Step Execution Phase Research Articles

Effect of increasing speed on whole-body angular momentum during stepping in the elderly

Recent evidence suggests that older adults may have difficulty controlling whole-body angular momentum (H) during volitional stepping, which could impose a major challenge for balance control and result in potential falls. However, it is not known if and how H is influenced by speed when stepping. This study aimed to investigate the effect on H of increasing speed during step initiation in older adults. Twenty-seven healthy individuals over 60 were enrolled in the current study and were instructed to perform a series of step initiations with their dominant leg under two speed conditions: at preferred speed and as fast as possible. Two force plates and a motion-capture system were used to record H and the components of the net external moment (moment arms and ground reaction forces) during the double support and step execution phases of stepping. Results revealed that increasing speed of stepping affected H differently in both stepping phases and in the different planes. H ranges in all three planes increased with speed during the double support phase. During the step execution phase, while H ranges in frontal and transversal planes decreased, sagittal plane H range significantly increased with speed. This increased H range in the sagittal plane, which may result from the task demands, could impose a greater challenge for balance control in the elderly.

Age-related changes in the control of whole-body angular momentum during stepping.

Appropriate control of whole-body angular momentum (H) is crucial to maintain dynamic balance and thus avoid falling during daily activities. Poor H control ability during locomotion has been found in people with an increased risk of falling, such as post-stroke patients and amputees. In contrast, little is known about the control of H during locomotion in the elderly. The aim of this study was to investigate whether and how aging influences three-dimensional H control during initiation of stepping. Twenty-two healthy old and 22 healthy young individuals were instructed to perform a series of initiation of stepping with their dominant leg and at their self-selected preferred pace. Two force plates and a motion capture system were used to record H, the net external moment about the body's center of mass and components of this net external moment (moment arms and ground reaction forces) during the double support and step execution phases of stepping. In the double support phase, older participants exhibited smaller peak-to-peak ranges of H in the sagittal and transversal planes compared to their younger counterparts. These results were explained by decreased net external moments in both planes in the older participants. Conversely, during the step execution phase, older adults had higher peak-to-peak ranges of H in the frontal and sagittal planes compared to the younger adults. These higher ranges of H were associated with a longer duration of the step execution phase. Furthermore, in the sagittal plane, a higher external moment also contributed to increasing peak-to-peak ranges of H in older adults. The current study revealed that older and younger adults exhibit different control strategies of H during initiation of stepping. The age-related changes, which may emphasize a higher difficulty to control H in the older adults, could impose a higher challenge for balance control and a potentially higher risk of falling during the step execution phase in this population.

O-19 Impact of attentional abilities on step initiation in healthy people: An EEG study

Background Step initiation can vary according to attentional resources. Components of visuospatial attention (alerting, orienting, and executive control or conflict resolution) have shown to be able to modulate specifically anticipatory postural adjustments (APAs) and step execution phase, and can be assessed simultaneously within a single paradigm: the “Attention Network Test” (ANT). The present study aims to a better understanding of the interaction between visuospatial attention and step initiation in a healthy population through the observation of: (1) the modulation of step initiation while performing the ANT and (2) the potential modifications of cortical motor preparation. Material and methods Thirty-two healthy subjects performed an attentional task adapted from the ANT and combined with step initiation. Cortical activity during movement preparation (APA) and execution was assessed via high-resolution encephalography. Results Step initiation was particularly perturbed in conflict resolution condition. Conflict resolution also affected event-related spectral perturbations (ERSPs) in alpha and beta bands: movement-related desynchronization over the sensorimotor cortex occurred earlier and was longer and of higher amplitude after response to a target with incongruent flankers. Conclusions Tasks with attentional load such as conflict resolution lead to an alteration of step initiation and concomitant modifications of ERSPs during APAs. The ANT combined with step initiation could be therefore used to better understand pathologies such as Parkinson’s disease with gait akinesia.

