Contributions of feed-forward and feedback strategies at the human ankle during control of unstable loads

Aging Does Not Alter Ankle, Muscle, and Tendon Stiffness at Low Loads Relevant to Stance

Older adults have difficulty maintaining balance when faced with postural disturbances, a task that is influenced by the stiffness of the triceps surae and Achilles tendon. Age-related changes in Achilles tendon stiffness have been reported at matched levels of effort, but measures typically have not been made at matched loads, which is important due to age-dependent changes in strength. Moreover, there has been limited investigation into age-dependent changes in muscle stiffness. Here, we investigate how age alters muscle and tendon stiffness and their influence on ankle stiffness. We hypothesized that age-related changes in muscle and tendon contribute to reduced ankle stiffness in older adults and evaluated this hypothesis when either load or effort were matched. We used B-mode ultrasound with joint-level perturbations to quantify ankle, muscle, and tendon stiffness across a range of loads and efforts in seventeen healthy younger and older adults. At matched loads relevant to standing and the stance phase of walking, there was no significant difference in ankle, muscle, or tendon stiffness between groups (all p > 0.13). However, at matched effort, older adults exhibited a significant decrease in ankle (27%; p = 0.008), muscle (37%; p = 0.02), and tendon stiffness (22%; p = 0.03) at 30% of maximum effort. This is consistent with our finding that older adults were 36% weaker than younger adults in plantarflexion (p = 0.004). Together, these results indicate that, at the loads tested in this study, there are no age-dependent changes in the mechanical properties of muscle or tendon, only differences in strength that result in altered ankle, muscle, and tendon stiffness at matched levels of effort.

A single sequence of intermittent hypoxia does not alter stretch reflex excitability in able-bodied individuals

Spasticity attributable to exaggerated stretch reflex pathways, particularly affecting the ankle plantar flexors, often impairs overground walking in persons with incomplete spinal cord injury. Compelling evidence from rodent models underscores how exposure to acute intermittent hypoxia (AIH) can provide a unique medium to induce spinal plasticity in key inhibitory pathways mediating stretch reflex excitability and potentially affect spasticity. In this study, we quantify the effects of a single exposure to AIH on the stretch reflex in able-bodied individuals. We hypothesized that a single sequence of AIH will increase the stretch reflex excitability of the soleus muscle during ramp-and-hold angular perturbations applied to the ankle joint while participants perform passive and volitionally matched contractions. Our results revealed that a single AIH exposure did not significantly change the stretch reflex excitability during both passive and active matching conditions. Furthermore, we found that able-bodied individuals increased their stretch reflex response from passive to active matching conditions after both sham and AIH exposures. Together, these findings suggest that a single AIH exposure might not engage inhibitory pathways sufficiently to alter stretch reflex responses in able-bodied persons. However, the generalizability of our present findings requires further examination during repetitive exposures to AIH along with potential reflex modulation during functional movements, such as overground walking.

At matched loads, aging does not alter ankle, muscle, or tendon stiffness

Older adults have difficulty maintaining balance when faced with postural disturbances, a task that is influenced by the stiffness of the triceps surae and Achilles tendon. Age-related changes in Achilles tendon stiffness have been reported at matched levels of effort, but measures typically have not been made at matched loads, which is important due to age-dependent changes in strength. Moreover, age-dependent changes in muscle stiffness have yet to be tested. Here, we investigate how age alters muscle and tendon stiffness and their influence on ankle stiffness. We hypothesized that age-related changes in muscle and tendon contribute to reduced ankle stiffness in older adults and evaluated this hypothesis when either load or effort were matched. We used B-mode ultrasound with joint-level perturbations to quantify ankle, muscle, and tendon stiffness across a range of loads and efforts in seventeen healthy younger and older adults. At matched loads, there was no significant difference in ankle, muscle, or tendon stiffness between groups (all p>0.13). However, at matched effort, older adults exhibited a significant decrease in ankle (27%; p=0.008), muscle (37%; p=0.02), and tendon stiffness (22%; p=0.03) at 30% of maximum effort. This is consistent with our finding that older adults were 36% weaker than younger adults in plantarflexion (p=0.004). Together these results indicate that, at the loads tested in this study, there are no age-dependent changes in the mechanical properties of muscle or tendon, only differences in strength that result in altered ankle, muscle, and tendon stiffness at matched levels of effort.

Movement predictability modulates sensorimotor processing

Introduction

An important factor for optimal sensorimotor control is how well we are able to predict sensory feedback from internal and external sources during movement. If predictability decreases due to external disturbances, the brain is able to adjust muscle activation and the filtering of incoming sensory inputs. However, little is known about sensorimotor adjustments when predictability is increased by availability of additional internal feedback. In the present study we investigated how modifications of internal and external sensory feedback influence the control of muscle activation and gating of sensory input.

Methods

Co-activation of forearm muscles, somatosensory evoked potentials (SEP) and short afferent inhibition (SAI) were assessed during three object manipulation tasks designed to differ in the predictability of sensory feedback. These included manipulation of a shared object with both hands (predictable coupling), manipulation of two independent objects without (uncoupled) and with external interference on one of the objects (unpredictable coupling).

Results

We found a task-specific reduction in co-activation during the predictable coupling compared to the other tasks. Less sensory gating, reflected in larger subcortical SEP amplitudes, was observed in the unpredictable coupling task. SAI behavior was closely linked to the subcortical SEP component indicating an important function of subcortical sites in predictability related SEP gating and their direct influence on M1 inhibition.

Discussion

Together, these findings suggest that the unpredictable coupling task cannot only rely on predictive forward control and is compensated by enhancing co-activation and increasing the saliency for external stimuli by reducing sensory gating at subcortical level. This behavior might serve as a preparatory step to compensate for external disturbances and to enhance processing and integration of all incoming external stimuli to update the current sensorimotor state. In contrast, predictive forward control is accurate in the predictable coupling task due to the integrated sensory feedback from both hands where sensorimotor resources are economized by reducing muscular co-activation and increasing sensory gating.

