Principles underlying the organization of movement initiation from quiet stance

Color-induced cognitive conflicts affect muscle activity prior to gait initiation in the Go/No-go task

Introduction

In traffic rule, green/blue means go, and red means stop. It has been shown that this prior knowledge about traffic signal colors can affect reaction times (RTs). For example, RTs are longer when responding to a red "Go" signal and withholding the response to a blue "No-go" signal (Red Go/Blue No-go task) than when responding to a blue "Go" signal and withholding the response to a red "No-go" signal (Blue Go/Red No-go task), when responses are provided by button press. However, it remains unknown whether this holds in different actions. The aim of this study was to investigate the effect of prior knowledge of color on gait initiation in a Go/No-go task.

Methods

Seventeen participants performed Green Go/Red No-go and Red Go/Green No-go tasks, in which they stepped forward from a force plate in response to a green or red signal and withhold the response to red or green signal, respectively. We recorded the center of pressure (COP) and electromyogram (EMG) from the bilateral tibialis anterior muscles during gait initiation.

Results

The onset of COP movement and toe-off time as well as COP displacements did not differ between the Go/No-go tasks. The EMG onset for the stance leg was delayed in the Red Go/Green No-go than Green Go/Red No-go task.

Discussion

These findings suggest that the conflict between prior knowledge of color related to traffic rule and the meaning of the stimulus color affects muscle activity but not COP characteristics during gait initiation, highlighting two distinct motor control mechanisms, where the initial phase is influenced by cognitive load while the subsequent phase remains unaffected. This dissociation suggests that the later phase of gait initiation relies on robust spinal loops and central pattern generators, which are less influenced by cognitive factors such as prior knowledge.

"Posture first": Interaction between posture and locomotion in people with low back pain during unexpectedly cued modification of gait initiation motor command

The ability to adapt anticipatory postural adjustments (APAs) in response to perturbations during single-joint movements is altered in people with chronic low back pain (LBP), but a comprehensive analysis during functional motor tasks is still missing. This study aimed to compare APAs and stepping characteristics during gait initiation between people with LBP and healthy controls, both in normal (without cue occurrence) condition and when an unexpected visual cue required to switch the stepping limb. Fourteen individuals with LPB and 10 healthy controls performed gait initiation in normal and switch conditions. The postural responses were evaluated through the analysis of center of pressure, propulsive ground reaction forces, trunk and whole-body kinematics, and activation onsets of leg and back muscles. During normal gait initiation, participants with LBP exhibited similar APAs and stepping characteristics to healthy controls. In the switch condition, individuals with LBP were characterized by greater mediolateral postural stability but decreased forward body motion and propulsion before stepping. The thorax motion was associated with forward propulsion parameters in both task conditions in people with LBP but not healthy controls. No between-group differences were found in muscle activation onsets. The results suggest that postural stability is prioritized over forward locomotion in individuals with LBP. Furthermore, the condition-invariant coupling between thorax and whole-body forward propulsion in LBP suggests an adaptation in the functional use of the thorax within the postural strategy, even in poor balance conditions.

Theoretical discrimination index of postural instability in amyotrophic lateral sclerosis

To assess the usefulness of a theoretical postural instability discrimination index (PIth) in amyotrophic lateral sclerosis (ALS). Prospective regression analyzes were performed to identify the biomechanical determinants of postural instability unrelated to lower limb motor deficits from gait initiation factors. PIth was constructed using a logit function of biomechanical determinants. Discriminatory performance and performance differences were tested. Backward displacement of the pression center (APAamplitude) and active vertical braking of the mass center (Braking-index) were the biomechanical determinants of postural instability. PIth = − 0.13 × APAamplitude − 0.12 × Braking-index + 5.67, (P < 0.0001, RSquare = 0.6119). OR (APAamplitude) and OR (Braking-index) were 0.878 and 0.887, respectively, i.e., for a decrease of 10 mm in APAamplitude or 10% in Braking-index, the postural instability risk was 11.391 or 11.274 times higher, respectively. PIth had the highest discriminatory performance (AUC 0.953) with a decision threshold value $$\ge$$ ≥ 0.587, a sensitivity of 90.91%, and a specificity of 83.87%, significantly increasing the sensitivity by 11.11%. PIth, as objective clinical integrator of gait initiation biomechanical processes significantly involved in dynamic postural control, was a reliable and performing discrimination index of postural instability with a significant increased sensitivity, and may be useful for a personalized approach to postural instability in ALS.

The mechanisms and mechanical energy of human gait initiation from the lower-limb joint level perspective

This study aims to improve our understanding of gait initiation mechanisms and the lower-limb joint mechanical energy contributions. Healthy subjects were instructed to initiate gait on an instrumented track to reach three self-selected target velocities: slow, normal and fast. Lower-limb joint kinematics and kinetics of the first five strides were analyzed. The results show that the initial lateral weight shift is achieved by hip abduction torque on the lifting leg (leading limb). Before the take-off of the leading limb, the forward body movement is initiated by decreasing ankle plantarflexion torque, which results in an inverted pendulum-like passive forward fall. The hip flexion/extension joint has the greatest positive mechanical energy output in the first stride of the leading limb, while the ankle joint contributes the most positive mechanical energy in the first stride of the trailing limb (stance leg). Our results indicate a strong correlation between control of the frontal plane and the sagittal plane joints during gait initiation. The identified mechanisms and the related data can be used as a guideline for improving gait initiation with wearable robots such as exoskeletons and prostheses.

Kinematic effects of different gait speeds during gait initiation movement

[Purpose] We investigated the influence of gait speed on the movement strategy during gait initiation. [Participants and Methods] This study included 21 young healthy individuals (11 males and 10 females; mean age, 21.7 ± 0.5 years; mean height, 166.1 ± 9.8 cm; and mean weight, 57.3 ± 11.2 kg). A three-dimensional motion analyzer and strain gauge force platform were used in this study. The measurement task consisted of gait initiation from the quiet stance; the two measurement conditions were normal gait and the highest speed. The analysis interval was from the start of the center of pressure migration to the heel contact at the first step of the swing limb. The center of gravity, center of pressure, joint movements, step length, and step time during the anticipatory postural control (from the start of center of pressure migration to swing leg-heel off) and swing (swing leg-heel off to swing leg-heel contact) phases were analyzed. [Results] Significant differences were observed in the center of gravity, center of pressure, hip flexion, abduction movement, stance-limb ankle dorsiflexion movement during the anticipatory postural control phase, and step time during the anticipatory postural control and swing phases. The stance-limb ankle plantar flexion movement and step length did not differ significantly in the swing phase. [Conclusion] When the gait speed increases, fluctuations in the joint movements increase as the center of pressure displacement increases, thus requiring complex control.