P30-S Impact of attentional abilities on step initiation in Parkinsonian patients with freezing of gait

Background Components of visuospatial attention (alertness, orienting and executive control) have shown to be able to modulate specifically step initiation, and can be assessed simultaneously within a single paradigm: the “Attention Network Test” (ANT). However, patients with Parkinson’s disease (PD) with freezing of gait (FoG) present a deficit in executive control that could deteriorate step initiation. The present study aims to a better understanding of the interaction between visuospatial attention and step initiation in PD patients with and without FoG through the observed modulation of (1) step initiation while performing the ANT and (2) cortical motor preparation. Material and methods Fourteen freezers (PD + FoG), fifteen non-freezers (PD-FoG) and fifteen healthy controls (HC) performed an attentional task adapted from the ANT and combined with step initiation. Cortical activity during movement preparation (anticipatory postural adjustments, APAs) and step execution phase was assessed via high-resolution encephalography. Results Although step execution time was longer for PD patients compared to HC partly due to a higher occurrence of multiple APAs, the effects of cue and target were not significantly different between groups. Subjects with PD presented abnormalities regarding movement-related desynchronization over the sensorimotor cortex after APA onset as a response to a target with incongruent flankers. Conclusions Tasks with attentional load such as conflict resolution lead to an alteration of step initiation in the same way in PD + FoG, PD-FoG and HC. The concomittant EEG recording during the ANT combined with step initiation seems promising to better understand the influence of attention on gait initiation in PD.

Adaptation of anticipatory postural adjustments during gait initiation induced by short-term whole-body vibration: A randomized sham intervention study in young adults and elderly

Whole-body vibration (WBV) is a training method that exposes the entire body to mechanical oscillations while standing on a vibrating platform. WBV are used by clinicians to improve balance and gait. However, WBV may also induce an increase in leg stiffness through a “tonic reflex vibration”, which may potentially hinder balance control during locomotor tasks. Gait initiation (GI) is a functional task that is classically used to investigate balance control. It includes two phases, the “Anticipatory Postural Adjustments” (APAs) and step-execution phase. During GI, stability is known to depend on the capacity of the central nervous system (CNS) to generate adequate APAs along the mediolateral direction. This study tested the hypothesis that, in young and elderly adults, the CNS programs mediolateral APAs in function of the changes in stance leg stiffness induced by WBV so as to maintain (or even improve) stability. Participants (i.e. 23 young adults and 20 elderly) were randomly divided in two subgroups and exposed to a single WBV session or a sham intervention. Series of ten GI trials were performed on a force-plate before and after WBV or sham intervention. In young adults, the amplitude of mediolateral APAs (peak center-of-pressure shift and center-of-mass velocity at heel-off) and stance leg stiffness significantly increased following WBV. Mechanical modelling of the body showed that the increased leg stiffness was detrimental to stability. However, experimental data showed that stability remained unchanged following WBV, and even tended to be improved. Results further showed that center-of-mass velocity at heel-off and stance leg stiffness were positively correlated. In elderly, there was no significant effect of WBV on GI parameters. Young adults compensate the negative effect of increased leg stiffness on stability by larger mediolateral APAs. In elderly, the absence of WBV effects may possibly be due to proprioceptive and/or plantar inputs alterations.

Comparative gait initiation kinematics between simulated unilateral and bilateral ankle hypomobility: Does bilateral constraint improve speed performance?