Mountain Hiking: Prolonged Eccentric Muscle Contraction during Simulated Downhill Walking Perturbs Sensorimotor Control Loops Needed for Safe Dynamic Foot-Ground Interactions

Safe mountain hiking requires precise control of dynamic foot-ground interactions. In addition to vision and vestibular afferents, limb proprioception, sensorimotor control loops, and reflex responses are used to adapt to the specific nature of the ground contact. Diminished leg dexterity and balance during downhill walking is usually attributed to fatigue. We investigated the supplementary hypothesis that the eccentric contractions inherent to downhill walking can also disrupt muscle proprioception, as well as the sensorimotor control loops and reflex responses that depend on it. In this study, we measured leg dexterity (LD), anterior-posterior (AP) and medio-lateral (ML) bipedal balance, and maximal voluntary leg extension strength in young and healthy participants before and after 30 min of simulated downhill walking at a natural pace on a treadmill at a 20° decline. Post-pre comparisons of LD (p < 0.001) and AP balance (p = 0.001) revealed significant reductions in dynamic foot-ground interactions after eccentric exercise without an accompanying reduction in leg extension strength. We conclude that eccentric contractions during downhill walking can disrupt the control of dynamic foot-ground interactions independently of fatigue. We speculate that mountaineering safety could be improved by increasing conscious attention to compensate for unadjusted proprioception weighting, especially in the descent.

Lifespan changes in postural control

Lifespan development of postural control shows as an inverted U-shaped function with optimal performance in young adults and similar levels of underperformance in children and older adults. However, similarities in children and older adults might conceal differences in underlying control processes. We mapped out age-related differences in postural control using center-of-pressure trajectories of 299 participants ranging from 7 to 81 years old in three tasks: stable stance, compromised vision, and narrowed base of support. Summary statistics (path length, ellipse area) replicated the well-known U-shape function also showing that compromising vision and narrowing the base of support affected older adults more than children. Stabilogram diffusion analysis (SDA) allows to assess postural control performance in terms of diffusion at short (< 1 s) and longer timescales. SDA parameters showed the strongest short-term drift in older adults, especially under compromised vision or narrowed base of support conditions. However, older adults accommodated their poor short-term control by corrective adjustments as reflected in long-term diffusion under eyes closed conditions and initiating anti-persistent behavior earlier compared with children and young adults in tandem stance. We argue that these results highlight the adaptability of the postural control system and warrant a reinterpretation of previous postural control frameworks.

Coupling of sensorimotor and cognitive functions in middle- and late adulthood

Introduction

The present study explored age effects and the coupling of sensorimotor and cognitive functions in a stratified sample of 96 middle-aged and older adults (age 45-86 years) with no indication of mild cognitive decline. In our sensorimotor tasks, we had an emphasis on listening in noise and postural control, but we also assessed functional mobility and tactile sensitivity.

Methods

Our cognitive measures comprised processing speed and assessments of core cognitive control processes (executive functions), notably inhibition, task switching, and working memory updating. We explored whether our measures of sensorimotor functioning mediated age differences in cognitive variables and compared their effect to processing speed. Subsequently, we examined whether individuals who had poorer (or better) than median cognitive performance for their age group also performed relatively poorer (or better) on sensorimotor tasks. Moreover, we examined whether the link between cognitive and sensorimotor functions becomes more pronounced in older age groups.

Results

Except for tactile sensitivity, we observed substantial age-related differences in all sensorimotor and cognitive variables from middle age onward. Processing speed and functional mobility were reliable mediators of age in task switching and inhibitory control. Regarding coupling between sensorimotor and cognition, we observed that individuals with poor cognitive control do not necessarily have poor listening in noise skills or poor postural control.

Discussion

As most conditions do not show an interdependency between sensorimotor and cognitive performance, other domain-specific factors that were not accounted for must also play a role. These need to be researched in order to gain a better understanding of how rehabilitation may impact cognitive functioning in aging persons.

Leveraging Joint Mechanics Simplifies the Neural Control of Movement

Behaviors we perform each day, such as manipulating an object or walking, require precise control of the interaction forces between our bodies and the environment. These forces are generated by muscle contractions, specified by the nervous system, and by joint mechanics, determined by the intrinsic properties of the musculoskeletal system. Depending on behavioral goals, joint mechanics might simplify or complicate control of movement by the nervous system. Whether humans can exploit joint mechanics to simplify neural control remains unclear. Here we evaluated if leveraging joint mechanics simplifies neural control by comparing performance in three tasks that required subjects to generate specified torques about the ankle during imposed sinusoidal movements; only one task required torques that could be generated by leveraging the intrinsic mechanics of the joint. The complexity of the neural control was assessed by subjects' perceived difficulty and the resultant task performance. We developed a novel approach that used continuous estimates of ankle impedance, a quantitative description of the joint mechanics, and measures of muscle activity to determine the mechanical and neural contributions to the net ankle torque generated in each task. We found that the torque resulting from changes in neural control was reduced when ankle impedance was consistent with the task being performed. Subjects perceived this task to be easier than those that were not consistent with the impedance of the ankle and were able to perform it with the highest level of consistency across repeated trials. These results demonstrate that leveraging the mechanical properties of a joint can simplify task completion and improve performance.

Copers adopt an altered dynamic postural control compared to individuals with chronic ankle instability and controls in unanticipated single-leg landing

BACKGROUND

Several prior studies involving "expected" single-leg landings have not succeeded in establishing a difference between copers and a control group.

RESEARCH QUESTION

Does expected and unanticipated single-leg landing affect dynamic postural stability in lateral ankle sprain individuals with chronic ankle instability (CAI), copers, and controls?

METHODS

In this prospective cross-sectional study, physically active adults with CAI (n = 12), copers (n = 12), and controls (n = 12) were included. Participants performed expected single-leg landing by stepping off a 30-cm box. They also performed unanticipated landings including side-step cutting, side-step cutting at 60°, single-leg landing, and forward stepping. The expected and unanticipated conditions of each groups were compared in terms of time to stabilization (TTS) and center of pressure (COP) for the anterior-posterior (AP) and medial-lateral (ML) conditions. To analyze the data, a mixed-model one-way analysis of variance and a Tukey-Kramer post hoc test were performed.

RESULTS

A significant condition × group interaction was observed in only TTS ML, with the CAI group demonstrating a significantly longer TTS ML than the coper (p < 0.001) and control (p < 0.001) groups during unanticipated trials. In addition, group interaction effects were observed for COP AP and TTS AP. The coper group demonstrated significantly longer COP AP and TTS AP than the control group (p < 0.001).

SIGNIFICANCE

The CAI group demonstrated a significantly longer TTS ML than the coper and control groups during the unanticipated condition, and the coper group demonstrated significantly longer TTS AP and COP AP than the control group. Thus, longer COP AP and TTS AP sway time in the coper group may be a protection mechanism, allowing greater freedom in the AP plane while quickly controlling ML sway and preventing lateral ankle sprains. These findings can help in the prevention of lateral ankle sprains and assessment of dynamic postural control.