Anticipatory postural adjustments in forward and backward single stepping: Task variability and effects of footwear

A single step is usually preceded by the so-named anticipatory postural adjustments (APAs). These are normally described through the observation of the trajectory of the center of pressure (CoP). Even though, external factors such as stepping direction and footwear are known to modify APAs, quantitative investigations regarding their relevant effects are understudied in the literature. Therefore, this study aims at characterizing APAs patterns prior to forward and backward stepping when performed either in barefoot or shod condition and explores their variability. Twenty-eight young healthy volunteers participated in the study. CoP trajectories were recorded using a force plate and relevant spatio-temporal parameters extracted (i.e. duration, amplitude, and mean speed). Results showed distinct effects of both direction of the step and footwear on APAs: the first mainly induces variations of APAs along the anteroposterior direction, whereas the latter in the mediolateral direction. In addition, variability indices exhibited lower values for the APAs along the mediolateral axis which was affected by neither footwear nor direction of the step. This study extends previous literature by revealing significant direction X footwear interactions on APAs. Furthermore, regardless of these factors, the medio-lateral strategy is still well preserved, highlighting the prioritization of balance control over motor performance. In conclusion, both direction and footwear have a major effect on postural preparation therefore both factors should be included when evaluating APAs in real-life condition.

Gait initiation and lateral wedge insole for individuals with early knee osteoarthritis

BACKGROUND

People with knee osteoarthritis are at higher risk of postural deficits and locomotor adaptations which could be manifested in transient tasks like gait initiation. To better understand the effect of early knee osteoarthritis on gait initiation, we measured the gait initiation in two groups of early knee osteoarthritis and healthy age-matched controls and assessed the effect of lateral wedge insole in knee osteoarthritis group.

METHODS

Twenty-one individuals with early knee osteoarthritis, mean (SD) age = 53.1 (7.4, years), and 19 age- and sex-matched asymptomatic healthy individuals, mean (SD) age = 47.5 (11.2, years), participated. Centre of pressure trajectories were used to quantify the two phases of gait initiation in barefoot condition: anticipatory postural adjustment, and locomotor phase. The immediate and four-week effect of lateral wedge insole and shod condition were also compared for individuals in knee osteoarthritis group.

FINDINGS

Longer duration of anticipatory postural adjustment phase (P = 0.046), locomotor phase (P = 0.049), and total duration of gait initiation (P = 0.035) with lower length and velocity of the center of pressure trajectories in anticipatory postural adjustment phase (P = 0.002, and 0.008, respectively) were observed in knee osteoarthritis group compared to controls. Lateral wedge insole could increase the length and velocity of the centre of pressure in locomotor phase compared to barefoot condition (P = 0.001, and 0.002, respectively).

INTERPRETATION

Our study outlined that people with early knee osteoarthritis adopt different gait initiation strategies mainly in postural preparation phase. The potential effect of lateral wedge insole was shown with better performance of the locomotor phase.

CLINICAL TRIAL REGISTRY NUMBER

IRCT2016060628310N1.

Differential Effects of Acute and Multiple Concussions on Gait Initiation Performance

OBJECTIVE

To identify the effect of acute and multiple concussions on gait initiation performance.

DESIGN

Cohort study.

SETTING

University research center.

PARTICIPANTS

A population-based sample of participants (N=45) divided into 3 groups: No Prior Concussion, ≥3 Prior Concussions, and Acute Concussion. The Acute Concussion participants were assessed within 24 hours of their concussion. Participants were matched based on (1) sport, (2) position, and (3) anthropometric measures.

INTERVENTIONS

Participants were tested on a single occasion and performed 5 trials of gait initiation on 4 force plates. The No Prior Concussion and ≥3 Prior Concussions groups were tested out of their primary athletic season.

MAIN OUTCOME MEASURES

The dependent variables were the posterior and lateral displacement and velocity of the center of pressure (COP) during the anticipatory postural adjustment phase and initial step kinematics (step length and step velocity). Comparisons between groups used a 1-way analysis of variance with Tukey post hoc when significant effects were identified and effect sizes were calculated.

RESULTS

There were significant effects for group for all 6 outcome measures with large effect sizes. Post hoc tests identified differences between Acute Concussion and No Prior Concussion groups for all measures. The ≥3 Prior Concussions group and No Prior Concussion group were different for COP posterior displacement (4.91±1.09 and 4.91±1.09cm, respectively, P=.032, d=0.91) and velocity (0.18±0.06 and 0.27±0.08m/s, respectively, P=.002, d=1.27).

CONCLUSION

There was continuum of performance identified whereby the Acute Concussion participants performed worse and the No Prior Concussion participants performed the best. The ≥3 Prior Concussions generally fell between these 2 groups, but only statistical significance on COP posterior displacement and velocity. These results suggest there may be subtle neurophysiological deficits present in collegiate student-athletes with ≥3 Prior Concussions group, and further investigation over the lifespan is warranted.

Accuracy and Reliability of Onset Detection Algorithms in Gait Initiation for Healthy Controls and Participants With Parkinson's Disease

Accurate and reliable detection of the onset of gait initiation is essential for the correct assessment of gait. Thus, this study was aimed at evaluation of the reliability and accuracy of 3 different center of pressure-based gait onset detection algorithms: A displacement baseline-based algorithm (method 1), a velocity baseline-based algorithm (method 2), and a velocity extrema-based algorithm (method 3). The center of pressure signal was obtained during 10 gait initiation trials from 16 healthy participants and 3 participants with Parkinson's disease. Intrasession and absolute reliability of each algorithm was assessed using the intraclass correlation coefficient and the coefficient of variation of center of pressure displacement during the postural phase of gait initiation. The accuracy was evaluated using the time error of the detected onset by each algorithm relative to that of visual inspection. The authors' results revealed that although all 3 algorithms had high to very high intrasession reliabilities in both healthy subjects and subjects with Parkinson's disease, methods 2 and 3 showed significantly better absolute reliability than method 1 in healthy controls (P = .001). Furthermore, method 2 outperformed the other 2 algorithms in both healthy subjects and subjects with Parkinson's disease with an overall accuracy of 0.80. Based on these results, the authors recommend using method 2 for accurate and reliable gait onset detection.

Sagittal and Frontal Plane Gait Initiation Kinetics in Healthy, Young Subjects

The study purposes were to record the lower extremity sagittal and frontal joint moments and powers during gait initiation (GI); evaluate GI support moments in both planes; and analyze planar energy patterns in a group of 15 healthy, young adults. 3D motion and ground reaction force data were used to calculate support moments (SM) and joint moments and powers as well as center of mass (COM) kinematics. STEP1 had no visible SM. It appeared in STEP2 and, by STEP3, resembled that seen in steady-state gait. Joint moments demonstrated a similar development towards typical patterns over the three steps. Correlations of moment data between planes indicate that the frontal plane component of the SM acts to keep the COM centered. It is suggested that Winter’s 1980 SM definition be extended to include both a support (sagittal) component and a centering (frontal) component. Energy was calculated for individual bursts of joint powers in both planes and each step had characteristic patterns in each plane, with patterns resembling steady-state gait appearing in the third step. Test-retest reliability (ICC range: 0.796 – 0.945) was high with CV values in the sagittal plane (36.6 – 37.5%) being less variable than in the frontal plane (39.0 – 82.0%). COM kinematics revealed that acceleration peaked in STEP2 (ICC range: 0.950 – 0.980, CV < 20.0%). Data supported hypotheses regarding the dominance of the frontal plane power in STEP1, with substantial power coming from hip flexors. As well, powers in the sagittal plane were generally of larger magnitude than in the frontal plane.

