Phase-dependent changes in local dynamic stability of human gait

Several methods derived from nonlinear time series analysis have been suggested to quantify stability in human gait kinematics. One of these methods is the definition of the maximum finite time Lyapunov exponent (λ) that quantifies how the system responds to infinitesimal perturbations. However, there are fundamental limitations to the conventional definition of λ for gait kinematics. First, exponential increase in initial perturbations cannot be assumed since real-life perturbations of gait kinematics are finite sized. Second, the transitions between single and double support phase within each stride cycle define two distinct dynamical regimes that may not be captured by a single λ. The present article presents a new method to quantify intra-stride changes λ(t) in local dynamical stability and employs the method to 3D lower extremity gait kinematics in 10 healthy adults walking on a treadmill at 3 different speeds. All participants showed an intra-stride change in λ(t) in the transition between single and double support phase. The intra-stride change reflected an both a increase and decrease in λ(t) at heel strike and toe off, respectively, with increased gait speed. Furthermore, a close relationship was found between the intra-stride change in standard deviation of foot velocity in the anterior-posterior direction and the intra-stride change of the initial perturbations. The present results indicate that local dynamical stability has gait phase-dependent changes that are not identified by conventional computation of a single λ.     

Influence of gait speed on the control of mediolateral dynamic stability during gait initiation

This study investigated the influence of gait speed on the control of mediolateral dynamic stability during gait initiation. Thirteen healthy young adults initiated gait at three self-selected speeds: Slow, Normal and Fast. The results indicated that the duration of anticipatory postural adjustments (APA) decreased from Slow to Fast, i.e. the time allocated to propel the centre of mass (COM) towards the stance-leg side was shortened. Likely as an attempt at compensation, the peak of the anticipatory centre of pressure (COP) shift increased. However, COP compensation was not fully efficient since the results indicated that the mediolateral COM shift towards the stance-leg side at swing foot-off decreased with gait speed. Consequently, the COM shift towards the swing-leg side at swing heel-contact increased from Slow to Fast, indicating that the mediolateral COM fall during step execution increased as gait speed rose. However, this increased COM fall was compensated by greater step width so that the margin of stability (the distance between the base-of-support boundary and the mediolateral component of the “extrapolated centre of mass”) at heel-contact remained unchanged across the speed conditions. Furthermore, a positive correlation between the mediolateral extrapolated COM position at heel-contact and step width was found, indicating that the greater the mediolateral COM fall, the greater the step width. Globally, these results suggest that mediolateral APA and step width are modulated with gait speed so as to maintain equivalent mediolateral dynamical stability at the time of swing heel-contact.     

Local dynamic stability in turning and straight-line gait

Successful community and household ambulation require the ability to navigate corners and maneuver around obstacles, posing unique challenges compared to straight-line walking. The challenges associated with turning may contribute to an increased incidence of falling and the occurrence of fall-related injuries. A measure of stability applied to turning gait may be able to quantify a system's response to naturally occurring disturbances associated with turning and identify subjects at greater risk of falling. An index of stability has been used previously to assess the rate of kinematic separation (local dynamic stability) during straight-line gait. The purpose of this study was to determine if local dynamic stability during constant speed turning is reduced compared to straight-line treadmill walking. Maximum finite-time Lyapunov exponents (λ) were used to estimate the local stability of able-bodied subjects’ (n=19) sagittal plane hip, knee, and ankle trajectories for turning compared to straight-line walking at two different walking speeds. Turning λ was greater than straight λ for the hip, right knee, and ankle (p<0.05). Turning λ for the left knee angle was similar to straight λ. There were no differences in λ between left and right limbs for the hip and ankle and also no differences between the inside and outside limbs during turning for all joints. These findings indicate able-bodied subjects’ hip, right knee, and ankle kinematics are less locally stable while turning than walking in a straight line and may be used as a comparative tool for determining the efficacy of therapeutic interventions for mobility-impaired populations.     

Cardiopulmonary baroreflex is reset during dynamic exercise.

