Learning to balance on a slackline: Development of coordinated multi-joint synergies.

Previous research has investigated synergies involved in locomotion and balance reactions; however, there is limited insight into the emergence of skilled balance control with practice of challenging tasks. We explored motor learning of tandem and single leg stance on an unstable surface-a slackline. Balance was tested in 10 naïve healthy adults at four time points: baseline, after one slackline practice session, after 1 week of practice, and 1 week following the final practice session. We recorded kinematics of the upper and lower arms bilaterally, trunk, and thigh and foot unilaterally while participants balanced in tandem and single leg stance on a slackline and narrow rigid beam (transfer task). When participants first attempted to stand on the slackline, they exhibited fast and frequent movements across all joints with actions along the frontal plane (particularly the hip) and fell after a short period (~3 seconds). Performance improved rapidly (fewer falls), and this was accompanied by dampened trunk and foot oscillations and the development of coordinated movement patterns with a progressive emphasis on more distal upper body segments. Continuous relative phase angles between joint pairs began to cluster around either 0° (indicating in-phase movement) or 180° (indicating anti-phase movement). Participants also began to demonstrate coordinated upper body synergies and performance improvements (fewer falls) on the transfer task, while a control group (n = 10) did not exhibit similar synergies or performance improvements. Our findings describe the emergence of coordinated movement synergies involving the upper body as healthy adults learn a challenging balance task.

Is there a decline in bovine spongiform encephalopathy cases born after reinforced feed bans? A modelling study in EU member states.

Occasional cases of classical bovine spongiform encephalopathy (BSE) still continue to occur within the European Union (EU) for animals born after reinforced feed bans (BARBs), which should in theory have eliminated all risk of infection. The study aimed to determine (i) whether a common rate of decline of BSE infection was evident across EU member states, i.e. to determine whether control measures have been equally effective in all member states, (ii) whether there was any evidence of spontaneous occurrence of BSE in the data and (iii) the expected date for the last BSE case in UK. It was found that there was no significant difference in the rate of decline of BSE prevalence between member states, with a common rate of decline of 33·9% per annum (95% CI 30·9-37%) in successive annual birth cohorts. Trend analysis indicated an ultimate decline to 0 prevalence, suggesting that spontaneous occurrence does not explain the majority of cases. Projecting forward the trends from the back-calculation model indicated that there was approximately a 50% probability of further cases in the UK, and should the current rate of decline continue, there remains the possibility of further occasional cases up until 2026.

Does increased postural threat lead to more conscious control of posture?

Although it is well established that postural threat modifies postural control, little is known regarding the underlying mechanism(s) responsible for these changes. It is possible that changes in postural control under conditions of elevated postural threat result from a shift to a more conscious control of posture. The purpose of this study was to determine the influence of elevated postural threat on conscious control of posture and to determine the relationship between conscious control and postural control measures. Forty-eight healthy young adults stood on a force plate at two different surface heights: ground level (LOW) and 3.2-m above ground level (HIGH). Centre of pressure measures calculated in the anterior-posterior (AP) direction were mean position (AP-MP), root mean square (AP-RMS) and mean power frequency (AP-MPF). A modified state-specific version of the Movement Specific Reinvestment Scale was used to measure conscious motor processing (CMP) and movement self-consciousness (MSC). Balance confidence, fear of falling, perceived stability, and perceived and actual anxiety indicators were also collected. A significant effect of postural threat was found for movement reinvestment as participants reported more conscious control and a greater concern about their posture at the HIGH height. Significant correlations between CMP and MSC with AP-MP were observed as participants who consciously controlled and were more concerned for their posture leaned further away from the platform edge. It is possible that changes in movement reinvestment can influence specific aspects of posture (leaning) but other aspects may be immune to these changes (amplitude and frequency).

Is fall prevention by vitamin D mediated by a change in postural or dynamic balance?