Gait initiation and partial body weight unloading for functional improvement in post-stroke individuals

BACKGROUND

To better understand gait initiation in individuals with stroke and suggest possible training strategies, we compared the gait initiation of individuals with stroke and age-matched controls, and we examined the influence of different amounts of body weight support (BWS) during the execution of gait initiation in individuals with stroke.

MATERIALS AND METHODS

Twelve individuals with stroke and 12 age-matched controls initiated gait after a verbal command at a self-selected and comfortable speed, and individuals with stroke also initiated gait wearing a harness with 0%, 15%, and 30% of BWS. Length and velocity of the first step, distance between heels, and weight bearing in both lower limbs in the initial position were calculated. We also assessed the displacement and average velocity of the center of pressure (CoP) in the medial-lateral (ML) and anterior-posterior (AP) directions in 3 distinct sections during gait initiation, which correspond to the CoP position toward the swing limb, stance limb and progression line, respectively.

RESULTS

Individuals with stroke presented shorter and slower step, shorter and slower CoP-ML and CoP-AP toward swing limb and Cop-ML towards stance limb, and longer and faster CoP-AP toward stance limb compared to their peers. The BWS lead individuals with stroke to decrease step length and to increase CoP-ML displacement and average velocity toward stance limb.

CONCLUSION

Individuals with stroke present impairments in executing gait initiation mainly during the preparation period and the employment of an overground BWS system promotes a better performance. These results suggest that BWS is a functional strategy that enables individuals with stroke to modulate gait initiation and it could be adopted for gait intervention.

The role of the ipsilesional side in the rehabilitation of post-stroke subjects

Most stroke lesions occur in the middle cerebral artery territory, presenting a high probability of damage of pathways with predominant ipsilesional disposition, mainly related to postural control. Despite the high probability of bilateral postural control dysfunction based on neuroanatomical fundaments, both research and clinical rehabilitation involving stroke subjects have been focused on contralesional side (also named affected side) impairments, while ipsilesional side (also named non-affected side) impairments have been attributed to an adaptive strategy. This paper aims to present a critical understanding about the state-of-the-art that sustains the hypothesis that stroke subjects with middle cerebral artery territory lesion at the subcortical level show an atypical behaviour in the ipsilateral side associated with the lesion itself and the possible implications.

A little trouble getting started: Initial slowness in Parkinson's disease step negotiation

Bradykinesia is a prominent problem for persons with Parkinson's disease (PD) and has been studied extensively with upper extremity tasks; however there is a lack of research examining bradykinesia in targeted lower extremity tasks related to mobility. Navigating steps and curbs are challenging tasks for older adults and neurologically impaired and thus utilizing these behaviors provides ecological validity to the study of bradykinesia. Herein we assess differences in step negotiation performance between individuals with PD and aged matched older adults. Three-dimensional kinematics and ground reaction forces were collected while 12 participants with PD and 12 older adults performed a single step up onto a platform. Persons with PD spent a significantly greater amount of time in the heel lift phase (P=0.0003, d=1.80). Peak vertical foot velocity of the lead foot was also significantly less in PD (P=0.02, d=1.05). Lastly, persons with PD displayed reduced sagittal hip and knee range of motion during the trail step (P=0.01, d=1.20 and P=0.02, d=1.05, respectively). Parkinson's participants exhibited slight decrement in step negotiation execution. Increased step time and decreased foot velocity and range of motion were attributes associated with Parkinson's step negotiation performance. Contrary to our hypothesis, in many comparisons, persons with PD during their best medicated state performed comparable to older adults, indicative of successful pharmacotherapy. Rehabilitation efforts can seek to improve performance in motor control tasks such as step negotiation, by restoring the relationship between perceived and actual motor output and enhancing muscle coordination and output as well as ranges of motion.

Electromyographic Pattern during Gait Initiation Differentiates Yoga Practitioners among Physically Active Older Subjects

During gait initiation, postural adjustments are needed to deal with balance and movement. With aging, gait initiation changes and reflects functional degradation of frailty individuals. However, physical activities have demonstrated beneficial effects of daily motor tasks. The aim of our study was to compare center of pressure (COP) displacement and ankle muscle co-activation during gait initiation in two physically active groups: a group of walkers (n = 12; mean age ± SD 72.6 ± 3.2 years) and a yoga group (n = 11; 71.5 ± 3.8 years). COP trajectory and electromyography of leg muscles were recorded simultaneously during five successive trials of gait initiation. Our main finding was that yoga practitioners had slower COP displacements (p < 0.01) and lower leg muscles % of coactivation (p < 0.01) in comparison with walkers. These parameters which characterized gait initiation control were correlated (r = 0.76; p < 0.01). Our results emphasize that lengthy ankle muscle co-activation and COP path in gait initiation differentiate yoga practitioners among physically active subjects.

Gait initiation and termination strategies in patients with Prader-Willi syndrome

BACKGROUND

Gait Initiation (GI) is a functional task representing one of the first voluntary destabilizing behaviours observed in the development of a locomotor pattern as the whole body centre of mass transitions from a large to a small base of support. Conversely, Gait Termination (GT) consists in the transition from walking to standing which, in everyday life, is a very common movement. Compared to normal walking, it requires higher control of postural stability. For a safe GT, the forward movement of the body has to be slowed down to achieve a stable upright position. Stability requirements have to be fulfilled for safe GT. In individuals with Prader-Willi syndrome (PWS), excessive body weight negatively affects the movement, such as walking and posture, but there are no experimental studies about GI and GT in these individuals. The aim of this study was to quantitatively characterise the strategy of patients with PWS during GI and GT using parameters obtained by the Center of Pressure (CoP) track.

METHODS

Twelve patients with PWS, 20 obese (OG) and 19 healthy individuals (HG) were tested using a force platform during the GI and GT tasks. CoP plots were divided into different phases, and duration, length and velocity of the CoP trace in these phases were calculated and compared for each task.

RESULTS

As for GI, the results showed a significant reduction of the task duration and lower velocity and CoP length parameters in PWS, compared to OG and HG. In PWS, those parameters were reduced to a higher degree with respect to the OG. During GT, longer durations, similar to OG, were observed in PWS than HG. Velocity is reduced when compared to OG and HG, especially in medio-lateral direction and in the terminal part of GT.

CONCLUSIONS

From these data, GI appears to be a demanding task in most of its sub-phases for PWS individuals, while GT seems to require caution only towards the end of the task. Breaking the cycle of gait into the phases of GI and GT and implementing specific exercises focusing on weight transfer and foot clearance during the transition phase from the steady condition to gait will possibly improve the effectiveness of rehabilitation and fall and injury prevention.

