOAs walked for an additional minute while speed-matched to an ET participant. The second purpose was to describe the clinical correlates of gait variability in this population. To address this aim, 31 persons with ET walked on a treadmill for 5 min and completed the Fahn–Tolosa–Marin Tremor Rating Scale. Gait variability magnitude was derived by calculating coefficients of variation in stride length, stride time, step length, step time, and step width. Gait variability structure was derived using a detrended fluctuation analysis technique. At preferred walking speeds, ET participants walked significantly slower with significantly increased variability magnitude in all five spatiotemporal gait parameters. At speed-matched walking, ET participants exhibited significantly higher step width variability. Gait variability structure was not different between groups. We also observed that gait variability magnitude was predicted by severity of upper extremity and midline tremors. This study revealed that self-selected gait in ET is characterized by high variability that is associated with tremor severity in the upper extremity and midline.     

Faster walking speeds increase local instability among people with peripheral neuropathy

Individuals with peripheral neuropathy (PN) may compensate for decreased somatosensation by reducing walking speed. Predisposition to falls may therefore arise from an inability to adapt to challenging walking speeds. The purpose of this study was to examine the effects of PN on the magnitude of variability and local instability on walking at different speeds. Twelve individuals with PN and 12 controls completed a 6-min walk test to determine fast walking speed (FWS). Sagittal plane hip, knee, and ankle joint angles were then calculated during 3min of treadmill walking at 100%, 80%, and 60% FWS. The magnitudes of stride duration variability (SDvar), joint angle variability (JTvar), and both short- and long-term Lyapunov exponents (used to estimate local instability) were calculated. The PN group walked slower than the control group (p<.001). With groups combined, walking faster led to increased local instability and increased variability (p<.001). The PN group exhibited increased variability (SDvar, p=.02; JTvar, p=.01) over all speeds, and exaggerated local instability (p<.05) when walking at the fastest speed. PN leads to increased walking variability and local instability, particularly when walking at challenging speeds. These results are important to consider in future patient education and rehabilitation programs.     

Effect of prolonged free-walking fatigue on gait and physiological rhythm

This study examined the ways in which gait patterns and physiological rhythms such as those of muscle activity (tibialis anterior (TA) and biceps femoris (BF)) and cardiac activity are affected by the fatigue induced by prolonged free walking. Twelve normal subjects who walked for 3 h at their preferred pace were divided into two groups according to whether their mean gait cycle time (reciprocal of stride rate) during the second 90 min was higher (Group A: n=8) or lower (Group B: n=4) than that during the first 90 min. For Group A, the level of subjective fatigue during the walking task was significantly higher and the heart rate at rest was significantly lower than Group B. In Group A, prolonged walking significantly decreased the mean power frequency of the electromyography from TA, increased the variability of gait rhythm, decreased the largest Lyapunov exponent of the vertical component of back-waist acceleration, and decreased the amplitude of the vertical component of back-waist acceleration. Taking the onset timings of these changes into account, we propose that subjects who tire easily during prolonged walking first show local muscle fatigue at TA followed by instability of gait rhythm and then they slow their gait rhythm to enhance local dynamic stability. For both groups we constructed a physical fatigue index described by linear regression of gait and physiological variables. When we compared the subjective fatigue level with the fatigue level predicted using the index, we obtained a relatively high correlation coefficient for both groups (r=0.77).     

Effects of Aging on Arm Swing during Gait: The Role of Gait Speed and Dual Tasking

Healthy walking is characterized by pronounced arm swing and axial rotation. Aging effects on gait speed, stride length and stride time variability have been previously reported, however, less is known about aging effects on arm swing and axial rotation and their relationship to age-associated gait changes during usual walking and during more challenging conditions like dual tasking. Sixty healthy adults between the ages of 30–77 were included in this study designed to address this gap. Lightweight body fixed sensors were placed on each wrist and lower back. Participants walked under 3 walking conditions each of 1 minute: 1) comfortable speed, 2) walking while serially subtracting 3’s (Dual Task), 3) walking at fast speed. Aging effects on arm swing amplitude, range, symmetry, jerk and axial rotation amplitude and jerk were compared between decades of age (30–40; 41–50; 51–60; 61–77 years). As expected, older adults walked slower (p = 0.03) and with increased stride variability (p = 0.02). Arm swing amplitude decreased with age under all conditions (p = 0.04). In the oldest group, arm swing decreased during dual task and increased during the fast walking condition (p<0.0001). Similarly, arm swing asymmetry increased during the dual task in the older groups (p<0.004), but not in the younger groups (p = 0.67). Significant differences between groups and within conditions were observed in arm swing jerk (p<0.02), axial rotation amplitude (p<0.02) and axial jerk (p<0.001). Gait speed, arm swing amplitude of the dominant arm, arm swing asymmetry and axial rotation jerk were all independent predictors of age in a multivariate model. These findings suggest that the effects of gait speed and dual tasking on arm swing and axial rotation during walking are altered among healthy older adults. Follow-up work is needed to examine if these effects contribute to reduced stability in aging.     