The purpose of this study was to examine the hypothesis that the operating point of the cardiopulmonary baroreflex resets to the higher cardiac filling pressure of exercise associated with the increased cardiac filling volumes. Eight men (age 26 +/- 1 yr; height 180 +/- 3 cm; weight 86 +/- 6 kg; means +/- SE) participated in the present study. Lower body negative pressure (LBNP) was applied at 8 and 16 Torr to decrease central venous pressure (CVP) at rest and during steady-state leg cycling at 50% peak oxygen uptake (104 +/- 20 W). Subsequently, two discrete infusions of 25% human serum albumin solution were administered until CVP was increased by 1.8 +/- 0.6 and 2.4 +/- 0.4 mmHg at rest and 2.9 +/- 0.9 and 4.6 +/- 0.9 mmHg during exercise. During all protocols, heart rate, arterial blood pressure, and CVP were recorded continuously. At each stage of LBNP or albumin infusion, forearm blood flow and cardiac output were measured. During exercise, forearm vascular conductance increased from 7.5 +/- 0.5 to 8.7 +/- 0.6 U (P = 0.024) and total systemic vascular conductance from 7.2 +/- 0.2 to 13.5 +/- 0.9 l.min(-1).mmHg(-1) (P < 0.001). However, there was no significant difference in the responses of both forearm vascular conductance and total systemic vascular conductance to LBNP and the infusion of albumin between rest and exercise. These data indicate that the cardiopulmonary baroreflex had been reset during exercise to the new operating point associated with the exercise-induced change in cardiac filling volume.     

Peak dynamic force in human gait

Studies were made of the forces generated at heel stroke in human gait using both force plates having a high resonant frequencies (capable of picking up high frequency components in the contact force) as well as a force transducer inserted into the heel of the shoe of the subjects. The output traces were analyzed for the existence of high frequency impulsive loads during a normal walking cycle. The effect of the complicance of the foot and floor was studied with the force transducers. The results showed that during normal human gait the lower limb is subjected to a high frequency impulsive load at heel strike. The severity of this impulse varied with the individual, the velocity and angle with which the limb aproached the ground and the compliance of the two materials coming in contact at heel strike. The magnitude of this peak force varied from 0.5 to 1.25 times body weight and its frequency components from 10 to 75 Hz.     

Stumbling with optimal phase reset during gait can prevent a humanoid from falling

The human biped walking shows phase- dependent transient changes in gait trajectory in response to external brief force perturbations. Such responses, referred to as the stumbling reactions, are usually accompanied with phase reset of the walking rhythm. Our previous studies provided evidence, based on a human gait experiment and analyses of mathematical models of gait in the sagittal plane, that an appropriate amount of phase reset in response to a perturbation depended on the gait phase at the perturbation and could play an important role for preventing the walker from a fall, thus increasing gait stability. In this paper, we provide a further material that supports this evidence by a gait experiment on a biped humanoid. In the experiment, the impulsive force perturbations were applied using push-impacts by a pendulum-like hammer to the back of the robot during gait. The responses of the external perturbations were managed by resetting the gait phase with different delays or advancements. The results showed that appropriate amounts of phase resetting contributed to the avoidance of falling against the perturbation during the three-dimensional robot gait. A parallelism with human gait stumbling reactions was discussed.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

Differences between local and orbital dynamic stability during human walking

Currently there is no commonly accepted way to define, much less quantify, locomotor stability. In engineering, "orbital stability" is defined using Floquet multipliers that quantify how purely periodic systems respond to perturbations discretely from one cycle to the next. For aperiodic systems, "local stability" is defined by local divergence exponents that quantify how the system responds to very small perturbations continuously in real time. Triaxial trunk accelerations and lower extremity sagittal plane joint angles were recorded from ten young healthy subjects as they walked for 10 min over level ground and on a motorized treadmill at the same speed. Maximum Floquet multipliers (Max FM) were computed at each percent of the gait cycle (from 0% to 100%) for each time series to quantify the orbital stability of these movements. Analyses of variance comparing Max FM values between walking conditions and correlations between Max FM values and previously published local divergence exponent results were computed. All subjects exhibited orbitally stable walking kinematics (i.e., magnitudes of Max FM < 1.0), even though these same kinematics were previously found to be locally unstable. Variations in orbital stability across the gait cycle were generally small and exhibited no systematic patterns. Walking on the treadmill led to small, but statistically significant improvements in the orbital stability of mediolateral (p = 0.040) and vertical (p = 0.038) trunk accelerations and ankle joint kinematics (p = 0.002). However, these improvements were not exhibited by all subjects (p < or = 0.012 for subject x condition interaction effects). Correlations between Max FM values and previously published local divergence exponents were inconsistent and 11 of the 12 comparisons made were not statistically significant (r2 < or = 19.8%; p > or = 0.049). Thus, the variability inherent in human walking, which manifests itself as local instability, does not substantially adversely affect the orbital stability of walking. The results of this study will allow future efforts to gain a better understanding of where the boundaries lie between locally unstable movements that remain orbitally stable and those that lead to global instability (i.e., falling).     