Effects of Changing Body Weight Distribution on Mediolateral Stability Control during Gait Initiation

During gait initiation, anticipatory postural adjustments (APA) precede the execution of the first step. It is generally acknowledged that these APA contribute to forward progression but also serve to stabilize the whole body in the mediolateral direction during step execution. Although previous studies have shown that changes in the distribution of body weight between both legs influence motor performance during gait initiation, it is not known whether and how such changes affect a person's postural stability during this task. The aim of this study was to investigate the effects of changing initial body weight distribution between legs on mediolateral postural stability during gait initiation. Changes in body weight distribution were induced under experimental conditions by modifying the frontal plane distribution of an external load located at the participants' waists. Fifteen healthy adults performed a gait initiation series at a similar speed under three conditions: with the overload evenly distributed over both legs; with the overload strictly distributed over the swing-limb side; and with the overload strictly distributed over the stance-leg side. Our results showed that the mediolateral location of center-of-mass (CoM) during the initial upright posture differed between the experimental conditions, indicating modifications in the initial distribution of body weight between the legs according to the load distribution. While the parameters related to the forward progression remained unchanged, the alterations in body weight distribution elicited adaptive changes in the amplitude of APA in the mediolateral direction (i.e., maximal mediolateral shift of the center of pressure (CoP)), without variation in their duration. Specifically, it was observed that the amplitude of APA was modulated in such a way that mediolateral dynamic stability at swing foot-contact, quantified by the margin of stability (i.e., the distance between the base of support boundary and the extrapolated CoM position), did not vary between the conditions. These findings suggest that APA seem to be scaled as a function of the initial body weight distribution between both legs so as to maintain optimal conditions of stability during gait initiation.

A modelling approach to the dynamics of gait initiation

Gait initiation is an integral and complex part of human locomotion. In this paper, we present a novel compliant-leg model-based approach to understanding the key phases of initiation, the nature of the effective forces involved in initiation, and the importance of the anticipatory postural adjustments (APAs). The results demonstrate that in the presence of APAs, we observe a change in the characteristic of forcing required for initiation, and the energetic cost of gait initiation is also reduced by approximately 58%. APAs also result in biologically relevant leg landing angles and trajectories of motion. Furthermore, we find that a sublinear functional relationship with the velocity error from steady state predicts the required force, consistent with an open loop control law basis for gait initiation.

Control of vertical posture while elevating one foot to avoid a real or virtual obstacle

The purpose of this study is to investigate the control of vertical posture during obstacle avoidance in a real versus a virtual reality (VR) environment. Ten healthy participants stood upright and lifted one leg to avoid colliding with a real obstacle sliding on the floor toward a participant and with its virtual image. Virtual obstacles were delivered by a head mounted display (HMD) or a 3D projector. The acceleration of the foot, center of pressure, and electrical activity of the leg and trunk muscles were measured and analyzed during the time intervals typical for early postural adjustments (EPAs), anticipatory postural adjustments (APAs), and compensatory postural adjustments (CPAs). The results showed that the peak acceleration of foot elevation in the HMD condition decreased significantly when compared with that of the real and 3D projector conditions. Reduced activity of the leg and trunk muscles was seen when dealing with virtual obstacles (HMD and 3D projector) as compared with that seen when dealing with real obstacles. These effects were more pronounced during APAs and CPAs. The onsets of muscle activities in the supporting limb were seen during EPAs and APAs. The observed modulation of muscle activity and altered patterns of movement seen while avoiding a virtual obstacle should be considered when designing virtual rehabilitation protocols.

Elderly Fallers Enhance Dynamic Stability Through Anticipatory Postural Adjustments during a Choice Stepping Reaction Time

In the case of disequilibrium, the capacity to step quickly is critical to avoid falling in elderly. This capacity can be simply assessed through the choice stepping reaction time test (CSRT), where elderly fallers (F) take longer to step than elderly non-fallers (NF). However, the reasons why elderly F elongate their stepping time remain unclear. The purpose of this study is to assess the characteristics of anticipated postural adjustments (APA) that elderly F develop in a stepping context and their consequences on the dynamic stability. Forty-four community-dwelling elderly subjects (20 F and 24 NF) performed a CSRT where kinematics and ground reaction forces were collected. Variables were analyzed using two-way repeated measures ANOVAs. Results for F compared to NF showed that stepping time is elongated, due to a longer APA phase. During APA, they seem to use two distinct balance strategies, depending on the axis: in the anteroposterior direction, we measured a smaller backward movement and slower peak velocity of the center of pressure (CoP); in the mediolateral direction, the CoP movement was similar in amplitude and peak velocity between groups but lasted longer. The biomechanical consequence of both strategies was an increased margin of stability (MoS) at foot-off, in the respective direction. By elongating their APA, elderly F use a safer balance strategy that prioritizes dynamic stability conditions instead of the objective of the task. Such a choice in balance strategy probably comes from muscular limitations and/or a higher fear of falling and paradoxically indicates an increased risk of fall.

Anticipatory Postural Control of Stability during Gait Initiation Over Obstacles of Different Height and Distance Made Under Reaction-Time and Self-Initiated Instructions

Despite the abundant literature on obstacle crossing in humans, the question of how the central nervous system (CNS) controls postural stability during gait initiation with the goal to clear an obstacle remains unclear. Stabilizing features of gait initiation include anticipatory postural adjustments (APAs) and lateral swing foot placement. To answer the above question, 14 participants initiated gait as fast as possible in three conditions of obstacle height, three conditions of obstacle distance and one obstacle-free (control) condition. Each of these conditions was performed with two levels of temporal pressure: reaction-time (high-pressure) and self-initiated (low-pressure) movements. A mechanical model of the body falling laterally under the influence of gravity and submitted to an elastic restoring force is proposed to assess the effect of initial (foot-off) center-of-mass position and velocity (or “initial center-of-mass set”) on the stability at foot-contact. Results showed that the anticipatory peak of mediolateral (ML) center-of-pressure shift, the initial ML center-of-mass velocity and the duration of the swing phase, of gait initiation increased with obstacle height, but not with obstacle distance. These results suggest that ML APAs are scaled with swing duration in order to maintain an equivalent stability across experimental conditions. This statement is strengthened by the results obtained with the mechanical model, which showed how stability would be degraded if there was no adaptation of the initial center-of-mass set to swing duration. The anteroposterior (AP) component of APAs varied also according to obstacle height and distance, but in an opposite way to the ML component. Indeed, results showed that the anticipatory peak of backward center-of-pressure shift and the initial forward center-of-mass set decreased with obstacle height, probably in order to limit the risk to trip over the obstacle, while the forward center-of-mass velocity at foot-off increased with obstacle distance, allowing a further step to be taken. These effects of obstacle height and distance were globally similar under low and high-temporal pressure. Collectively, these findings imply that the CNS is able to predict the potential instability elicited by the obstacle clearance and that it scales the spatiotemporal parameters of APAs accordingly.