INTRODUCTION: The objectives were:(1) to validate a quantitative balance assessment method for fall risk prediction; (2) to investigate whether the effect of vitamin D and calcium on the risk of falling is mediated through postural or dynamic balance, as assessed by this method. MATERIALS AND METHODS: A secondary analysis of a double blind randomized controlled trial was employed, which included 64 institutionalized elderly women with complete balance assessment (age range: 65-97; mean 25-hydroxyvitamin D levels: 16.4 ng/ml (SD +/-9.9). Participants received 1,200 mg calcium plus 800 IU cholecalciferol (n=33) or 1,200 mg calcium (n=31) per day over a 3-month treatment period. Using an electronic device attached to the lower back of the participant, balance was assessed as the degree of trunk angular displacement and angular velocity during a postural task (standing on two legs, eyes open, for 20 s) and a dynamic task (get up from a standard height chair with arm rests, sit down and then stand up again and remain standing). RESULTS: It was found that both postural and dynamic balance independently and significantly predicted the rate of falling within the 3-month follow-up. Vitamin D plus calcium reduced the rate of falls by 60% [relative risk (RR)=0.40; 95% CI: 0.17, 0.94] if compared with calcium alone. Once postural and dynamic balance were added to the regression analysis, they both attenuated the effect of vitamin D plus calcium on the rate of falls. For postural balance, the RR changed by 22% from 0.40 to 0.62 if angular displacement was added to the model, and by 9% from 0.40 to 0.49 if angular velocity was added. For dynamic balance, it changed by 1% from 0.40 to 0.41 if angular displacement was added, and by 14% from 0.40 to 0.54 if angular velocity was added. DISCUSSION: Thus, balance assessment using trunk angular displacement is a valid method for the prediction of falls in older women. Of the observed 60% reduction in the rate of falls by vitamin D plus calcium supplementation compared with calcium alone, up to 22% of the treatment effect was explained by a change in postural balance and up to 14% by dynamic balance.

Trunk sway measurements during stance and gait tasks in Parkinson's disease.

To achieve a unified assessment of postural instability in Parkinson's disease (PD) over a range of clinical stance and gait tasks, which may provide an insight into a tendency to fall, we measured trunk sway in the anterior-posterior and medial-lateral directions in freely moving PD patients and age-matched controls. We also measured task duration as time to complete the task or time to loss of balance. Patients had larger amplitudes of trunk sway velocities for stance tasks (e.g. mean pitch velocity when standing on two-legs eyes closed equalled 19.1 +/- 6.4 for PD patients on medication versus 4.8 +/- 0.3 degrees/s for controls, p = 0.0003) and for an expected (following prior warning) retropulsion test (mean roll angle equalled 4.3 +/- 0.5 degrees for PD patients versus 2.2 +/- 0.6 degrees for controls, p = 0.0003) than controls. Patients were more likely to fall earlier for stance tasks, and took longer to complete gait tasks (e.g. walking 3 m eyes closed, mean time 6.8 +/- 0.6 sees versus 4.9 +/- 0.1 sees, p = 0.0001). These differences between patients and controls were, in most cases, independent of medication. Based on these results we defined a simple test battery of stance and gait tasks that could discriminate between PD patients who had recent falls and controls. These results indicate that trunk sway measures recorded during stance and gait tasks provide useful information on balance deficits leading to falls in PD patients.

Influence of postural anxiety on postural reactions to multi-directional surface rotations.

Previous studies have shown significant effects of increased postural anxiety in healthy young individuals when standing quietly or performing voluntary postural tasks. However, little is known about the influence of anxiety on reactive postural control. The present study examined how increased postural anxiety influenced postural reactions to unexpected surface rotations in multiple directions. Ten healthy young adults (mean age: 25.5 yr, range: 22-27 yr) were required to recover from unexpected rotations of the support surface (7.5 degrees amplitude, 50 degrees/s velocity) delivered in six different directions while standing in a low postural threat (surface height: 60 cm above ground) or high postural threat (surface height: 160 cm above ground) condition. Electromyographic data from 12 different postural leg, hip, and trunk muscles was collected simultaneously. Full body kinematic data were also used to determine total body center of mass (COM) and segment displacements. Four distinct changes were observed with increased postural anxiety: increased amplitude in balance-correcting responses (120-220 ms) in all leg, trunk, and arm muscles; decreased onset latency of deltoid responses; reduced magnitude of COM displacement; and reduced angular displacement of leg, pelvis, and trunk. These observations suggest that changes in dynamic postural responses with increased anxiety are mediated by alterations in neuro-muscular control mechanisms and thus may contribute significantly to the pathophysiology of balance deficits associated with aging or neurological disease.

Postural abnormalities to multidirectional stance perturbations in Parkinson's disease.