Phase reset affects auditory-visual simultaneity judgment

We continuously receive the external information from multiple sensors simultaneously. The brain must judge a source event of these sensory informations and integrate them. It is thought that judging the simultaneity of such multisensory stimuli is an important cue when we discriminate whether the stimuli are derived from one event or not. Although previous studies have investigated the correspondence between an auditory-visual (AV) simultaneity perceptions and the neural responses, there are still few studies of this. Electrophysiological studies have reported that ongoing oscillations in human cortex affect perception. Especially, the phase resetting of ongoing oscillations has been examined as it plays an important role in multisensory integration. The aim of this study was to investigate the relationship of phase resetting for the judgment of AV simultaneity judgement tasks. The subjects were successively presented with auditory and visual stimuli with intervals that were controlled as SOA_50% and they were asked to report whether they perceived them simultaneously or not. We investigated the effects of the phase of ongoing oscillations on simultaneity judgments with AV stimuli with SOAs in which the detection rate of asynchrony was 50 %. It was found that phase resetting at the beta frequency band in the brain area that related to the modality of the following stimulus occurred after preceding stimulus onset only when the subjects perceived AV stimuli as simultaneous. This result suggested that beta phase resetting occurred in areas that are related to the subsequent stimulus, supporting perception multisensory stimuli as simultaneous.     

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

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

Postural stability during gait for adults with hereditary spastic paraparesis

Individuals with hereditary spastic paraparesis (HSP) are often impaired in their ability to control posture as a result of the neurological and musculoskeletal implications of their condition. This research aimed to assess postural stability during gait in a group of adults with HSP. Ten individuals with HSP and 10 healthy controls underwent computerized gait analysis while walking barefoot along a 10-m track. Two biomechanics methods were used to assess stability: the center of pressure and center of mass separation (COP-COM) method, and the extrapolated center of mass (XCOM) method. Spatiotemporal and kinematic variables were also investigated. The XCOM method identified deficits in mediolateral stability for the HSP group at both heel strike and mid-stance. The group with HSP also had slower walking velocity, lower cadence, more time spent in double stance, larger step widths, and greater lateral trunk flexion than the control group. These results suggest that individuals with HSP adjust characteristics of their gait to minimize the instability arising from their impairments but have residual deficits in mediolateral stability. This may result in an increased risk of falls, particularly in the sideways direction.     

A computational model for dynamic analysis of the human gait

Biomechanical models are important tools in the study of human motion. This work proposes a computational model to analyse the dynamics of lower limb motion using a kinematic chain to represent the body segments and rotational joints linked by viscoelastic elements. The model uses anthropometric parameters, ground reaction forces and joint Cardan angles from subjects to analyse lower limb motion during the gait. The model allows evaluating these data in each body plane. Six healthy subjects walked on a treadmill to record the kinematic and kinetic data. In addition, anthropometric parameters were recorded to construct the model. The viscoelastic parameter values were fitted for the model joints (hip, knee and ankle). The proposed model demonstrated that manipulating the viscoelastic parameters between the body segments could fit the amplitudes and frequencies of motion. The data collected in this work have viscoelastic parameter values that follow a normal distribution, indicating that these values are directly related to the gait pattern. To validate the model, we used the values of the joint angles to perform a comparison between the model results and previously published data. The model results show a same pattern and range of values found in the literature for the human gait motion.     