Relationship between gait initiation and disability in individuals affected by multiple sclerosis

This study analyzes how multiple sclerosis (MS) does affect one of the most common voluntary activities in life: the gait initiation (GI). The main aim of the work is to characterize the execution of this task by measuring and comparing relevant parameters based on center of pressure (COP) patterns and to study the relationship between these and the level of expanded disability status scale (EDSS). To this aim, 95 MS subjects with an average EDSS score of 2.4 and 35 healthy subjects were tested using a force platform during the transition from standing posture to gait. COP time-series were acquired and processed to extract a number of parameters related to the trajectory followed by the COP. The statistical analysis revealed that only a few measurements were statistically different between the two groups and only these were subsequently correlated with EDSS score. The correlation analysis underlined that a progressive alteration of the task execution can be directly related with the increase of EDSS score. These finding suggest that most of the impairment found in people with MS comes from the first part of the COP pattern, the anticipatory postural adjustments (APAs). The central nervous system performs APAs before every voluntary movement to minimize balance perturbation due to the movement itself. Gait Initiation's APAs consist in some ankle muscles contractions that induce a backward COP shift to the swing limb. The analysis here performed highlighted that MS affected patients have a reduced posterior COP shift that reveals that the anticipatory mechanism is impaired.

Ankle anticipatory postural adjustments during gait initiation in healthy and post-stroke subjects

BACKGROUND

Anticipatory postural adjustments during gait initiation have an important role in postural stability but also in gait performance. However, these first phase mechanisms of gait initiation have received little attention, particularly in subcortical post-stroke subjects, where bilateral postural control pathways can be impaired. This study aims to evaluate ankle anticipatory postural adjustments during gait initiation in chronic post-stroke subjects with lesion in the territory of middle cerebral artery.

METHODS

Eleven subjects with post-stroke hemiparesis with the ability to walk independently and twelve healthy controls participated in this study. Bilateral electromyographic activity of tibialis anterior, soleus and medial gastrocnemius was collected during gait initiation to assess the muscle onset timing, period of activation/deactivation and magnitude of muscle activity during postural phase of gait initiation. This phase was identified through centre of pressure signal.

FINDINGS

Post-stroke group presented only half of the tibialis anterior relative magnitude observed in healthy subjects in contralesional limb (t=2.38, P=0.027) and decreased soleus deactivation period (contralesional limb, t=2.25, P=0.04; ipsilesional limb, t=3.67, P=0.003) as well its onset timing (contralesional limb, t=3.2. P=0.005; ipsilesional limb, t=2.88, P=0.033) in both limbs. A decreased centre of pressure displacement backward (t=3.45, P=0.002) and toward the first swing limb (t=3.29, P=0.004) was observed in post-stroke subjects.

INTERPRETATION

These findings indicate that chronic post-stroke subjects with lesion at middle cerebral artery territory present dysfunction in ankle anticipatory postural adjustments in both limbs during gait initiation.

The influence of accuracy constraints on EMG and kinetic variables during gait initiation

[Purpose] This study investigated the effects of accuracy constraints (targets) placed on the stepping-limb heel-strike (HS) on the electromyogram (EMG) and ground reaction forces (GRFs) during gait initiation. [Subjects and Methods] Twenty healthy subjects (29.2 ± 2.9 years) were asked to begin walking or stepping over a 10-cm-high obstacle at a fast speed. A 3-cm-diameter target was placed on the ground to dictate the position and accuracy of the stepping-limb HS. [Results] The results showed that the initiation velocity increase in the no-target conditions was due to modulation of the stance- and stepping-limb GRFs and a corresponding increase in the tibialis anterior (TA) activities of both limbs before stepping-limb toe-off. This was achieved by significantly increasing the stepping- and stance-limb TAEMG1 (determined between the onset of movement and time to peak anteroposterior (A-P) GRF of the stepping- and stance- limb) for the no-target conditions. It seems, therefore, that TAEMG1 and the slope to stepping-limb peak A-P GRF contributed to the intended velocity of initiation. [Conclusion] These data indicate that gait initiation and/or stepping over an obstacle may prove to be tasks by which motor control can be measured. The present study provides insight into the working mechanisms of the stepping and stance limbs and shows a clear need to further investigate whether the intact or affected limb should be used to initiate gait during rehabilitation and prosthetic training.

Influence of temporal pressure constraint on the biomechanical organization of gait initiation made with or without an obstacle to clear

Many daily motor tasks have to be performed under a temporal pressure constraint. This study aimed to explore the influence of such constraint on motor performance and postural stability during gait initiation. Young healthy participants initiated gait at maximal velocity under two conditions of temporal pressure: in the low-pressure condition, gait was self-initiated (self-initiated condition, SI); in the high-pressure condition, it was initiated as soon as possible after an acoustic signal (reaction-time condition, RT). Gait was initiated with and without an environmental constraint in the form of an obstacle to be cleared placed in front of participants. Results showed that the duration of postural adjustments preceding swing heel-off ("anticipatory postural adjustments", APAs) was shorter, while their amplitude was larger in RT compared to SI. These larger APAs allowed the participants to reach equivalent postural stability and motor performance in both RT and SI. In addition, the duration of the execution phase of gait initiation increased greatly in the condition with an obstacle to be cleared (OBST) compared to the condition without an obstacle (NO OBST), thereby increasing lateral instability and thus involving larger mediolateral APA. Similar effects of temporal pressure were obtained in NO OBST and OBST. This study shows the adaptability of the postural system to temporal pressure in healthy young adults initiating gait. The outcome of this study may provide a basis for better understanding the aetiology of balance impairments with the risk of falling in frail populations while performing daily complex tasks involving a whole-body progression.

Effect of natural trunk inclination on variability in soleus inhibition and tibialis anterior activation during gait initiation in young adults

BACKGROUND

Anticipatory postural adjustments (APAs) of gait initiation (GI) permit first step execution. APAs are characterized by a structured pattern of soleus (SOL) inhibition followed by tibialis anterior (TA) activation. This pattern shows variability among young adults where SOL is not always inhibited before TA activation. Initial posture preceding GI could explain a part of this variability. The aim of the study was to investigate the effect of natural trunk inclination on APAs during GI.

METHODS

Two groups of twelve subjects divided by natural trunk inclination angle performed five gait initiation trials. A regression model was computed to predict SOL inhibition and TA activation.

RESULTS

Backward leaners showed less SOL inhibition in stance leg (25.8% of trials) compared to forward leaners (55.6% of trials). Regression model revealed that high tonic EMG activity in SOL in the stance leg is the variable that best explains SOL inhibition variation within trials but not TA activation.

CONCLUSION

Slight variations in APAs are due to natural trunk inclination but more contribution in APAs is due to initial posture, future step speed and initial tonic soleus activity. Absence of SOL inhibition could be in part explained by natural trunk inclination, where the backward inclination leads to lower tonic SOL activity in quiet standing. These effects could be due to inherent and functional variability, which depend on postural variation, muscular coordination and limb roles.

Muscle synergies in preparation to a step made with and without obstacle

PURPOSE

To study multi-muscle synergies during preparation in making a step (self-paced level stepping vs. obstacle crossing stepping).

METHODS

The uncontrolled manifold hypothesis was used to explore the organization of leg and trunk muscles into groups (M-modes) and co-variation of M-mode involvement (M-mode synergies) during stepping tasks. Subjects performed two tasks: (1) making a comfortable step from quiet stance (STCS), (2) stepping over an obstacle of 15% body height from quiet stance, STOS. Electromyographic (EMG) signals of 10 postural muscles were recorded and analyzed. Principal component analysis was used to identify M-modes within the space of integrated indices of muscle activity. Variance in the M-mode space across stepping trials was partitioned into two components, one that did not affect the average value of center of pressure (COP) shift and the other that did. An index (ΔV) corresponding to the normalized difference between two components of variance was computed.