OBJECTIVE: We investigated trunk control, protective arm movements, and electromyographic responses to multidirectional support-surface rotations in patients with Parkinson's disease (PD), aiming to better understand the pathophysiology underlying postural instability in PD, on and off antiparkinson medication. METHODS: Ten patients with PD were compared with 11 age matched healthy controls. Seven patients were also tested without (OFF) antiparkinson medication. All subjects received rotational perturbations (7.5 deg amplitude) that were randomly delivered in six different directions. RESULTS: The PD patients had decreased trunk rotation and ankle torque changes, consistent with a stiffening response. Stiffness appeared to be caused by the combined action of three factors: co-contraction that interfered in particular with the normal response asymmetry in trunk muscles; increased response amplitudes in agonist and antagonist muscles at both medium (approximately 80 ms) and balance correcting (approximately 120 ms) response latencies; and increased background activity in lower leg, hip, and trunk muscles. Although the patients had significantly earlier onset of deltoid muscle responses, this gave no functional protection because the arm movements were abnormally directed. Most instability in PD occurred for backward falls, with or without a roll component. Medication provided partial improvement in arm responses and trunk roll instability. CONCLUSIONS: Our results confirm previous findings in ankle muscles, and provide new information on balance impairments in hip, trunk, and arm responses in PD.

Cortical activation following a balance disturbance.

Although recent work suggests that cortical processing can be involved in the control of balance responses, the central mechanisms involved in these reactions remain unclear. We presently investigated the characteristics of scalp-recorded perturbation-evoked responses (PERs) following a balance disturbance. Eight young adults stabilized an inverted pendulum using their ankle musculature while seated. When perturbations were applied to the pendulum, subjects were instructed to return (active condition) or not return (passive condition) the pendulum to its original stable position. Primary measures included peak latency and amplitude of early PERs (the first negative peak between 100 and 150 ms, N1), amplitude of late PERs (between 200 and 400 ms) and onset and initial amplitude of ankle muscle responses. Based on the timing of PERs, we hypothesized that N1 would represent sensory processing of the balance disturbance and that late PERs would be linked to the sensorimotor processing of balance corrections. Our results revealed that N1 was maximal over frontal-central electrode sites (FCz and Cz). Average N1 measures at FCz, Cz, and CPz were comparable between active and passive tasks ( p>0.05). In contrast, the amplitude of late PERs at Cz was less positive for the active condition than for the passive ( p<0.05). The similarity in N1 between tasks suggests a sensory representation of early PERs. Differences in late PERs may represent sensorimotor processing related to the execution of balance responses.

Differences between trunk sway characteristics on a foam support surface and on the Equitest ankle-sway-referenced support surface.

Clinicians have sought ways to increase trunk sway so that it is easily observed and a balance deficit more easily identified. One technique often used for this purpose is to reduce the efficacy of ankle proprioceptive inputs on sway. To achieve this reduction either a foam mat is used as an unstable support surface or the subject stands on a surface made unstable with servo-driven ankle-sway-referencing. The purpose of the current study was to investigate differences in trunk pitch and roll sway characteristics using these techniques. Trunk sway while standing quietly on two legs was measured in 25 normal subjects in the age range 20-35 years for three support-surface conditions. Each condition was tested twice for 20 s, once with eyes open and once with eyes closed. The three conditions were standing on a foam support surface, standing on a support surface with pitch (fore-aft) ankle-sway-referencing as used for the standard Sensory Organization Test (SOT) of the Neurocom Equitest System (SOT 4 and 5), and standing with roll (lateral) ankle-sway-referencing. The latter was achieved by having the subjects stand turned 90 degrees to the standard SOT position. Two angular velocity sensors mounted on a belt measured trunk sway in the pitch and roll directions. Trunk roll angle and angular velocity amplitudes for pitch sway-referencing were reduced compared to either the foam or roll sway-referencing conditions, but trunk pitch angle and angular velocities amplitudes were greater. For roll sway-referencing, the trunk roll angle was greater than for the other stimulus conditions. Analyses of the trunk sway velocity in the frequency domain indicated that ankle-sway-referencing in the pitch direction increased trunk pitch sway at 1 Hz and decreased trunk roll sway between 2 and 5 Hz compared to foam support frequency spectra. Roll ankle-sway-referencing decreased trunk roll between 2 and 4 Hz only. These results indicate that using a foam support surface provides multidirectional trunk sway with velocity content across all frequencies in the range 0.8-5.2 Hz. Roll ankle-sway-referencing, but not pitch ankle-sway-referencing, yields trunk sway with similar characteristics to those with foam. Pitch ankle-sway-referencing forces pitch trunk resonance to be around 1 Hz and yields very different trunk sway from that obtained with a foam support surface. Roll sway-referencing is an alternative means to test multidirectional control of sway. Clinically though, foam is simpler to use and provides a more difficult balance task for the patient.