Improvement of motor performance in unilateral upper limb motor disability has been shown when utilizing inter-limb coupling strategies during physical rehabilitation. This suggests that ‘default’ bilateral central motor commands are facilitated. Here, we tested whether this bilateral motor control principle may be generalized to the lower limbs during gait initiation, which involves alternate bilateral actions. Disability was simulated by strapping to produce ankle hypomobility. Healthy adult subjects initiated gait at a self-paced speed with no ankle constraint (control), or with the stance, swing or bilateral ankles strapped. The duration of the anticipatory postural adjustments lengthened and the center of mass instantaneous progression velocity at foot-off decreased when the ankle was strapped. During the step execution phase, progression velocity at foot-contact was higher when both ankles were strapped compared to unilateral strapping of the stance ankle. These findings suggest that bilateral central motor commands are favored during walking tasks. Indeed, unilateral constraint of the stance ankle should compel the central nervous system to adapt specific commands to the constraint and normal sides whereas the ‘default’ bilateral motor commands would be utilized when both ankles are strapped leading to better kinematics performance. Bilateral in-phase upper limb coordination and bilateral alternating lower limb locomotor movements may share similar control mechanisms.

Relative and absolute reliability of the new “Step-Ex” step-execution test in elderly people with and without balance problems

Purpose: To evaluate the relative and absolute test–re-test reliability of a new step-execution test, “Step-Ex”, for clinical use in elderly with and without balance problems. Method: Test–re-test design to assess intrarater reliability. Thirty-four healthy community-dwelling elderly (65–87 years), 16 with balance problems, were tested twice two weeks apart. Step-Ex consists of two portable force platforms that register vertical ground reaction forces connected to a computer for easy detection of temporal events and phases (reaction-, preparation-, stepping- and step execution phase). Standing with one foot on each platform, the subjects were given a tactile stimulus on the heel to initiate rapid steps forward. Results: Test–re-test agreement was good to very good: ICC2.6 0.83–0.87 (without balance problems) and 0.71–0.83 (with balance problems) with no apparent systematic differences between the tests. The SEM, i.e. the smallest detectable change that may indicate a real clinical improvement for a group of individuals was small, 4.6–8.6%. The smallest real difference, representing the smallest change that reveals clinical improvement for a single individual, was 13–24%. Conclusion: Step-Ex is a highly reliable instrument and can be recommended as an outcome measure evaluating the effects of balance training in elderly people with and without balance deficits.Implications for RehabilitationBalance deficits are common in the elderly population.To evaluate the effects of treatment or changes over time it is of great importance to have reliable instruments.Step-Ex is a highly reliable instrument and can be recommended as an outcome measure evaluating the effects of treatment or changes over time in elderly people with and without balance deficits.

Effects of new, individually adjusted, progressive balance group training for elderly people with fear of falling and tend to fall: a randomized controlled trial

Objective: To evaluate the effects of a new, individually adjusted, progressive and specific balance group training programme on fear of falling, step execution, and gait in healthy elderly people with fear of falling and tend to fall. Design: Randomized controlled trial. Setting: The study was conducted in Stockholm County, Sweden. Subjects: Fifty-nine community dwelling elderly people were recruited by advertisement, and allocated at random to an intervention group (n = 38) or a control group (n = 21). Intervention: Individually adjusted, progressive and specific balance group training was given three times a week for three months. The training incorporated elements included in, and required for, independent activities of daily living, and for reactions to loss of balance during dual or multiple tasks. Main measures: Fear of falling was assessed with Falls Efficacy Scale International (FES-I). The reaction time of step execution was measured with the step-execution test, and gait was measured with GAITRite®. Results: After three months the intervention group showed significant positive changes in the FES-I (P = 0.008), in the step-execution phase of dual-task performance (P = 0.012), and in gait at preferred speed during single-task performance; in cadence (P = 0.030) and, at fast speed, in velocity (P = 0.004) and cadence (P = 0.001). Significant decreases were also found for the likelihood of depression after participating in the training programme. Conclusion: This new balance training programme is feasible and leads to decreased fear of falling, decreased time for step execution during dual-task performance and increased velocity during fast walking.