RESULTS

Under the two tasks, strong multi-M-mode synergies stabilizing trajectories of the COP in the anterior-posterior direction were found. Despite the significant differences in the COP shifts and EMG patterns of postural adjustments, the synergies showed only minor differences across the conditions.

CONCLUSIONS

These findings demonstrate the robustness of multi-M-mode synergies across different manners of making a step.

Center of pressure displacements during gait initiation in individuals with obesity

Background

Obesity is known to affect balance and gait pattern increasing the risk of fall and injury as compared to the lean population. Such risk is particularly high during postural transitions. Gait initiation (GI) is a transient procedure between static upright posture and steady-state locomotion, which includes anticipatory antero-posterior and lateral movements. GI requires propulsion and balance control. The aim of this study was to characterise quantitatively the strategy of obese subjects during GI using parameters obtained by the Center of Pressure (CoP) track.

Methods

20 obese individuals and 15 age-matched healthy subjects were tested using a force platform during the initiation trials. CoP plots were divided in different phases, which identified the anticipatory postural adjustments (APA₁, APA₂) and a movement phase (LOC). Duration, length and velocity of the CoP trace in these phases were calculated and compared.

Results and discussion

The results show that the main characteristic of GI in obese participants is represented by a higher excursion in medio-lateral direction. This condition lead to longer APA length and duration, which are statistical significant during APA2 when compared to control subjects. We also found longer duration of APA1 and LOC phases. In terms of velocity, most of the phases were characterised by a reduced CoP velocity in antero-posterior direction and faster movement in medio-lateral direction as compared to the control group.

Conclusions

Our findings provide novel evidence in GI in obese subjects that may serve for developing exercise programs aimed at specifically improving balance in both the antero-posterior and lateral directions. Such programs together with weight management may be beneficial for improving stability during postural transitions and reducing risk of fall in this population.

Effect of bilateral step-up and -down training on motor function in a person with hemiparesis: a case report

Gait training to facilitate the use of the paretic limb for persons with hemiparesis continues to be of interest to those in the clinical research domain. The purpose of this case report was to assess the outcomes of a repeated step-up and -down treatment, initiating with the paretic limb, on functional mobility, endurance and gait kinematic parameters in a person with hemiparesis. The participant was an 85-year-old female 3 years status post left hemiparesis, who reported overall good health. The participant was asked to step up on a 1-inch height wood box with her paretic limb. Once both feet were on top of the box, the participant initiated descent also with her paretic limb. The height of the box gradually progressed to 5 inches based on the participant's performance and tolerance. A metronome was used to facilitate rhythmic lower extremity movement patterns. The training duration for each treatment session was 7-15 min/day. The participant completed nine sessions spanning over 3 weeks. The outcome measure used to identify motor recovery was the Fugl-Myer (lower extremity). In addition, the timed up and go (TUG), the 6-min walk test (6 MWT) and gait kinematics were assessed to examine mobility and gait. The Fugl-Myer score and 6 MWT did not reflect a meaningful change (0% and +2.6%, respectively). However, TUG scores did show a meaningful change (+31.9%). With respect to gait kinematics, hip flexion on the paretic limb was improved from 11° to 18°, which indicates the normal range of hip motion during the initial swing phase in post-test.

Uphill racewalking at iso-efficiency speed

The aim of this study was to investigate the effects of gradients (0, 2, and 7%) on biomechanical parameters during racewalking (RW) at iso-efficiency speed (IES). During the experiment, 12 high-level athletes performed at IES on different slopes. The parameters studied were surface muscular activity (EMG) of 5 muscles of the leg using Muscle Lab (Boscosystem); kinematic parameters were collected using a high-speed camera (210 Hz) analyzed with Dartfish 5.5 Pro; and heart rate (HR) was monitored with a Cardio Polar. The results showed step length (SL), step frequency (SF), and internal work (WINT) decreased with increased treadmill gradient: SL = ([0-2% = 3.48%, p = 0.158], [0-7% = 12.17%, p < 0.001]); SF = ([0-2% = 2.38%, p = 0.173], [0-7% = 6.07%, p < 0.01]); WINT = ([0-2% = 8.34%, p < 0.001], [0-7% = 22.81%, p < 0.0001]). Conversely, contact time (CT) and HR increased less significantly with the increased gradients: CT = ([0-2% = 2.46%, p = 0.198], [0-7% = 6.56%, p < 0.01]); HR = ([0-2% = 0.62%, p = 0.652], [0-7% = 3.25%, p < 0.05]). The knee angle (KE) increased, whereas ankle angle (AK) and hip angle (HP) decreased with the increased gradients: AK = ([0-2% = 1.69%, p < 0.001], [0-7% = 1.13%, p < 0.01]); HP = ([0-2% = 0.22%, p < 0.03], [0-7% = 0.16%, p = 0.456]); KE = ([0-2% = 1.01%, p < 0.001], [0-7% = 1.60%, p < 0.001]). Electromyography (EMG) significantly decreased with the increased gradients in the: tibialis anterior ([0-2% = 22.49%, p < 0.0001], [0-7% = 41.18%, p < 0.0001]) and rectus femoris ([0-2% = 15.35%, p < 0.0001], [0-7% = 29.13%, p< 0.0001]). In contrast, EMG activity was significantly increased in the vastus lateralis ([0-2% = 22.95%, p < 0.0001], [0-7% = 31.15%, p < 0.0001]), gastrocnemius medialis ([0-2% = 21.40%, p < 0.001], [0-7% = 48.37%, p < 0.0001]), and biceps femoris ([0-2% = 190.78%, p < 0.0001], [0-7% = 201.37%, p < 0.0001]). The results indicate that increasing the gradient to 2% did not elicit an increased HR in racewalkers; however, at a 7% gradient, greater muscle activity was required.

A new method to assess temporal features of gait initiation with a single force plate

The aim of this study was to investigate whether time of toe-off and heel-contact during gait initiation could be assessed with a single force plate. Twenty subjects performed ten self-paced gait initiations and seven other subjects performed ten gait initiations in four new conditions (slow, fast, obstacle and splint). Several force-plate parameters were measured with a single force plate, and actual toe-off and heel-contact were assessed with a motion analysis system. Results showed strong temporal correlations and closeness (r=.86-.99, mean error=3-86 ms) between two force-plate parameters and the kinematics events (toe-off and heel-contact). These new parameters may be of interest to easily measure duration of anticipatory postural adjustments and swing phase during clinical assessments.