The metabolic and mechanical costs of step time asymmetry in walking

Animals use both pendular and elastic mechanisms to minimize energy expenditure during terrestrial locomotion. Elastic gaits can be either bilaterally symmetric (e.g. run and trot) or asymmetric (e.g. skip, canter and gallop), yet only symmetric pendular gaits (e.g. walk) are observed in nature. Does minimizing metabolic and mechanical power constrain pendular gaits to temporal symmetry? We measured rates of metabolic energy expenditure and calculated mechanical power production while healthy humans walked symmetrically and asymmetrically at a range of step and stride times. We found that walking with a 42 per cent step time asymmetry required 80 per cent (2.5 W kg⁻¹) more metabolic power than preferred symmetric gait. Positive mechanical power production increased by 64 per cent (approx. 0.24 W kg⁻¹), paralleling the increases we observed in metabolic power. We found that when walking asymmetrically, subjects absorbed more power during double support than during symmetric walking and compensated by increasing power production during single support. Overall, we identify inherent metabolic and mechanical costs to gait asymmetry and find that symmetry is optimal in healthy human walking.     

Influence of simulated neuromuscular noise on movement variability and fall risk in a 3D dynamic walking model

People at risk of falling exhibit increased gait variability, which may predict future falls. However, the causal mechanisms underlying these correlations are not well known. Increased neuronal noise associated with aging likely leads to increased gait variability, which could in turn lead to increased fall risk. This paper presents a model of how changes in neuromuscular noise independently affect gait variability and probability of falling, and aims to determine the extent to which changes in gait variability directly predict fall risk. We used a dynamic walking model that incorporates a lateral step controller to maintain lateral stability. Noise was applied to this controller to approximate neuromuscular noise in humans. Noise amplitude was varied between low amplitudes that did not induce falls and high amplitudes for which the model always fell. With increases in noise amplitude, the model fell more often and after fewer steps. Gait variability increased with noise amplitude and predicted increased probability of falling. Importantly, these relationships were not linear. At either low gait variability or very high gait variability, small increases in noise and variability affected probability of falling very little. Conversely, at intermediate noise and/or variability levels, the same small increases resulted in large increases in probability of falling. Our results validate the idea that age-related increases in neuromuscular noise likely play a direct contributing role in increasing fall risk. However, neuromuscular noise remains only one of many important factors that need to be considered. These findings have important implications for fall prevention research and practice.     

The effects of rhythmic sensory cues on the temporal dynamics of human gait

Walking is a complex, rhythmic task performed by the locomotor system. However, natural gait rhythms can be influenced by metronomic auditory stimuli, a phenomenon of particular interest in neurological rehabilitation. In this paper, we examined the effects of aural, visual and tactile rhythmic cues on the temporal dynamics associated with human gait. Data were collected from fifteen healthy adults in two sessions. Each session consisted of five 15-minute trials. In the first trial of each session, participants walked at their preferred walking speed. In subsequent trials, participants were asked to walk to a metronomic beat, provided through visually, aurally, tactile or all three cues (simultaneously and in sync), the pace of which was set to the preferred walking speed of the first trial. Using the collected data, we extracted several parameters including: gait speed, mean stride interval, stride interval variability, scaling exponent and maximum Lyapunov exponent. The extracted parameters showed that rhythmic sensory cues affect the temporal dynamics of human gait. The auditory rhythmic cue had the greatest influence on the gait parameters, while the visual cue had no statistically significant effect on the scaling exponent. These results demonstrate that visual rhythmic cues could be considered as an alternative cueing modality in rehabilitation without concern of adversely altering the statistical persistence of walking.     