A review of center of pressure (COP) variables to quantify standing balance in elderly people: Algorithms and open-access code

Postural control is often quantified by recording the trajectory of the center of pressure (COP)-also called stabilogram-during human quiet standing. This quantification has many important applications, such as the early detection of balance degradation to prevent falls, a crucial task whose relevance increases with the aging of the population. Due to the complexity of the quantification process, the analyses of sway patterns have been performed empirically using a number of variables, such as ellipse confidence area or mean velocity. This study reviews and compares a wide range of state-of-the-art variables that are used to assess the risk of fall in elderly from a stabilogram. When appropriate, we discuss the hypothesis and mathematical assumptions that underlie these variables, and we propose a reproducible method to compute each of them. Additionally, we provide a statistical description of their behavior on two datasets recorded in two elderly populations and with different protocols, to hint at typical values of these variables. First, the balance of 133 elderly individuals, including 32 fallers, was measured on a relatively inexpensive, portable force platform (Wii Balance Board, Nintendo) with a 25-s open-eyes protocol. Second, the recordings of 76 elderly individuals, from an open access database commonly used to test static balance analyses, were used to compute the values of the variables on 60-s eyes-open recordings with a research laboratory standard force platform.

Effect of unplanned athletic movement on knee mechanics: a systematic review with multilevel meta-analysis

OBJECTIVE

To compare the effects of pre-planned and unplanned movement tasks on knee biomechanics in uninjured individuals.

DESIGN

Systematic review with meta-analysis.

DATA SOURCES

Five databases (PubMed, Google Scholar, Cochrane Library, ScienceDirect and Web of Science) were searched from inception to November 2020. Cross-sectional, (randomised) controlled/non-controlled trials comparing knee angles/moments of pre-planned and unplanned single-leg landings/cuttings were included. Quality of evidence was assessed using the tool of the Grading of Recommendations Assessment, Development and Evaluation working group.

METHODS

A multilevel meta-analysis with a robust random-effects meta-regression model was used to pool the standardised mean differences (SMD) of knee mechanics between pre-planned and unplanned tasks. The influence of possible effect modifiers (eg, competitive performance level) was examined in a moderator analysis.

RESULTS

Twenty-five trials (485 participants) with good methodological quality (Downs and Black) were identified. Quality of evidence was downgraded due to potential risk of bias (eg, confounding). Moderate-quality evidence indicates that unplanned tasks evoked significantly higher external knee abduction (SMD: 0.34, 95% CI: 0.16 to 0.51, 14 studies) and tibial internal rotation moments (SMD: 0.51, 95% CI: 0.23 to 0.79, 11 studies). No significant between-condition differences were detected for sagittal plane mechanics (p>0.05). According to the moderator analysis, increased abduction moments particularly occurred in non-professional athletes (SMD: 0.55, 95% CI: 0.14 to 0.95, 5 studies).

CONCLUSION

Unplanned movement entails higher knee abduction and tibial internal rotation moments, which could predispose for knee injury. Exercise professionals designing injury-prevention protocols, especially for non-elite athletes, should consider the implementation of assessments and exercises requiring time-constrained decision-making.

PROSPERO REGISTRATION NUMBER

CRD42019140331.

Standing on unstable surface challenges postural control of tracking tasks and modulates neuromuscular adjustments specific to task complexity

Understanding the modulations of motor control in the presence of perturbations in task conditions of varying complexity is a key element towards the design of effective perturbation-based balance exercise programs. In this study we investigated the effect of mechanical perturbations, induced by an unstable surface, on muscle activation and visuo-postural coupling, when actively tracking target motion cues of different complexity. Four postural tasks following a visual oscillating target of varying target complexity (periodic-sinusoidal vs. chaotic-Lorenz) and surface (stable-floor vs. unstable-foam) were performed. The electromyographic activity of the main plantarflexor and dorsiflexor muscles was captured. The coupling between sway and target was assessed through spectral analysis and the system's local dynamic stability through the short-term maximum Lyapunov exponent. We found that external perturbations increased local instability and deteriorated visuo-motor coupling. Visuo-motor deterioration was greater for the chaotic target, implying that the effect of the induced perturbations depends on target complexity. There was a modulation of the neuromotor system towards amplification of muscle activity and coactivation to compensate surface-related perturbations and to ensure robust motor control. Our findings provide evidence that, in the presence of perturbations, target complexity induces specific modulations in the neuromotor system while controlling balance and posture.

Varied movement errors drive learning of dynamic balance control during walking in people with incomplete spinal cord injury: a pilot study

The purpose of this study was to determine whether the application of a varied pelvis perturbation force would improve dynamic balance control and gait stability of people with incomplete spinal cord injury (iSCI). Fourteen participants with iSCI completed the test in two conditions, i.e., walking paired with pelvis perturbation force and treadmill walking only, with 1-week interval in between. The order of the testing condition was randomized across participants. For the pelvis pertubation condition, subjects walked on a treadmill with no force for 1 min, with a varied pelvis perturbation force that was bilaterally applied in the medial-lateral direction for 10 min, without force for 1 min, and then with the perturbation for another 10 min after a sitting break. For the treadmill only condition, a protocol that was similar to the perturbation condition was used but no force was applied. Margin of stability (MoS), weight shifting, and other spatiotemporal gait parameters were calculated. Compared to treadmill training only, participants showed significant smaller MoS and double-leg support time after treadmill walking with pelvis perturbation. In addition, participants showed significantly greater improvements in overground walking speed after treadmill walking with pelvis perturbation than treadmill only (p = 0.021). Results from this study suggest that applying a varied pelvis perturbation force during treadmill walking could improve dynamic balance control in people with iSCI, which could be transferred to overground walking. These findings may be used to develop a new intervention to improve balance and walking function in people with iSCI.

Characteristics of Postural Muscle Activity in Response to A Motor-Motor Task in Elderly

The purpose of the current study was to evaluate postural muscle performance of older adults in response to a combination of two motor tasks perturbations. Fifteen older participants were instructed to perform a pushing task as an upper limb perturbation while standing on a fixed or sliding board as a lower limb perturbation. Postural responses were characterized by onsets and magnitudes of muscle activities as well as onsets of segment movements. The sliding board did not affect the onset timing and sequence of muscle initiations and segment movements. However, significant large muscle activities of tibialis anterior and erector spinae were observed in the sliding condition (p < 0.05). The co-contraction values of the trunk and shank segments were significantly larger in the sliding condition through the studied periods (p < 0.05). Lastly, heavy pushing weight did not change the timing, magnitude, sequence of all studied parameters. Older adults enhanced postural stability by increasing the segment stiffness then started to handle two perturbations. In conclusion, they were able to deal with a dual motor-motor task after having secured their balance but could not make corresponding adjustments to the level of the perturbation difficulty.