Trunk's natural inclination influences stance limb kinetics, but not body kinematics, during gait initiation in able men

The imposing mass of the trunk in relation to the whole body has an important impact on human motion. The objective of this study is to determine the influence of trunk's natural inclination--forward (FW) or backward (BW) with respect to the vertical--on body kinematics and stance limb kinetics during gait initiation.Twenty-five healthy males were divided based on their natural trunk inclination (FW or BW) during gait initiation. Instantaneous speed was calculated at the center of mass at the first heel strike. The antero-posterior impulse was calculated by integrating the antero-posterior ground reaction force in time. Ankle, knee, hip and thoraco-lumbar (L5) moments were calculated using inverse dynamics and only peaks of the joint moments were analyzed. Among all the investigated parameters, only joint moments present significant differences between the two groups. The knee extensor moment is 1.4 times higher (P<0.001) for the BW group, before the heel contact. At the hip, although the BW group displays a flexor moment 2.4 times higher (P<0.001) before the swing limb's heel-off, the FW group displays an extensor moment 3.1 times higher (P<0.01) during the swing phase. The three L5 extensor peaks after the toe-off are respectively 1.7 (P<0.001), 1.4 (P<0.001) and 1.7 (P<0.01) times higher for the FW group. The main results support the idea that the patterns described during steady-state gait are already observable during gait initiation. This study also provides reference data to further investigate stance limb kinetics in specific or pathologic populations during gait initiation. It will be of particular interest for elderly people, knowing that this population displays atypical trunk postures and present a high risk of falling during this forward stepping.

TauG-guidance of dynamic balance control during gait initiation across adulthood

Measurements from force plates were investigated to identify the life-span developmental course of dynamic balance control during gait initiation across adulthood. Center of pressure (CoP) data of the initial weight shift onto the supporting foot in the mediolateral (CoP(x)) direction were tauG analyzed, investigating the hypothesis that tau of the CoP(x) motion gap (τ(CoPx)) is tau-coupled onto an intrinsic tauG-guide (τ(G)), by maintaining the relation τ(CoPx)=Kτ(G), for a constant K. Participants were in their twenties, forties, sixties, and eighties. As regression analysis suggested a strong linear relationship between τ(CoPx) and τ(G), an investigation of the regression slope as an estimate of the coupling constant K in the tau-coupling equation was justified. Mean K values increased significantly with age from 0.40, 0.47, 0.67, to 0.79, suggesting that control of dynamic balance deteriorates from participants in their twenties making touch contact (K≤0.5) to participants in their sixties and eighties colliding with the boundaries of the base of support (K>0.5). The findings may prove useful as a measure for testing prospective balance control, a helpful tool for early detection of elderly people at increased risk of falling.

Progressive resistance training improves gait initiation in individuals with Parkinson's disease

An impaired ability to initiate walking is a common feature of postural instability and gait impairment in Parkinson's disease. While progressive resistance training (PRT) has been proposed to be an effective modality to improve balance and gait function in people with Parkinson's disease, there are a limited number of randomized trials and no studies have evaluated gait initiation performance. Thus, the purpose of this study was to examine the potential benefits PRT on GI performance in people with Parkinson's disease. Eighteen individuals with idiopathic PD were randomly assigned to either a twice weekly PRT program or a non-contact control group for 10 weeks. Biomechanical analysis of GI was performed pre- and post-intervention. Dependent variables of interest included the displacement of the center-of-pressure (COP) during the anticipatory postural phase of GI as well as the initial stride length and velocity. The PRT group demonstrated improvements in the posterior displacement of the COP and the initial stride length and velocity. There were no improvements in any variables for the control subjects. These results suggest that PRT may be an effective non-pharmacological and nonsurgical treatment to improve GI performance in PWP.

Does lower limb preference influence gait initiation?

Gait initiation (GI) has been the focus of many investigations in order to determine the kinematic and kinetic parameters associated with this process. In these studies, the parameters are observed during GI with the preferential lower limb. However, none of these studies have looked at the impact on GI parameters when the start is achieved with the non-preferential limb. This investigation focused on the effects of lower limb preference on the kinematic and kinetic parameters of GI. Upon display of a visual cue, subjects stepped with preferential limb or non-preferential limb at natural speed. The duration of GI phases, the medio-lateral component of the center of mass (CM) displacement, the medio-lateral distance between the center of pressure and the CM, the step width as well as the medio-lateral impulse, were observed. When subjects started with the non-preferential limb, the bodyweight transfer was facilitated by a greater impulse during the anticipatory postural adjustment (APA) phase. Conversely, a more lateral CM displacement during the execution phase and a more lateral step in preferential start were observed. Asymmetry in frontal plane body motion was observed during weight transfer following APA, as well as during assistive control of ballistic body motion during the execution phase of the first step. In both conditions, the non-preferential limb provided the greater lateral impulse on the ground. This may have clinical relevance especially in individuals with unilateral limb dyscontrol and postural asymmetry that may require rehabilitation.

Does movement planning follow Fitts' law? Scaling anticipatory postural adjustments with movement speed and accuracy

We wanted to determine whether movement planning followed Fitts' law by investigating the relationship between movement planning and movement performance in experienced dancers executing a typical classical ballet step in which the big toe was pointed to targets at different distances and of different widths so as to obtain several indices of difficulty (ID). Movement time, velocity and variability at the target were the variables of movement performance kinematics; movement planning was evaluated by analysis of anticipatory postural adjustments (APAs) to assess their modulation at different IDs. Movement time and peak of velocity were found to scale with the ID only when individual movement distance across target widths was entered into the analysis. APA magnitude and duration both scaled according to movement parameters but not in the same way. APA magnitude scaled with movement velocity, while APA duration was sensitive to the amplitude-to-accuracy ratio following the ID for movements performed in the shortest time interval when on-line feedback control is probably not available. Here we show that timing of muscle activation acts as an independent central command that triggers fine-tuning for speed-accuracy trade-off.

Does footwear type impact the number of steps required to reach gait steady state?: an innovative look at the impact of foot orthoses on gait initiation

Many studies have attempted to better elucidate the effect of foot orthoses on gait dynamics. To our knowledge, most previous studies exclude the first few steps of gait and begin analysis at steady state walking. These unanalyzed steps of gait may contain important information about the dynamic and complex processes required to achieve equilibrium for a given gait velocity. The purpose of this study was to quantify gait initiation and determine how many steps were required to reach steady state walking under three footwear conditions: barefoot, habitual shoes, and habitual shoes with a prefabricated foot orthoses. Fifteen healthy subjects walked 50m at habitual speed in each condition. Wearing habitual shoes with the prefabricated orthoses enabled subjects to reach steady state walking in fewer steps (3.5 steps+/-2.0) compared to the barefoot condition (5.2 steps+/-3.0; p=0.02) as well as compared to the habitual shoes condition (4.7 steps+/-1.6; p=0.05). Interestingly, the subjects' dynamic medial-lateral balance was significantly improved (22%, p<0.05) by using foot orthoses compared to other footwear conditions. These findings suggest that foot orthoses may help individuals reach steady state more quickly and with a better dynamic balance in the medial-lateral direction, independent of foot type. The findings of this pilot study may open new avenues for objectively assessing the impact of prescription footwear on dynamic balance and spatio-temporal parameters of gait. Further work to better assess the impact of foot orthoses on gait initiation in patients suffering from gait and instability pathologies may be warranted.