Local dynamic stability versus kinematic variability of continuous overground and treadmill walking

This study quantified the relationships between local dynamic stabiliht and variabilitr during continuous overground and treadmill walking. Stride-to-stride standard deviations were computed from temporal and kinematic data. Marimum finite-time Lyapunov exponents were estimated to quantify local dynamic stability. Local stability of gait kinematics was shown to be achieved over multiple consecutive strides. Traditional measures of variability poorly predicted local stability. Treadmill walking was associated with significant changes in both variability and local stability. Thus, motorized treadmills may produce misleading or erroneous results in situations where changes in neuromuscular control are likely to affect the variability and/or stability of locomotion.     

Dynamic stability of individuals with transtibial amputation walking in destabilizing environments

Lower limb amputation substantially disrupts motor and proprioceptive function. People with lower limb amputation experience considerable impairments in walking ability, including increased fall risk. Understanding the biomechanical aspects of the gait of these patients is crucial in improving their gait function and their quality of life. In the present study, 9 persons with unilateral transtibial amputation and 13 able-bodied controls walked on a large treadmill in a Computer Assisted Rehabilitation Environment (CAREN). While walking, subjects were either not perturbed, or were perturbed either by continuous mediolateral platform movements or by continuous mediolateral movements of the visual scene. Means and standard deviations of both step lengths and step widths increased significantly during both perturbation conditions (all p<0.001) for both groups. Measures of variability, local and orbital dynamic stability of trunk movements likewise exhibited large and highly significant increases during both perturbation conditions (all p<0.001) for both groups. Patients with amputation exhibited greater step width variability (p=0.01) and greater trunk movement variability (p=0.04) during platform perturbations, but did not exhibit greater local or orbital instability than healthy controls for either perturbation conditions. Our findings suggest that, in the absence of other co-morbidities, patients with unilateral transtibial amputation appear to retain sufficient sensory and motor function to maintain overall upper body stability during walking, even when substantially challenged. Additionally, these patients did not appear to rely more heavily on visual feedback to maintain trunk stability during these walking tasks.     

Local dynamic stability of amputees wearing a torsion adapter compared to a rigid adapter during straight-line and turning gait

Lower limb amputees have decreased balance during daily ambulation compared to nonamputees. An optimally compliant torsion adapter, which enables transverse plane rotation at the socket–pylon junction may reduce limb asymmetries and improve comfort leading to increased confidence and stability during gait. The purpose of this study was to determine if the presence of a torsion adapter affects amputee sensitivity to local perturbations (local dynamic stability) during straight-line walking and during a turning task. Ten unilateral transtibial amputees were fit with a torsion and rigid adapter in random order and blinded to the condition. After a 3-week acclimation period, kinematic data were collected while subjects walked in a straight-line on a treadmill and around a 1-m radius circular path at constant speed. Maximum finite-time Lyapunov exponents (λ), an estimator of local dynamic stability, were calculated for the amputee’s sagittal plane hip, knee and ankle angles for each condition. The prosthetic limb λ was greater during a turn compared to straight-line walking, suggesting amputees are less stable while turning. There were no statistically significant differences found in λ between adapters during both walking conditions, suggesting the torsion adapter had no effect on amputee stability; however, high inter-subject variability due to the examined population and turning task may have masked a small decrease in prosthetic limb hip and knee stability for the torsion adapter during straight-line gait. Therefore, the torsion adapter’s added degree of freedom may have a small adverse effect on prosthetic limb stability during straight-line walking and no effect on turning.     

Dynamic balance in persons with multiple sclerosis who have a falls history is altered compared to non-fallers and to healthy controls

Around 60% of persons with multiple sclerosis (MS) experience falls, however the dynamic balance differences between those who fall and those who don’t are not well understood. The purpose of this study is to identify distinct biomechanical features of dynamic balance during gait that are different between fallers with MS, non-fallers with MS, and healthy controls. 27 recurrent fallers with MS, 28 persons with MS with no falls history, and 27 healthy controls walked on a treadmill at their preferred speed for 3 min. The variability of trunk accelerations and the average and variability of minimum toe clearance, spatiotemporal parameters, and margin of stability were compared between groups. Fallers with MS exhibited a slower cautious gait compared to non-fallers and healthy controls, but had decreased anterior-posterior margin of stability and minimum toe clearance. Fallers walked with less locally stable and predictable trunk accelerations, and increased variability of step length, stride time, and both anterior-posterior and mediolateral margin of stability compared to non-fallers and healthy controls. The present work provides evidence that within a group of persons with MS, there are gait differences that are influenced by falls history. These differences indicate that in persons with MS who fall, the center of mass is poorly controlled through base of support placement and the foot is closer to the ground during swing phase relative to the non-fallers. These identified biomechanical differences could be used to evaluate dynamic balance in persons with MS and to help improve fall prevention strategies.     