Agonist-Antagonist Coactivation Enhances Corticomotor Excitability of Ankle Muscles

Spinal pathways underlying reciprocal flexion-extension contractions have been well characterized, but the extent to which cortically evoked motor-evoked potentials (MEPs) are influenced by antagonist muscle activation remains unclear. A majority of studies using transcranial magnetic stimulation- (TMS-) evoked MEPs to evaluate the excitability of the corticospinal pathway focus on upper extremity muscles. Due to functional and neural control differences between lower and upper limb muscles, there is a need to evaluate methodological factors influencing TMS-evoked MEPs specifically in lower limb musculature. If and to what extent the activation of the nontargeted muscles, such as antagonists, affects TMS-evoked MEPs is poorly understood, and such gaps in our knowledge may limit the rigor and reproducibility of TMS studies. Here, we evaluated the effect of the activation state of the antagonist muscle on TMS-evoked MEPs obtained from the target (agonist) ankle muscle for both tibialis anterior (TA) and soleus muscles. Fourteen able-bodied participants (11 females, age: 26.1 ± 4.1 years) completed one experimental session; data from 12 individuals were included in the analysis. TMS was delivered during 4 conditions: rest, TA activated, soleus activated, and TA and soleus coactivation. Three pairwise comparisons were made for MEP amplitude and coefficient of variability (CV): rest versus coactivation, rest versus antagonist activation, and agonist activation versus coactivation. We demonstrated that agonist-antagonist coactivation enhanced MEP amplitude and reduced MEP CVs for both TA and soleus muscles. Our results provide methodological considerations for future TMS studies and pave the way for future exploration of coactivation-dependent modulation of corticomotor excitability in pathological cohorts such as stroke or spinal cord injury.

Time-dependent tuning of balance control and aftereffects following optical flow perturbation training in older adults

Background

Walking balance in older adults is disproportionately susceptible to lateral instability provoked by optical flow perturbations. The prolonged exposure to these perturbations could promote reactive balance control and increased balance confidence in older adults, but this scientific premise has yet to be investigated. This proof of concept study was designed to investigate the propensity for time-dependent tuning of walking balance control and the presence of aftereffects in older adults following a single session of optical flow perturbation training.

Methods

Thirteen older adults participated in a randomized, crossover design performed on different days that included 10 min of treadmill walking with (experimental session) and without (control session) optical flow perturbations. We used electromyographic recordings of leg muscle activity and 3D motion capture to quantify foot placement kinematics, lateral margin of stability, and antagonist coactivation during normal walking (baseline), early (min 1) and late (min 10) responses to perturbations, and aftereffects immediately following perturbation cessation (post).

Results

At their onset, perturbations elicited 17% wider and 7% shorter steps, higher step width and length variability (+171% and +132%, respectively), larger and more variable margins of stability (MoS), and roughly twice the antagonist leg muscle coactivation (p-values<0.05). Despite continued perturbations, most outcomes returned to values observed during normal, unperturbed walking by the end of prolonged exposure. After 10 min of perturbation training and their subsequent cessation, older adults walked with longer and more narrow steps, modest increases in foot placement variability, and roughly half the MoS variability and antagonist lower leg muscle coactivation as they did before training.

Conclusions

Findings suggest that older adults: (i) respond to the onset of perturbations using generalized anticipatory balance control, (ii) deprioritize that strategy following prolonged exposure to perturbations, and (iii) upon removal of perturbations, exhibit short-term aftereffects that indicate a lessening of anticipatory control, an increase in reactive control, and/or increased balance confidence. We consider this an early, proof-of-concept study into the clinical utility of prolonged exposure to optical flow perturbations as a training tool for corrective motor adjustments relevant to walking balance integrity toward reinforcing task-specific, reactive control and/or improving balance confidence in older adults.

Trial registration

clinicaltrials.gov (NCT03341728). Registered 14 November 2017.

Electronic supplementary material

The online version of this article (10.1186/s12984-019-0555-3) contains supplementary material, which is available to authorized users.

Age-related changes in leg proprioception: implications for postural control

In addition to being a prerequisite for many activities of daily living, the ability to maintain steady upright standing is a relevant model to study sensorimotor integrative function. Upright standing requires managing multimodal sensory inputs to produce finely tuned motor output that can be adjusted to accommodate changes in standing conditions and environment. The sensory information used for postural control mainly arises from the vestibular system of the inner ear, vision, and proprioception. Proprioception (sense of body position and movement) encompasses signals from mechanoreceptors (proprioceptors) located in muscles, tendons, and joint capsules. There is general agreement that proprioception signals from leg muscles provide the primary source of information for postural control. This is because of their exquisite sensitivity to detect body sway during unperturbed upright standing that mainly results from variations in leg muscle length induced by rotations around the ankle joint. However, aging is associated with alterations of muscle spindles and their neural pathways, which induce a decrease in the sensitivity, acuity, and integration of the proprioceptive signal. These alterations promote changes in postural control that reduce its efficiency and thereby may have deleterious consequences for the functional independence of an individual. This narrative review provides an overview of how aging alters the proprioceptive signal from the legs and presents compelling evidence that these changes modify the neural control of upright standing.

The center of pressure and ankle muscle co-contraction in response to anterior-posterior perturbations

Though both contraction of agonist muscles and co-contraction of antagonistic muscle pairs across the ankle joint are essential to postural stability, they are perceived to operate independently of each other, In an antagonistic setup, agonist muscles contract generating moment about the joint, while antagonist muscles contract generating stiffness across the joint. While both work together in maintaining robustness in the face of external perturbations, contractions of agonist muscles and co-contractions of antagonistic muscle pairs across the ankle joint play different roles in responding to and adapting to external perturbations. To determine their respective roles, we exposed participants to repeated perturbations in both large and small magnitudes. The center of pressure (COP) and a co-contraction index (CCI) were used to quantify the activation of agonist muscles and antagonistic muscle pairs across the ankle joint. Our results found that participants generated moment of a large magnitude across the ankle joint—a large deviation in the COP curve—in response to perturbations of a large magnitude (p <0.05), whereas the same participants generated higher stiffness about the ankle—a larger value in CCI—in response to perturbations of a small magnitude (p <0.05). These results indicate that participants use different postural strategies pertaining to circumstances. Further, the moment across the ankle decreased with repetitions of the same perturbation (p <0.05), and CCI tended to remain unchanged even in response to a different perturbation following repetition of the same perturbation (p <0.05). These findings suggest that ankle muscle contraction and co-contraction play different roles in regaining and maintaining postural stability. This study demonstrates that ankle moment and stiffness are not correlated in response to external perturbations.