Age-differences in the free vertical moment during step descent

Background This study utilises a rarely examined biomechanical parameter – the free vertical moment to determine age-related differences in rotational kinetics of the body about the vertical-axis when stepping down from a stationary position. Methods Ten older and 10 young adults completed step-downs from three heights. Free vertical moment impulse and peak during step-initiation double-support and the subsequent step-execution phase, and vertical-axis pelvis angular displacement and velocity at instant of landing were compared. Findings The free vertical moment during double-support was directed away from the intended lead-limb side, producing a change in vertical-axis rotational momentum that moved the lead-limb in a forwards-medial direction about the stationary support/trailing limb during the subsequent step-execution phase. The free vertical moment during step-execution was directed towards the lead-limb side and acted to slow/halt the body’s vertical-axis rotation away from lead-limb side. Free vertical moment impulse and peak during double-support were similar between groups ( P > 0.05), but during step-execution were significantly reduced in older adults ( P = 0.002). As a result older adults had greater vertical-axis pelvis angular displacement and velocity at instant of landing (directed away from lead-limb side), with significant ( P < 0.001) group-by-step height interactions indicating that differences between groups became more pronounced with increasing step-height. Interpretation These findings highlight that older adults were unable to exert the same vertical-axis control during single-support as young subjects did. Findings also highlight that the analysis of free vertical moment data can be a useful biomechanical tool to highlight age-related differences in how steps/stairs are negotiated.

Controlling Propulsive Forces in Gait Initiation in Transfemoral Amputees

During prosthetic gait initiation, transfemoral (TF) amputees control the spatial and temporal parameters that modulate the propulsive forces, the positions of the center of pressure (CoP), and the center of mass (CoM). Whether their sound leg or the prosthetic leg is leading, the TF amputees reach the same end velocity. We wondered how the CoM velocity build up is influenced by the differences in propulsive components in the legs and how the trajectory of the CoP differs from the CoP trajectory in able bodied (AB) subjects. Seven TF subjects and eight AB subjects were tested on a force plate and on an 8 m long walkway. On the force plate, they initiated gait two times with their sound leg and two times with their prosthetic leg. Force measurement data were used to calculate the CoM velocity curves in horizontal and vertical directions. Gait initiated on the walkway was used to determine the leg preference. We hypothesized that because of the differences in propulsive components, the motions of the CoP and the CoM have to be different, as ankle muscles are used to help generate horizontal ground reaction force components. Also, due to the absence of an active ankle function in the prosthetic leg, the vertical CoM velocity during gait initiation may be different when leading with the prosthetic leg compared to when leading with the sound leg. The data showed that whether the TF subjects initiated a gait with their prosthetic leg or with their sound leg, their horizontal end velocity was equal. The subjects compensated the loss of propulsive force under the prosthesis with the sound leg, both when the prosthetic leg was leading and when the sound leg was leading. In the vertical CoM velocity, a tendency for differences between the two conditions was found. When initiating gait with the sound leg, the downward vertical CoM velocity at the end of the gait initiation was higher compared to when leading with the prosthetic leg. Our subjects used a gait initiation strategy that depended mainly on the active ankle function of the sound leg; therefore, they changed the relative durations of the gait initiation anticipatory postural adjustment phase and the step execution phase. Both legs were controlled in one single system of gait propulsion. The shape of the CoP trajectories, the applied forces, and the CoM velocity curves are described in this paper.

BALANCE CONTROL DURING STAIR NEGOTIATION

This study utilises a rarely examined biomechanical parameter – the free vertical moment to determine age-related differences in rotational kinetics of the body about the vertical-axis when stepping down from a stationary position.Ten older and 10 young adults completed step-downs from three heights. Free vertical moment impulse and peak during step-initiation double-support and the subsequent step-execution phase, and vertical-axis pelvis angular displacement and velocity at instant of landing were compared.The free vertical moment during double-support was directed away from the intended lead-limb side, producing a change in vertical-axis rotational momentum that moved the lead-limb in a forwards-medial direction about the stationary support/trailing limb during the subsequent step-execution phase. The free vertical moment during step-execution was directed towards the lead-limb side and acted to slow/halt the body’s vertical-axis rotation away from lead-limb side. Free vertical moment impulse and peak during double-support were similar between groups (P > 0.05), but during step-execution were significantly reduced in older adults (P = 0.002). As a result older adults had greater vertical-axis pelvis angular displacement and velocity at instant of landing (directed away from lead-limb side), with significant (P < 0.001) group-by-step height interactions indicating that differences between groups became more pronounced with increasing step-height.These findings highlight that older adults were unable to exert the same vertical-axis control during single-support as young subjects did. Findings also highlight that the analysis of free vertical moment data can be a useful biomechanical tool to highlight age-related differences in how steps/stairs are negotiated.