Comparison of trunk activity during gait initiation and walking in humans

To understand the role of trunk muscles in maintenance of dynamic postural equilibrium we investigate trunk movements during gait initiation and walking, performing trunk kinematics analysis, Erector spinae muscle (ES) recordings and dynamic analysis. ES muscle expressed a metachronal descending pattern of activity during walking and gait initiation. In the frontal and horizontal planes, lateroflexion and rotation occur before in the upper trunk and after in the lower trunk. Comparison of ES muscle EMGs and trunk kinematics showed that trunk muscle activity precedes corresponding kinematics activity, indicating that the ES drive trunk movement during locomotion and thereby allowing a better pelvis mobilization. EMG data showed that ES activity anticipates propulsive phases in walking with a repetitive pattern, suggesting a programmed control by a central pattern generator. Our findings also suggest that the programs for gait initiation and walking overlap with the latter beginning before the first has ended.

Altered muscle activation characteristics associated with single volitional forward stepping in middle-aged adults

BACKGROUND

Middle-aged adults show a higher incidence of falls compared to young adults when performing outdoor physical activities. This study investigated whether or not the patterns and quantitative characteristics of the trunk and lower extremity muscle activations associated with stepping, which represents an important movement for arresting falls, differ between middle-aged adults and young adults.

METHODS

Nine healthy young adults (age=22[3] years) and nine healthy middle-aged adults (age=52[8] years), performed a single-step, volitional, fast forward stepping movement with each leg. The stepping movement was divided into the step-initiation, single-leg-support, and landing phases based on foot-switch signals. The activation sequence, occurrence rate, onset latency, burst duration, and normalized co-contraction duration of the tibialis anterior, medial gastrocnemius, rectus femoris, biceps femoris, and gluteus medius of the stance and swing legs and that of bilateral erector spinae muscles were analyzed using surface electromyography. We defined the essential muscle activation as exhibiting an occurrence rate of 90% or more in all of these trials.

FINDINGS

As compared to young adults, the middle-aged adults demonstrated several additional essential bursts throughout the stepping movement. Middle-aged adults also displayed significantly longer burst durations of the biceps femoris and medial gastrocnemius of the swing leg after landing, as well as longer co-contraction of the rectus femoris and biceps femoris of the stance leg in the single-leg-support phase (P<0.05).

INTERPRETATION

Age-related changes in step-related neuromuscular control exist in healthy middle-aged adults. We propose that training focused on improving or maintaining neuromuscular control associated with volitional leg movements may benefit middle-aged individuals.

Gait adjustments in obstacle crossing, gait initiation and gait termination after a recent lower limb amputation

OBJECTIVE

To describe the adjustments in gait characteristics of obstacle crossing, gait initiation and gait termination that occur in subjects with a recent lower limb amputation during the rehabilitation process.

DESIGN

Prospective and descriptive study.

SUBJECTS

Fourteen subjects with a recent transfemoral, knee disarticulation or transtibial amputation.

METHODS

Subjects stepped over an obstacle and initiated and terminated gait at four different times during the rehabilitation process.

OUTCOME MEASURES

Success rate, gait velocity and lower limb joint angles in obstacle crossing, centre of pressure shift and peak anteroposterior ground reaction force in gait initiation and termination.

RESULTS

In obstacle crossing amputees increased success rate, gait velocity and swing knee flexion of the prosthetic limb. Knee flexion in transfemoral and knee disarticulation amputees was not sufficient for safe obstacle crossing, which resulted in a circumduction strategy. In gait initiation and termination amputees increased the anteroposterior ground reaction force and the centre of pressure shift in the mediolateral direction in both tasks. Throughout the rehabilitation process the centre of pressure was shifted anteriorly before single-limb stance on the trailing prosthetic limb in gait initiation, whereas in gait termination the centre of pressure in single-limb stance remained posterior when leading with the prosthetic limb.

CONCLUSION

Subjects with a recent amputation develop adjustment strategies to improve obstacle crossing, gait initiation and gait termination. Innovations in prosthetic design or training methods may ease the learning process of these tasks.

Grip force control during gait initiation with a hand-held object

When walking with a hand-held object, grip force is coupled in an anticipatory manner to changes in inertial force resulting from the accelerations and decelerations of gait. However, it is not known how grip and inertial forces are organized at the onset of gait, and if the two forces are coupled in the early phases of gait initiation. Moreover, initiating walking with an object involves the coordination of anticipatory postural (e.g., ground reaction force changes) and grasping adjustments. The aim of this study was to investigate the relationship of ground reaction, grip, and inertial force onsets, and the subsequent development of the coupling of grip and inertial forces during gait initiation with a hand-held object. Ten subjects performed gait initiation with a hand-held object following predictable and unpredictable start signals. We found that ground reaction and grip force onsets were closely linked in time regardless of the predictability of the start signal. In the early period of gait initiation, the grip force started to increase prior to inertial force changes. While the strength of the coupling of grip and inertial forces was moderate in this early phase, it increased to values observed during steady-state gait after the swing foot left the ground. The early grip force increase and the coupling of grip and inertial forces represent an anticipatory control process. This process establishes an appropriate grip-inertial force ratio to ensure object stability during acceleration after foot-off and maintains this increased ratio thereafter. The results suggest that grasping and whole body movements are governed by a common internal representation.

Gait initiation in lower limb amputees

OBJECTIVE

To study limitations in function and adjustment strategies in lower limb amputees during gait initiation.

DESIGN

Observational cohort study.

SETTING

University Medical Center.

PARTICIPANTS

Amputees with a unilateral transfemoral or transtibial amputation, and able-bodied subjects.

MAIN OUTCOME MEASURES

Leading limb preference, temporal variables, ground reaction forces, and centre of pressure shift.

RESULTS

Amputees demonstrated a decrease in peak anterior ground reaction force, a smaller or absent posterior centre of pressure shift, and a lower gait initiation velocity. The main adjustments strategies in amputees were more limb-loading on the non-affected limb, prolonging the period of propulsive force production in the non-affected limb and initiating gait preferably with the prosthetic limb.

CONCLUSION

Since an intact ankle joint and musculature is of major importance in gait initiation, functional limitations and adjustment strategies in transfemoral and transtibial amputees were similar. Improving prosthetic ankle properties and initiating gait with the prosthetic limb may facilitate the gait initiation process in amputees.

Postural preparation to making a step: is there a 'motor program' for postural preparation?

We tested the hypothesis that a sequence of mechanical events occurs preceding a step that scales in time and magnitude as a whole in a task-specific manner, and is a reflection of a "motor program." Young subjects made a step under three speed instructions and four tasks: stepping straight ahead, down a stair, up a stair, and over an obstacle. Larger center-of-pressure (COP) and force adjustments in the anterior-posterior direction and smaller COP and force adjustments in the mediolateral direction were seen during stepping forward and down a stair, as compared with the tasks of stepping up a stair and over an obstacle. These differences were accentuated during stepping under the simple reaction time instruction. These results speak against the hypothesis of a single motor program that would underlie postural preparation to stepping. They are more compatible with the reference configuration hypothesis of whole-body actions.