Age and falls history effects on antagonist leg muscle coactivation during walking with balance perturbations

BACKGROUND

Inspired by a reliance on visual feedback for movement control in older age, optical flow perturbations provide a unique opportunity to study the neuromuscular mechanisms involved in walking balance control, including aging and falls history effects on the response to environmental balance challenges. Specifically, antagonist leg muscle coactivation, which increases with age during walking, is considered a neuromuscular defense against age-associated deficits in balance control. The purpose of this study was to investigate the effects of age and falls history on antagonist leg muscle coactivation during walking with and without optical flow perturbations of different amplitudes.

METHODS

Eleven young adults [mean (standard deviation) age: 24.8 (4.8) years], eleven older non-fallers [75.3 (5.4) years] and eleven older fallers [age: 78 (7.6) years] participated in this study. Participants completed 2-minute walking trials while watching a speed-matched virtual hallway that, in some conditions, included mediolateral optical flow perturbations designed to elicit the visual perception of imbalance.

FINDINGS

We first found that lower leg antagonist muscle coactivation during normal walking increased with age, independent of falls history. We also found that older but not young adults increased antagonist leg muscle coactivation in the presence of optical flow perturbations, with more pervasive effects in older adults with a history of falls.

INTERPRETATION

Our findings allude to a greater susceptibility to optical flow perturbations in older fallers during walking, which points to a higher potential for risk of instability in more complex and dynamic everyday environments. These findings may also have broader impacts related to the design of innovative training paradigms and neuromuscular targets for falls prevention.

Forecast or Fall: Prediction's Importance to Postural Control

To interact successfully with an uncertain environment, organisms must be able to respond to both unanticipated and anticipated events. For unanticipated events, organisms have evolved stereotyped motor behaviors mapped to the statistical regularities of the environment, which can be trigged by specific sensory stimuli. These “reflexive” responses are more or less hardwired to prevent falls and represent, maybe, the best available solution to maintaining posture given limited available time and information. With the gift of foresight, however, motor behaviors can be tuned or prepared in advance, improving the ability of the organism to compensate for, and interact with, the changing environment. Indeed, foresight's improvement of our interactive capacity occurs through several means, such as better action selection, processing, and conduction delay compensation and by providing a prediction with which to compare our actual behaviors to, thereby facilitating error identification and learning. Here we review the various roles foresight (prediction) plays in maintaining our postural equilibrium. We start by describing some of the more recent findings related to the prediction of instability. Specifically, we cover recent advancements in the understanding of anticipatory postural behaviors that are used broadly to stabilize volitional movement and compensate for impending postural disturbances. We also describe anticipatory changes in the state, or set, of the nervous system that may facilitate anticipatory behaviors. From changes in central set, we briefly discuss prediction of postural instability online before moving into a discussion of how predictive mechanisms, such as internal models, permit us to tune, perhaps our highest level predictive behaviors, namely the priming associated with motor affordances. Lastly, we explore methods best suited to expose the contribution of prediction to postural equilibrium control across a variety of contexts.

Neither total muscle activation nor co-activation explains the youthful walking economy of older runners

BACKGROUND

Older adults (≥65 years) exhibit greater metabolic rates during walking (worse walking economy) compared to young adults. Yet, previous research suggests that habitual running, but not habitual walking, exercise mitigates the age-related deterioration of walking economy.

RESEARCH QUESTION

Does total leg muscle activation and/or agonist-antagonist co-activation explain the superior walking economy of older runners versus older walkers?

METHODS

We quantified metabolic power, leg muscle activation, and co-activation in older walkers and older runners during walking at 0.75, 1.25, and 1.75 m/s.

RESULTS

While accounting for multiple comparisons, neither total stride (each speed p ≥ 0.024), stance- (each speed p ≥ 0.217), nor swing- (each speed p ≥ 0.170) phase EMG amplitude differed between older walkers and older runners at 0.75, 1.25, or 1.75 m/s. Stride averaged medial gastrocnemius and biceps femoris activation was lower in older runners than older walkers at 1.25 and 1.75 m/s (all p ≤ 0.025). We also calculated shank, thigh, and overall (shank and thigh) agonist-antagonist leg muscle co-activation over each stride, and the only difference between groups was a greater shank co-activation in older runners at 0.75 m/s (p = 0.024). Across groups, stride, stance-, and swing-phase total muscle activation positively correlated with gross metabolic power (R2 = 0.58-0.66; all p < 0.001). Paradoxically, across groups, stride, stance-, and swing-phase muscle co-activation indices were negatively correlated with gross metabolic power (R2 = 0.08-0.29; all p ≤ 0.007).

SIGNIFICANCE

Neither total leg muscle activation nor co-activation explains the superior walking economy of older runners versus older walkers.

Pre- and postoperative postural regulation following anterior cruciate ligament reconstruction

There are currently no longitudinal data describing the pre- and postoperative postural regulation and stability of patients with anterior cruciate ligament (ACL) damage. Therefore, the aim of this study was to evaluate postural regulation and stability prior to and during rehabilitation following surgery of the ACL. Fifty-four physically active subjects (age: 30.5±10.9 years, 29 male subjects) were examined with the Interactive Balance System pre-, 6, and at 12 weeks following surgical reconstruction of the ACL using a hamstring tendon graft. The average period of time from injury to surgery was 27 days. Data were calculated with unifactorial and univariate analysis of variance. Significant effects were found for the somatosensory system (η2=0.115), stability indicator (η2=0.123), weight distribution index (η2=0.176), and synchronization (foot coordination) (η2=0.249). Involved side weight distribution (parameter: left) increased significantly (patients with left-sided/right-sided injury: η2=0.234/0.272). Load distribution to the heel remained stable during all three examination periods (η2=0.035 and η2=0.071), although a remarkable load at forefoot was observed. In seven out of 10 parameters partial effects were seen during the first 6 weeks after surgery. The results of this study indicated that injury of the ACL and subsequent surgical reconstructions result in postural regulation, with improvements in somatosensory system function, postural stability, weight distribution index, and foot coordination. Also, overloading of the injured side on the feet reduces significantly during rehabilitation. Thus, the initial phase of rehabilitation (weeks 1 to 6) seems to be more effective than the second period (weeks 6 to 12) postoperatively.