Control of head stability during gait initiation in young and older women

Transition tasks between static and dynamic situations may challenge head stabilization and balance in older individuals. The study was designed to investigate differences between young and older women in the upper body motion during the voluntary task of gait initiation. Seven young (25 ± 2.3 years) and seven older healthy women (78 ± 3.4 years) were required to stand on a force platform and initiate walking at their self-selected preferred speed. Angles of head, neck and trunk were measured by motion analysis in the sagittal plane and a cross-correlation analysis was performed on segments pairs. Variability of head and neck angular displacements, as indicated by average standard deviation, was significantly greater in the older than in the young participants. The young women maintained dynamic stability of the upper body, as forward flexion of the trunk was consistently counteracted by coordinated head–neck extension. Differently, movement patterns employed by the older women also included a rigid motion of all upper body segments leaning forward as a single unit. These results demonstrated that older women perform the transition from standing to walking with greater variability in the patterns of upper body motion compared to young women.     

Cross-correlation as a method for comparing dynamic electromyography signals during gait

Current clinical interpretation of dynamic electromyography (EMG) data is usually based on qualitative assessments of muscle timing. Cross-correlation may provide a method for objectively comparing the timing and shape of EMG signals. This study used cross-correlation to compare EMG signals from different walking trials, different test sessions, and different individuals in able-bodied adults. Cross-correlation results (R-values) for different walking trials within a single test session were high, averaging > or = 0.90 for all muscles tested (R = 1.0 indicates exact agreement). Cross-correlation values were also high among trials from different test sessions conducted by the same and different examiners (average R > or = 0.78 for all muscles). R-values were much more variable when comparing different subjects (average 0.40-0.81, range 0.00-0.91). R-values were lower for the medial hamstrings and rectus femoris compared with the other muscles tested. These results suggest that cross-correlation may be useful for evaluating changes in an individual patient's muscle activation patterns, such as before and after surgery, but not for comparing EMG patterns among different individuals, such as between patients and normative data. This is especially true for biarticular muscles such as the hamstrings and rectus femoris, which may have variable activation patterns and/or increased sensitivity to electrode placement. Cross-correlation may also be useful for identifying appropriate muscles for transfer, identifying "outlier" trials within a test session, and selecting representative EMG curves for a given patient. The advantages of cross-correlation are that it considers shape of the EMG signal in addition to timing and that the assessments it provides are objective, rather than subjective.     

Quantification of the dynamic properties of EMG patterns during gait.

A technique for analyzing and comparing the dynamic properties of electromyographic (EMG) patterns collected during gait is presented. A gait metric is computed, consisting of both magnitude (amplitude) and phase (timing) components. For the magnitude component, the processed EMG pattern is compared to a normative EMG pattern obtained under similar walking conditions, where the metric is incremented if the muscle is firing during expected active regions or is silent during expected inactive regions. The magnitude metric is penalized when the EMG is silent during phases of expected activity or when the EMG is active in regions of expected inactivity. The phase component of the metric computes the percentage of the gait cycle when the muscle is firing appropriately, that is, active in expected active regions and silent in expected inactive regions. The magnitude and phase components of the metric are normalized and combined to yield the EMG pattern that demonstrates the closest characteristics compared to normative gait data collected under similar walking conditions. Using experimental data, the proposed gait metric was tested and accurately reflects the observed changes in the EMG patterns. Clinical uses for the gait metric are discussed in relation to gait therapies, such as determining optimal gait training conditions in individuals following stroke and spinal cord injury.     

Ground reaction forces during termination of human gait.

This is the first published report of the ground reaction forces during gait termination. Two mechanisms appear to be used to stop walking: increased braking forces and decreased push-off force. There appears to be a short interval of time during the gait cycle in which a decision to take an additional step is to be made.     

Multi-sensor assessment of dynamic balance during gait in patients with subacute stroke

The capacity to maintain upright balance by minimising upper body oscillations during walking, also referred to as gait stability, has been associated with a decreased risk of fall. Although it is well known that fall is a common complication after stroke, no study considered the role of both trunk and head when assessing gait stability in this population. The primary aim of this study was to propose a multi-sensor protocol to quantify gait stability in patients with subacute stroke using gait quality indices derived from pelvis, sternum, and head accelerations. Second, the association of these indices with the level of walking ability, with traditional clinical scale scores, and with fall events occurring within the six months after patients’ dismissal was investigated. The accelerations corresponding to the three abovementioned body levels were measured using inertial sensors during a 10-Meter Walk Test performed by 45 inpatients and 25 control healthy subjects. A set of indices related to gait stability were estimated and clinical performance scales were administered to each patient. The amplitude of the accelerations, the way it is attenuated/amplified from lower to upper body levels, and the gait symmetry provide valuable information about subject-specific motor strategies, discriminate between different levels of walking ability, and correlate with clinical scales. In conclusion, the proposed multi-sensor protocol could represent a useful tool to quantify gait stability, support clinicians in the identification of patients potentially exposed to a high risk of falling, and assess the effectiveness of rehabilitation protocols in the clinical routine.     