The Laetoli footprints and early hominin locomotor kinematics

A critical question in human evolution is whether the earliest bipeds walked with a bent-hip, bent-knee gait or on more extended hindlimbs. The differences between these gaits are not trivial, because the adoption of either has important implications for the evolution of bipedalism. In this study, we re-examined the Laetoli footprints to determine whether they can provide information on the locomotor posture of early hominins. Previous researchers have suggested that the stride lengths of Laetoli hominins fall within the range of modern human stride lengths and therefore, Laetoli hominins walked with modern-human-like kinematics. Using a dynamic-similarity analysis, we compared Laetoli hominin stride lengths with those of both modern humans and chimpanzees. Our results indicate that Laetoli hominins could have used either a bent-hip, bent-knee gait, similar to a chimpanzee, or an extended-hindlimb gait, similar to a human. In fact, our data suggest that the Laetoli hominins could have walked near their preferred speeds using either limb posture. This result contrasts with most previous studies, which suggest relatively slow walking speeds for these early bipeds. Despite the many attempts to discern limb-joint kinematics from Laetoli stride lengths, our study concludes that stride lengths alone do not resolve the debate over early hominin locomotor postures.     

Gait modification when decreasing double support percentage

Much is still unknown about walking stability, including which aspects of gait contribute to higher stability. Walking stability appears to be related to walking speed, although the exact relationship is unclear. As walking speed decreases, the double support (DS) period of gait increases both in time and as a percentage of the gait cycle. Because humans have more control over their center of mass movement during DS, increasing DS duration may alter stability. This study examined how human gait is affected by changing DS percentage independent of walking speed. Sixteen young, healthy adults walked on a treadmill at a single speed for six one-minute trials. These trials included normal gait as well as longer- and shorter-than-normal DS percentage gaits. Subjects were consistently able to decrease DS percentage but had difficulty increasing DS percentage. In some cases, subjects altered their cadence when changing DS percentage, particularly when attempting to increase DS percentage. The changes to gait when decreasing DS percentage were similar to changes when increasing walking speed but occurred mainly during the swing period. These changes include increased hip and knee flexion during the swing period, increased swing foot height, and larger magnitude peaks in ground reaction forces. The changes in gait when attempting to increase DS percentage trended toward changes when decreasing walking speed. Altering DS percentage induced gait changes that were similar to, yet clearly distinct from, gait changes due to walking speed. Further, the difficulty of increasing DS percentage when walking at a constant speed suggests that people walk more slowly when they want to increase time spent in DS.     

Effect of stable and unstable load carriage on walking gait variability,dynamic stability and muscle activity of older adults

Load carriage perturbs the neuromuscular system, which can be impaired due to ageing. The ability to counteract perturbations is an indicator of neuromuscular function but if the response is insufficient the risk of falls will increase. However, it is unknown how load carriage affects older adults. Fourteen older adults (65 ± 6 years) attended a single visit during which they performed 4 min of walking in 3 conditions, unloaded, stable backpack load and unstable backpack load. During each walking trial, 3-dimensional kinematics of the lower limb and trunk movements and electromyographic activity of 6 lower limb muscles were recorded. The local dynamic stability (local divergence exponents), joint angle variability and spatio-temporal variability were determined along with muscle activation magnitudes. Medio-lateral dynamic stability was lower (p = 0.018) and step width (p = 0.019) and step width variability (p = 0.015) were greater in unstable load walking and step width variability was greater in stable load walking (p = 0.009) compared to unloaded walking. However, there was no effect on joint angle variability. Unstable load carriage increased activity of the Rectus Femoris (p = 0.001) and Soleus (p = 0.043) and stable load carriage increased Rectus Femoris activity (p = 0.006). These results suggest that loaded walking alters the gait of older adults and that unstable load carriage reduces dynamic stability compared to unloaded walking. This can potentially increase the risk of falls, but also offers the potential to use unstable loads as part of fall prevention programmes.