Using Feedback Control to Reduce Limb Impedance during Forceful Contractions

Little is known about the ability to precisely regulate forces or torques during unexpected disturbances, as required during numerous tasks. Effective force regulation implies small changes in force responding to externally imposed displacements, a behavior characterized by low limb impedance. This task can be challenging, since the intrinsic impedance of muscles increases when generating volitional forces. The purpose of this study was to examine the ability to voluntarily reduce limb impedance during force regulation, and the neural mechanisms associated with that ability. Small displacement perturbations were used to quantify elbow impedance during the exertion of volitional elbow torques from 0% to 20% of maximum voluntary contraction. Subjects were instructed either to not intervene with the imposed perturbations or to explicitly intervene so as to minimize the influence of the perturbations on the elbow torque. Our results demonstrated that individuals can reduce the low frequency components of elbow impedance by 35%. Electromyographic analysis suggested that this behavior is mediated by volitional and possibly long-latency reflex pathways with delays of at least 120 ms. These results provide a context for understanding how feedback altered by aging or injuries may influence the ability to regulate forces precisely.

Control of locomotor stability in stabilizing and destabilizing environments

To develop effective interventions targeting locomotor stability, it is crucial to understand how people control and modify gait in response to changes in stabilization requirements. Our purpose was to examine how individuals with and without incomplete spinal cord injury (iSCI) control lateral stability in haptic walking environments that increase or decrease stabilization demands. We hypothesized that people would adapt to walking in a predictable, stabilizing viscous force field and unpredictable destabilizing force field by increasing and decreasing feedforward control of lateral stability, respectively. Adaptations in feedforward control were measured using after-effects when fields were removed. Both groups significantly (p<0.05) decreased step width in the stabilizing field. When the stabilizing field was removed, narrower steps persisted in both groups and subjects with iSCI significantly increased movement variability (p<0.05). The after-effect of walking in the stabilizing field was a suppression of ongoing general stabilization mechanisms. In the destabilizing field, subjects with iSCI took faster steps and increased lateral margins of stability (p<0.05). Step frequency increases persisted when the destabilizing field was removed (p<0.05), suggesting that subjects with iSCI made feedforward adaptions to increase control of lateral stability. In contrast, in the destabilizing field, non-impaired subjects increased movement variability (p<0.05) and did not change step width, step frequency, or lateral margin of stability (p>0.05). When the destabilizing field was removed, increases in movement variability persisted (p<0.05), suggesting that non-impaired subjects made feedforward decreases in resistance to perturbations.

The Neuromuscular Origins of Kinematic Variability during Perturbed Walking

We investigated the neuromuscular contributions to kinematic variability and thus step to step adjustments in posture and foot placement across a range of walking speeds in response to optical flow perturbations of different amplitudes using a custom virtual environment. We found that perturbations significantly increased step width, decreased step length, and elicited larger trunk sway compared to normal walking. However, perturbation-induced effects on the corresponding variabilities of these measurements were much more profound. Consistent with our hypotheses, we found that: (1) perturbations increased EMG activity of the gluteus medius and postural control muscles during leg swing, and increased antagonist leg muscle coactivation during limb loading in early stance, and (2) changes in the magnitude of step to step adjustments in postural sway and lateral foot placement positively correlated with those of postural control and gluteus medius muscle activities, respectively, in response to perturbations. However, (3) interactions between walking speed and susceptibility to perturbations, when present, were more complex than anticipated. Our study provides important mechanistic neuromuscular insight into walking balance control and important reference values for the emergence of balance impairment.

Unilateral Eccentric Contraction of the Plantarflexors Leads to Bilateral Alterations in Leg Dexterity

Eccentric contractions can affect musculotendon mechanical properties and disrupt muscle proprioception, but their behavioral consequences are poorly understood. We tested whether repeated eccentric contractions of plantarflexor muscles of one leg affected the dexterity of either leg. Twenty healthy male subjects (27.3 ± 4.0 yrs) compressed a compliant and slender spring prone to buckling with each isolated leg. The maximal instability they could control (i.e., the maximal average sustained compression force, or lower extremity dexterity force, LED_force) quantified the dexterity of each leg. We found that eccentric contractions did not affect LED_force, but reduced force variability (LED_SD). Surprisingly, LED_force increased in the non-exposed, contralateral leg. These effects were specific to exposure to eccentric contractions because an effort-matched exposure to walking did not affect leg dexterity. In the exposed leg, eccentric contractions (i) reduced voluntary error corrections during spring compressions (i.e., reduced 0.5–4 Hz power of LED_force); (ii) did not change spinal excitability (i.e., unaffected H-reflexes); and (iii) changed the structure of the neural drive to the α-motoneuron pool (i.e., reduced EMG power within the 4–8 Hz physiological tremor band). These results suggest that repeated eccentric contractions alter the feedback control for dexterity in the exposed leg by reducing muscle spindle sensitivity. Moreover, the unexpected improvement in LED_force in the non-exposed contralateral leg was likely a consequence of crossed-effects on its spinal and supraspinal feedback control. We discuss the implications of these bilateral effects of unilateral eccentric contractions, their effect on spinal and supraspinal control of dynamic foot-ground interactions, and their potential to facilitate rehabilitation from musculoskeletal and neuromotor impairments.

Evidence of impaired neuromuscular responses in the support leg to a destabilizing swing phase perturbation in hemiparetic gait

The neuromuscular mechanisms that underlie post-stroke impairment in reactive balance control during gait are not fully understood. Previous research has described altered muscle activations in the paretic leg in response to postural perturbations from static positions. Additionally, attenuation of interlimb reflexes after stroke has been reported. Our goal was to characterize post-stroke changes to neuromuscular responses in the stance leg following a swing phase perturbation during gait. We hypothesized that, following a trip, altered timing, sequence, and magnitudes of perturbation-induced activations would emerge in the paretic and nonparetic support legs of stroke survivors compared to healthy control subjects. The swing foot was interrupted, while subjects walked on a treadmill. In healthy subjects, a sequence of perturbation-induced activations emerged in the contralateral stance leg with mean onset latencies of 87-147 ms. The earliest latencies occurred in the hamstrings and hip abductor and adductors. The hamstrings, the adductor magnus, and the gastrocnemius dominated the relative balance of perturbation-induced activations. The sequence and balance of activations were largely preserved after stroke. However, onset latencies were significantly delayed across most muscles in both paretic and nonparetic stance legs. The shortest latencies observed suggest the involvement of interlimb reflexes with supraspinal pathways. The preservation of the sequence and balance of activations may point to a centrally programmed postural response that is preserved after stroke, while post-stroke delays may suggest longer transmission times for interlimb reflexes.