Detecting and quantifying global instability during a dynamic task using kinetic and kinematic gait parameters

ObjectivesInstability during gait can be identified in many different ways. Recent studies have suggested utilizing spatiotemporal parameters to detect instability during gait. Detecting instability using kinetic and kinematic gait parameters has not yet been examined fully. In addition, these studies have not yet identified measures that are capable of assessing the magnitude of instability. The objective of the present study was to identify kinetic and kinematic gait parameters that can best identify instability and quantify its magnitude.MethodsTen healthy men underwent successive gait analysis testing under three controlled settings: (1) Stage 0 instability (control setting), (2) Stage 1 instability and (3) Stage 2 instability. The levels of instability were precisely applied with the use of a controlled perturbation device (AposTherapy System). Differences between all stages and between stages were identified using Friedman and Wilcoxon tests.ResultsStride-to-stride variability (STSV) in kinetic and kinematic measures increased significantly between stages 0 and 1 or between stages 0 and 2 for almost all parameters (all P<0.05). A significant increase between stage 0 and both stages 1 and 2 was found for knee flexion moment, knee varus moment, knee flexion angle and hip adduction angle. The increase between stages 1 and 2 was variable. Only the knee varus moment parameter showed a significant increase in STSV between stages 1 and 2 (P=0.026).ConclusionsAlmost all kinetic and kinematic gait parameters are sensitive to changes in global instability in a dynamic task. The most sensitive are parameters measured at the knee. Of these, STSV in knee varus moment can be used to quantify the magnitude of dynamic instability.     

A superellipsoid-plane model for simulating foot-ground contact during human gait

Musculoskeletal models and forward dynamics simulations of human movement often include foot–ground interactions, with the foot–ground contact forces often determined using a constitutive model that depends on material properties and contact kinematics. When using soft constraints to model the foot–ground interactions, the kinematics of the minimum distance between the foot and planar ground needs to be computed. Due to their geometric simplicity, a considerable number of studies have used point–plane elements to represent these interacting bodies, but few studies have provided comparisons between point contact elements and other geometrically based analytical solutions. The objective of this work was to develop a more general-purpose superellipsoid–plane contact model that can be used to determine the three-dimensional foot–ground contact forces. As an example application, the model was used in a forward dynamics simulation of human walking. Simulation results and execution times were compared with a point-like viscoelastic contact model. Both models produced realistic ground reaction forces and kinematics with similar computational efficiency. However, solving the equations of motion with the surface contact model was found to be more efficient (~18% faster), and on average numerically ~37% less stiff. The superellipsoid–plane elements are also more versatile than point-like elements in that they allow for volumetric contact during three-dimensional motions (e.g. rotating, rolling, and sliding). In addition, the superellipsoid–plane element is geometrically accurate and easily integrated within multibody simulation code. These advantages make the use of superellipsoid–plane contact models in musculoskeletal simulations an appealing alternative to point-like elements.     

Regional neuromuscular regulation within human rectus femoris muscle during gait

The spatial distribution pattern of neuromuscular activation within the human rectus femoris (RF) muscle was investigated during gait by multi-channel surface electromyography (surface EMG). Eleven healthy men walked on a treadmill with three gait speeds (4, 5, and 6 km/h) and gradients (0°, 12.5°, and 25°). The spatial distribution of surface EMG was tested by central locus activation (CLA), which is calculated from 2-D multi-channel surface EMG with 46 surface electrodes. For all conditions, CLA was around the middle regions during the swing-to-stance transition and moved in a proximal direction during the stance phase and stance-to-swing transition (p<0.05). CLA during the stance-to-swing transition and early swing phase significantly moved to proximal site with increasing gait speed (p<0.05). During the early stance and swing phases, with increasing grade, CLA significantly moved distally (p<0.05). These results suggest that the RF muscle is regionally activated during a gait cycle and is non-uniformly regulated longitudinally.