Age-dependent variations in the directional sensitivity of balance corrections and compensatory arm movements in man.

We investigated the effects of ageing on balance corrections induced by sudden stance perturbations in different directions. Effects were examined in biomechanical and electromyographic (EMG) recordings from a total of 36 healthy subjects divided equally into three age groups (20-34, 35-55 and 60-75 years old). Perturbations consisted of six combinations of support-surface roll (laterally) and pitch (forward-backward) each with 7.5 deg amplitude (2 pure pitch, and 4 roll and pitch) delivered randomly. To reduce stimulus predictability further and to investigate scaling effects, perturbations were at either 30 or 60 deg s(-1). In the legs, trunk and arms we observed age-related changes in balance corrections. The changes that appeared in the lower leg responses included smaller stretch reflexes in soleus and larger reflexes in tibialis anterior of the elderly compared with the young. For all perturbation directions, onsets of balance correcting responses in these ankle muscles were delayed by 20-30 ms and initially had smaller amplitudes (between 120-220 ms) in the elderly. This reduced early activity was compensated by increased lower leg activity after 240 ms. These EMG changes were paralleled by comparable differences in ankle torque responses, which were initially (after 160 ms) smaller in the elderly, but subsequently greater (after 280 ms). Findings in the middle-aged group were generally intermediate between the young and the elderly groups. Comparable results were obtained for the two different stimulus velocities. Stimulus-induced trunk roll, but not trunk pitch, changed dramatically with increasing age. Young subjects responded with early large roll movements of the trunk in the opposite direction to platform roll. A similarly directed but reduced amplitude of trunk roll was observed in the middle-aged. The elderly had very little initial roll modulation and also had smaller stretch reflexes in paraspinals. Balance-correcting responses (over 120-220 ms) in gluteus medius and paraspinals were equally well tuned to roll in the elderly, as in the young, but were reduced in amplitude. Onset latencies were delayed with age in gluteus medius muscles. Following the onset of trunk and hip balance corrections, trunk roll was in the same direction as support-surface motion for all age groups and resulted in overall trunk roll towards the fall side in the elderly, but not in the young. Protective arm movements also changed with age. Initial arm roll movements were largest in the young, smaller in the middle aged, and smallest in the elderly. Initial arm roll movements were in the same direction as initial trunk motion in the young and middle aged. Thus initial roll arm movements in the elderly were directed oppositely to those in the young. Initial pitch motion of the arms was similar across age groups. Subsequent arm movements were related to the amplitude of deltoid muscle responses which commenced at 100 ms in the young and 20-30 ms later in the elderly. These deltoid muscle responses preceded additional arm roll motion which left the arms directed 'downhill' (in the direction of the fall) in the elderly, but 'uphill' (to counterbalance motion of the pelvis) in the young. We conclude that increased trunk roll stiffness is a key biomechanical change with age. This interferes with early compensatory trunk movements and leads to trunk displacements in the direction of the impending fall. The reversal of protective arm movements in the elderly may reflect an adaptive strategy to cushion the fall. The uniform delay and amplitude reduction of balance-correcting responses across many segments (legs, hips and arms) suggests a neurally based alteration in processing times and response modulation with age. Interestingly, the elderly compensated for these 'early abnormalities' with enlarged later responses in the legs, but no similar adaptation was noted in the arms and trunk. These changes with age provide an insight into possible mechanisms underlying falls in the elderly.

Balance control analysis as a method for screening and identifying balance deficits.

We propose a two-step clinical evaluation procedure to identify the possible etiology and laterality of a balance deficit. Step 1 employs a minimum clinical test battery, developed in our labs, to screen for the balance deficit by examining changes to trunk sway for standard clinical stance and gait tests. Step 2 characterizes pathophysiological components in balance corrections, as well as deficits brought about by aging, using biomechanical and electromyographic (EMG) responses to multidirectional stance perturbations. This is best accomplished by delivering stance perturbations to patients standing on a support surface that is tipped in four directions: forwards to the left and right, and backwards to the left and right. This review provides an overview of the two procedures and proposes for the screening procedure a minimum clinical test battery with a score, termed the balance control index (BCI).