Gait Characteristics When Walking on Different Slippery Walkways

OBJECTIVE

This study sought to determine the changes in muscle activity about the ankle, knee, and hip in able-bodied people walking at steady state on surfaces with different degrees of slipperiness.

METHODS

Muscle activity was measured through electromyographic signals from selected lower limb muscles and quantified to directly compare changes across surface conditions.

RESULTS

Our results showed distinct changes in the patterns of muscle activity controlling each joint. Muscles controlling the ankle showed a significant reduction in activity as the surface became more slippery, presumably resulting in a compliant distal joint to facilitate full contact with the surface. Select muscles about the knee and hip showed a significant increase in activity as the surface became more slippery. This resulted in increased knee and hip flexion likely contributing to a lowering of the body's center of mass and stabilization of the proximal leg and trunk.

CONCLUSION

These findings suggest a proximal-distal gradient in the control of muscle activity that could inform the future design of adaptable prosthetic controllers.

SIGNIFICANCE

Walking on a slippery surface is extremely difficult, especially for individuals with lower limb amputations because current prostheses do not allow the compensatory changes in lower limb dynamics that occur involuntarily in unimpaired subjects. With recent advances in prosthetic control, there is the potential to provide some of these compensatory changes; however, we first need to understand how able-bodied individuals modulate their gait under these challenging conditions.

Arm dominance affects feedforward strategy more than feedback sensitivity during a postural task

Handedness is a feature of human motor control that is still not fully understood. Recent work has demonstrated that the dominant and nondominant arm each excel at different behaviors and has proposed that this behavioral asymmetry arises from lateralization in the cerebral cortex: the dominant side specializes in predictive trajectory control, while the nondominant side is specialized for impedance control. Long-latency stretch reflexes are an automatic mechanism for regulating posture and have been shown to contribute to limb impedance. To determine whether long-latency reflexes also contribute to asymmetric motor behavior in the upper limbs, we investigated the effect of arm dominance on stretch reflexes during a postural task that required varying degrees of impedance control. Our results demonstrated slightly but significantly larger reflex responses in the biarticular muscles of the nondominant arm, as would be consistent with increased impedance control. These differences were attributed solely to higher levels of voluntary background activity in the nondominant biarticular muscles, indicating that feedforward strategies for postural stability may differ between arms. Reflex sensitivity, which was defined as the magnitude of the reflex response for matched levels of background activity, was not significantly different between arms for a broad subject population ranging from 23 to 51 years of age. These results indicate that inter-arm differences in feedforward strategies are more influential during posture than differences in feedback sensitivity, in a broad subject population. Interestingly, restricting our analysis to subjects under 40 years of age revealed a small increase in long-latency reflex sensitivity in the nondominant arm relative to the dominant arm. Though our subject numbers were small for this secondary analysis, it suggests that further studies may be required to assess the influence of reflex lateralization throughout development.

Running for exercise mitigates age-related deterioration of walking economy

INTRODUCTION

Impaired walking performance is a key predictor of morbidity among older adults. A distinctive characteristic of impaired walking performance among older adults is a greater metabolic cost (worse economy) compared to young adults. However, older adults who consistently run have been shown to retain a similar running economy as young runners. Unfortunately, those running studies did not measure the metabolic cost of walking. Thus, it is unclear if running exercise can prevent the deterioration of walking economy.

PURPOSE

To determine if and how regular walking vs. running exercise affects the economy of locomotion in older adults.

METHODS

15 older adults (69 ± 3 years) who walk ≥ 30 min, 3x/week for exercise, "walkers" and 15 older adults (69 ± 5 years) who run ≥ 30 min, 3x/week, "runners" walked on a force-instrumented treadmill at three speeds (0.75, 1.25, and 1.75 m/s). We determined walking economy using expired gas analysis and walking mechanics via ground reaction forces during the last 2 minutes of each 5 minute trial. We compared walking economy between the two groups and to non-aerobically trained young and older adults from a prior study.

RESULTS

Older runners had a 7-10% better walking economy than older walkers over the range of speeds tested (p = .016) and had walking economy similar to young sedentary adults over a similar range of speeds (p =  .237). We found no substantial biomechanical differences between older walkers and runners. In contrast to older runners, older walkers had similar walking economy as older sedentary adults (p =  .461) and ∼ 26% worse walking economy than young adults (p<.0001).

CONCLUSION

Running mitigates the age-related deterioration of walking economy whereas walking for exercise appears to have minimal effect on the age-related deterioration in walking economy.

Acceleration dependence and task-specific modulation of short- and medium-latency reflexes in the ankle extensors

Involuntary responses to muscle stretch are often composed of a short-latency reflex (SLR) and more variable responses at longer latencies such as the medium-latency (MLR) and long-latency stretch reflex (LLR). Although longer latency reflexes are enhanced in the upper limb during stabilization of external loads, it remains unknown if they have a similar role in the lower limb. This uncertainty results in part from the inconsistency with which longer latency reflexes have been observed in the lower limb. A review of the literature suggests that studies that only observe SLRs have used perturbations with large accelerations, possibly causing a synchronization of motoneuron refractory periods or an activation of force-dependent inhibition. We therefore hypothesized that the amplitude of longer latency reflexes would vary with perturbation acceleration. We further hypothesized that if longer latency reflexes were elicited, they would increase in amplitude during control of an unstable load, as has been observed in the upper limb. These hypotheses were tested at the ankle while subjects performed a torque or position control task. SLR and MLR reflex components were elicited by ankle flexion perturbations with a fixed peak velocity and variable acceleration. Both reflex components initially scaled with acceleration, however, while the SLR continued to increase at high accelerations, the MLR weakened. At accelerations that reliably elicited MLRs, both the SLR and MLR were reduced during control of the unstable load. These findings clarify the conditions required to elicit MLRs in the ankle extensors and provide additional evidence that rapid feedback pathways are downregulated when stability is compromised in the lower limb.