The gait initiation process in unilateral lower-limb amputees when stepping up and stepping down to a new level

Background. Unilateral lower-limb amputees lead with their intact limb when stepping up and with their prosthesis when stepping down; the gait initiation process for the different stepping directions has not previously been investigated. Methods. Ten unilateral amputees (5 transfemoral and 5 transtibial) and 8 able-bodied controls performed single steps up and single steps down to a new level (73 and 219 mm). Duration, a-p and m-l centre of mass and centre of pressure peak displacements and centre of mass peak velocity of the anticipatory postural adjustment and step execution phase were evaluated for each stepping direction by analysing data collected using a Vicon 3D motion analysis system. Findings. There were significant differences (in the phase duration, peak a-p and m-l centre of pressure displacement and peak a-p and m-l centre of mass velocity at heel-off and at foot-contact) between both amputee sub-groups and controls ( P < 0.05), but not between amputee sub-groups. These group differences were mainly a result of amputees adopting a different gait initiation strategy for each stepping direction. Interpretation. Findings indicate the gait initiation process utilised by lower-limb amputees was dependent on the direction of stepping and more particularly by which limb the amputee led with; this suggests that the balance and postural control of gait initiation is not governed by a fixed motor program, and thus that becoming an amputee will require time and training to develop alternative neuromuscular control and coordination strategies. These findings should be considered when developing training/rehabilitation programs.

The strategies to regulate and to modulate the propulsive forces during gait initiation in lower limb amputees.

We used the framework of motor program adaptability to examine how unilateral above-knee (AK) or below-knee (BK) amputee subjects organize the global and local biomechanical processes of generation of the propulsive forces during gait initiation to overcome the segmental and neuro-muscular asymmetry. The organization of the global biomechanical process refers to the kinematics behavior of the couple center of foot pressure (CoP) and center of mass (CoM); the organization of the local biomechanical process refers to the propulsive forces generated by the prosthetic or intact limb during the anticipatory postural adjustment phase and the step execution phase. Specifically, we examined: i) the strategy to regulate the progression velocity, i.e., to maintain it comparably when the leading limb changed from the prosthetic limb to the intact limb; and ii) the strategy to modulate the progression velocity, i.e., to increase it when gait was initiated with the prosthetic limb vs. intact limb. The kinematics of the CoM and CoP in the amputees showed the same global biomechanical organization that is typically observed in able-bodied subjects, i.e., the production of the forward disequilibrium torque was obtained by a backward shift of the CoP, followed by a forward acceleration of the CoM. However, gait initiation was achieved by using a different local strategy depending on which limb was used to initiate the step. For the regulation of the CoM progression velocity, when the gait was initiated with the intact limb, the slope of the progression velocity during the anticipatory postural adjustment phase (APA) was steeper and lasted longer, the step execution duration was shorter, and the variation of the CoM speed was lower. In other words, to regulate the speed of progression, the amputee subjects controlled the spatial and temporal parameters of the propulsive forces. In the modulation of the CoM progression velocity, when the gait was initiated with the intact limb, the amputees controlled only the intensity of the propulsive forces during both the APA and step execution phases. In contrast, when the gait was initiated with the prosthetic limb, the modulation resulted mainly from the propulsive forces generated during the step execution phase. These different strategies are discussed in terms of the subject's capacity to adapt the motor program for gait initiation to new constraints.