Trunk sway measures of postural stability during clinical balance tests: effects of a unilateral vestibular deficit.

This research evaluated whether quantified measures of trunk sway during clinical balance tasks are sensitive enough to identify a balance disorder and possibly specific enough to distinguish between different types of balance disorder. We used a light-weight, easy to attach, body-worn apparatus to measure trunk angular velocities in the roll and pitch planes during a number of stance and gait tasks similar to those of the Tinetti and CTSIB protocols. The tasks included standing on one or two legs both eyes-open and closed on a foam or firm support-surface, walking eight tandem steps, walking five steps while horizontally rotating or pitching the head, walking over low barriers, and up and down stairs. Tasks were sought, which when quantified might provide optimal screening for a balance pathology by comparing the test results of 15 patients with a well defined acute balance deficit (sudden unilateral vestibular loss (UVL)) with those of 26 patients with less severe chronic balance problems caused by a cerebellar-pontine-angle-tumour (CPAT) prior to surgery, and with those of 88 age- and sex-matched healthy subjects. The UVL patients demonstrated significantly greater than normal trunk sway for all two-legged stance tasks especially those performed with eyes closed on a foam support surface. Sway was also greater for walking while rotating or pitching the head, and for walking eight tandem steps on a foam support surface. Interestingly, the patients could perform gait tasks such as walking over barriers almost normally, however took longer. CPAT patients had trunk sway values intermediate between those of UVL patients and normals. A combination of trunk sway amplitude measurements (roll angle and pitch velocity) from the stance tasks of standing on two legs eyes closed on a foam support, standing eyes open on a normal support surface, as well as from the gait tasks of walking five steps while rotating, or pitching the head, and walking eight tandem steps on foam permitted a 97% correct recognition of a normal subject and a 93% correct recognition of an acute vestibular loss patient. Just over 50% of CPAT patients could be classified into a group with intermediate balance deficits, the rest were classified as normal. Our results indicate that measuring trunk sway in the form of roll angle and pitch angular velocity during five simple clinical tests of equilibrium, four of which probe both stance and gait control under more difficult sensory conditions, can reliably and quantitatively distinguish patients with a well defined balance deficit from healthy controls. Further, refinement of these trunk sway measuring techniques may be required if functions such as preliminary diagnosis rather than screening are to be attempted.

Trunk sway measures of postural stability during clinical balance tests: effects of age.

BACKGROUND: The major disadvantage of current clinical tests that screen for balance disorders is a reliance on an examiner's subjective assessment of equilibrium control. To overcome this disadvantage we investigated, using quantified measures of trunk sway, age-related differences of normal subjects for commonly used clinical balance tests. METHODS: Three age groups were tested: young (15-25 years; n = 48), middle-aged (45-55 years; n = 50) and elderly (65-75 years; n = 49). Each subject performed a series of fourteen tasks similar to those included in the Tinetti and Clinical Test of Sensory Interaction in Balance protocols. The test battery comprised stance and gait tasks performed under normal, altered visual (eyes closed), and altered proprioceptive (foam support surface) conditions. Quantification of trunk sway was performed using a system that measured trunk angular velocity and position in the roll (lateral) and pitch (fore-aft) planes at the level of the lower back. Ranges of sway amplitude and velocity were examined for age-differences with ANOVA techniques. RESULTS: A comparison between age groups showed several differences. Elderly subjects were distinguished from both middle-aged and young subjects by the range of trunk angular sway and angular velocity because both were greater in roll and pitch planes for stance and stance-related tasks (tandem walking). The most significant age group differences (F = 30, p <.0001) were found for standing on one leg on a normal floor or on a foam support surface with eyes open. Next in significance was walking eight tandem steps on a normal floor (F = 13, p <.0001). For gait tasks, such as walking five steps while rotating or pitching the head or with eyes closed, pitch and roll velocity ranges were influenced by age with middle-aged subjects showing the smallest ranges followed by elderly subjects and then young subjects (F = 12, p <.0001). Walking over a set of low barriers also yielded significant differences between age groups for duration and angular sway. In contrast, task duration was the only variable significantly influenced when walking up and down a set of stairs. An interesting finding for all tasks was the different spread of values for each population. Population distributions were skewed for all ages and broadened with age. CONCLUSIONS: Accurate measurement of trunk angular sway during stance and gait tasks provides a simple way of reliably measuring changes in balance stability with age and could prove useful when screening for balance disorders of those prone to fall.