Long-latency muscle activity reflects continuous, delayed sensorimotor feedback of task-level and not joint-level error

In both the upper and lower limbs, evidence suggests that short-latency electromyographic (EMG) responses to mechanical perturbations are modulated based on muscle stretch or joint motion, whereas long-latency responses are modulated based on attainment of task-level goals, e.g., desired direction of limb movement. We hypothesized that long-latency responses are modulated continuously by task-level error feedback. Previously, we identified an error-based sensorimotor feedback transformation that describes the time course of EMG responses to ramp-and-hold perturbations during standing balance (Safavynia and Ting 2013; Welch and Ting 2008, 2009). Here, our goals were 1) to test the robustness of the sensorimotor transformation over a richer set of perturbation conditions and postural states; and 2) to explicitly test whether the sensorimotor transformation is based on task-level vs. joint-level error. We developed novel perturbation trains of acceleration pulses such that perturbations were applied when the body deviated from the desired, upright state while recovering from preceding perturbations. The entire time course of EMG responses (∼4 s) in an antagonistic muscle pair was reconstructed using a weighted sum of center of mass (CoM) kinematics preceding EMGs at long-latency delays (∼100 ms). Furthermore, CoM and joint kinematic trajectories became decorrelated during perturbation trains, allowing us to explicitly compare task-level vs. joint feedback in the same experimental condition. Reconstruction of EMGs was poorer using joint kinematics compared with CoM kinematics and required unphysiologically short (∼10 ms) delays. Thus continuous, long-latency feedback of task-level variables may be a common mechanism regulating long-latency responses in the upper and lower limbs.

Vestibulo-ocular response and balance control in children and young adults with mild-to-moderate intellectual and developmental disability: a pilot study

The vestibulo-ocular response (VOR) may not be fully developed in children with an intellectual and developmental disability (IDD). This study aimed to identify the presence of VOR deficit in children and young adults with unspecified mild-to-moderate intellectual and developmental disability and its effect on balance control. Twenty-one children and young adults with IDD ranging in age from 8 to 22 years (mean 17.5 ± 3.9 years) were included in the study. The VOR was evaluated with the Head Impulse Test and the Static and Dynamic Visual Acuity Test (S&D-VAT). Postural stability was measured in an upright standing position by the Clinical Test for Sensory Interaction in Balance (CTSIB), single leg stance (SLS) during eyes open and eyes closed, and Romberg stance under eyes open and eyes closed conditions using a force platform. Reduced vestibulo-ocular responses were found in 13 of 21 (62%) participants who were able to complete testing. In the fifth condition of the CTSIB (standing on foam with eyes closed), those without VOR deficit were able to maintain balance longer than those with VOR deficit (29 s [median 30] vs. 12s [median 7.3], respectively; p=0.03). The study demonstrates potential effects of VOR deficit in children and young adults with IDD and some significant differences in balance control between those with and without a VOR deficit. VOR function in children and young adults with IDD should be routinely tested to enable early detection of deficits.

How does age affect leg muscle activity/coactivity during uphill and downhill walking?

Walking uphill and downhill can be challenging for community-dwelling old adults. We investigated the effects of age on leg muscle activity amplitudes and timing during level, uphill, and downhill walking. We hypothesized that old adults would exhibit smaller increases in ankle extensor muscle activities and greater increases in hip extensor muscle activities compared to young adults during uphill vs. level walking. We also hypothesized that, compared to level walking, antagonist leg muscle coactivation would be disproportionately greater in old vs. young adults during downhill walking. Ten old (72±5yrs) and ten young (25±4yrs) subjects walked at 1.25m/s on a treadmill at seven grades (0°, ±3°, ±6°, ±9°). We quantified the stance phase electromyographic activities of the gluteus maximus (GMAX), biceps femoris (BF), rectus femoris (RF), vastus medialis (VM), medial gastrocnemius (MG), soleus (SOL), and tibialis anterior (TA). Old adults exhibited smaller increases in MG activity with steeper uphill grade than young adults (e.g., +136% vs. +174% at 9°). A disproportionate recruitment of hip muscles led to GMAX activity approaching the maximum isometric capacity of these active old adults at steep uphill grades (e.g., old vs. young, 73% MVC vs. 33% MVC at +9°). Neither uphill nor downhill walking affected the greater coactivation of antagonist muscles in old vs. young adults. We conclude that the disproportionate recruitment of hip muscles with advanced age may have critical implications for maintaining independent mobility in old adults, particularly at steeper uphill grades.

Influence of environmental stability on the regulation of end-point impedance during the maintenance of arm posture

Many common tasks compromise arm stability along specific directions. Such tasks can be completed only if the impedance of the arm is sufficient to compensate for the destabilizing effects of the task. During movement, it has been demonstrated that the direction of maximal arm stiffness, the static component of impedance, can be preferentially increased to compensate for directionally unstable environments. In contrast, numerous studies have shown that such control is not possible during postural tasks. It remains unknown if these findings represent a fundamental difference in the control of arm mechanics during posture and movement or an involuntary response to the destabilizing environments used in the movement studies but not yet tested during posture maintenance. Our goal was to quantify how arm impedance is adapted during postural tasks that compromise stability along specific directions. Our results demonstrate that impedance can be modulated to compensate for these instabilities during postural tasks but that the changes are modest relative to those previously reported during reaching. Our observed changes were primarily in the magnitude of end-point stiffness, but these were not sufficient to alter the direction of maximal stiffness. Furthermore, there were no substantial changes in the magnitude of end-point viscosity or inertia, suggesting that the primary change to arm impedance was a selective increase in stiffness to compensate for the destabilizing stiffness properties of the environment. We suggest that these modest changes provide an initial involuntary response to destabilizing environments prior to the larger changes that can be affected through voluntary interventions.

Movement behavior of high-heeled walking: how does the nervous system control the ankle joint during an unstable walking condition?

The human locomotor system is flexible and enables humans to move without falling even under less than optimal conditions. Walking with high-heeled shoes constitutes an unstable condition and here we ask how the nervous system controls the ankle joint in this situation? We investigated the movement behavior of high-heeled and barefooted walking in eleven female subjects. The movement variability was quantified by calculation of approximate entropy (ApEn) in the ankle joint angle and the standard deviation (SD) of the stride time intervals. Electromyography (EMG) of the soleus (SO) and tibialis anterior (TA) muscles and the soleus Hoffmann (H-) reflex were measured at 4.0 km/h on a motor driven treadmill to reveal the underlying motor strategies in each walking condition. The ApEn of the ankle joint angle was significantly higher (p<0.01) during high-heeled (0.38±0.08) than during barefooted walking (0.28±0.07). During high-heeled walking, coactivation between the SO and TA muscles increased towards heel strike and the H-reflex was significantly increased in terminal swing by 40% (p<0.01). These observations show that high-heeled walking is characterized by a more complex and less predictable pattern than barefooted walking. Increased coactivation about the ankle joint together with increased excitability of the SO H-reflex in terminal swing phase indicates that the motor strategy was changed during high-heeled walking. Although, the participants were young, healthy and accustomed to high-heeled walking the results demonstrate that that walking on high-heels needs to be controlled differently from barefooted walking. We suggest that the higher variability reflects an adjusted neural strategy of the nervous system to control the ankle joint during high-heeled walking.