Control of steering in the presence of unexpected head yaw movements. Influence on sequencing of subtasks.

Control of the head during locomotion has been suggested as a means of facilitating overall postural control of the body. The control of online steering is challenging, as it requires the central nervous system (CNS) to simultaneously control body reorientation in a new direction while modifying the ongoing step cycle. Stable body posture during steering is maintained via appropriately organized postural responses to error signals detected by the visual, vestibular, and/or proprioceptive systems. Modifications to the gait cycle include step-width regulation and movement of body center of mass (COM) in the direction of travel, and may be preceded by independent control of head orientation to see where one is going. The purpose of this investigation was to examine how the ability to successfully steer is influenced by unexpected head perturbations and how various body segments are coordinated and controlled to successfully steer along different pathways. Body kinematics were monitored as participants changed their direction of travel by varying amounts when visually cued one stride before the turn. Perturbations to the head were applied to either assist or oppose the change in direction one step prior to initiation of the turn. Analyses focused on the timing of the changes in head yaw, trunk yaw, and COM trajectories in the mediolateral plane. Results indicate that the order of control over the body segments was head and trunk reorientation in the direction of travel and finally movement of the COM in the intended direction. Thus gaze, inferred from head movement, preceded changes in COM trajectory. This suggests that looking where you are going is critical for steering. When steering is potentially compromised by unexpected head movements, the CNS delays committing movement of the COM until it has a chance to look at the new travel path.

Postural control is scaled to level of postural threat.

This study investigated control of posture when standing at different surface heights above ground level. Alterations in surface height were used to modify threat to postural control. Sixty-two healthy adults (mean+/-S.D.=20.3+/-1.3 years) stood quietly on a force plate 40 cm (LOW threat), 100 cm (MEDIUM threat) or 160 cm (HIGH threat) above ground level. Each standing trial was performed with eyes open for 120 s. Postural threat was presented in ascending (n=31) or descending (n=31) order with the first threat condition in each series (LOW threat for ascending group, HIGH threat for descending group) repeated. This manipulation allowed for an examination of set effects (i.e. prior experience of postural threat) on postural control. The results demonstrated scaling of postural control variables to level of postural threat. Amplitude of centre of pressure (COP) displacement decreased and frequency of COP displacement increased linearly as postural threat increased from LOW to HIGH. The central nervous system progressively tightened control of posture as postural threat increased. Initial exposure to the HIGH or LOW threat condition influenced postural control differently. The group who received the HIGH threat condition first (descending) demonstrated increased amplitude of COP displacement in the anterior-posterior direction compared with the group who received the LOW threat condition first (ascending). A 'first trial' effect was observed when standing for two consecutive trials but only at the LOW threat condition. Decreased amplitude and increased frequency of COP displacement were observed on the first trial compared to the second trial. The results of this study demonstrated that control of posture is influenced not only by the threat to posture but also by the order in which the threat to posture is experienced.

Altered kinetic strategy for the control of swing limb elevation over obstacles in unilateral below-knee amputee gait.

Our goal was to document the kinetic strategies for obstacle avoidance in below-knee amputees. Kinematic data were collected as unilateral below-knee traumatic amputees stepped over obstacles of various heights in the walking path. Inverse dynamics were employed to calculate power profiles and work during the limb-elevation and limb-lowering phases. Limb elevation was achieved by employing a different strategy of intra-limb interaction for elevation of the prosthetic limb than for the sound limb, which was similar to that seen in healthy adult non-amputees. As obstacle height increased, prosthetic side knee flexion was increased by modulating the work done at the hip, and not the knee, as seen on the sound side. Although the strength of the muscles about the residual knee was preserved, the range of motion of that knee had previously been found to be somewhat limited. Perhaps more importantly, potential instability of the interface between the stump and the prosthetic socket, and associated discomfort at the stump could explain the altered limb-elevation strategy. Interestingly, the limb-lowering strategy seen in the sound limb and in non-amputees already features modulation of rotational and translational work at the hip, so an alternate strategy was not required. Thus, following a major insult to the sensory and neuromuscular system, the CNS is able to update the internal model of the locomotor apparatus as the individual uses the new limb in a variety of movements, and modify control strategies as appropriate.