Parkinson's Disease and Cognitive-Motor Dual-Task: Is Motor Prioritization Possible in the Early Stages of the Disease?

The authors aimed to compare the postural phase of gait initiation under single-task (gait initiation) and dual-task (gait initiation plus Stroop test) conditions in healthy subjects and in subjects with Parkinson's disease (PD) in the early stages (Hoehn and Yahr scale < 3). The postural phase of gait initiation was assessed through the centre of pressure in single and dual task in 10 healthy subjects and 9 with PD. The analysis indicated that in the early stages of PD, an additional cognitive task did not affect the displacement of the gait initiation. No significant effects occurred between the groups and within-subjects (p > .05). Also, no interaction was found between the groups and the conditions (single- and dual-task). Differences were found in the duration of the mediolateral postural phase (p = .003), which was higher in PD subjects than in healthy subjects. The findings suggest that subjects in the early stages of PD prioritize gait initiation, as their motor performance was similar to that of healthy subjects.

From normal to fast walking: Impact of cadence and stride length on lower extremity joint moments

This study aimed to clarify the influence of various speeding strategies (i.e. adjustments of cadence and stride length) on external joint moments. This study investigated the gait of 52 healthy subjects who performed self-selected normal and fast speed walking trials in a motion analysis laboratory. Subjects were classified into three separate groups based on how they increased their speed from normal to fast walking: (i) subjects who increased their cadence, (ii) subjects who increased their stride length and (iii) subjects who simultaneously increased both stride length and cadence. Joint moments were calculated using inverse dynamics and then compared between normal and fast speed trials within and between three groups using spatial parameter mapping. Individuals who increased cadence, but not stride length, to walk faster did not experience a significant increase in the lower limb joint moments. Conversely, subjects who increased their stride length or both stride length and cadence, experienced a significant increase in all joint moments. Additionally, our findings revealed that increasing the stride length had a higher impact on joint moments in the sagittal plane than those in the frontal plane. However, both sagittal and frontal plane moments were still more responsive to the gait speed change than transverse plane moments. This study suggests that the role of speed in altering the joint moment patterns depends on the individual's speed-regulating strategy, i.e. an increase in cadence or stride length. Since the confounding effect of walking speed is a major consideration in human gait research, future studies may investigate whether stride length is the confounding variable of interest.

Use of a backpack alters gait initiation of high school students

We assessed how backpack carriage influences the gait initiation (GI) process in high school students, who extensively use backpacks. GI involves different dynamics from gait itself, while the excessive use of backpacks can result in adverse effects. 117 high school students were evaluated in three experimental conditions: no backpack (NB), bilateral backpack (BB), and unilateral backpack (UB). Two force plates were used to acquire ground reaction forces (GRFs) and moments for each foot separately. Center of pressure (COP) scalar variables were extracted, and statistical parametric mapping analysis was performed over the entire COP/GRFs time series. GI anticipatory postural adjustments (APAs) were reduced and were faster in backpack conditions; medial-lateral COP excursion was smaller in this phase. The uneven distribution of the extra load in the UB condition led to a larger medial-lateral COP shift in the support-foot unloading phase, with a corresponding vertical GRF change that suggests a more pronounced unloading swing foot/loading support foot mechanism. The anterior-posterior GRFs were altered, but the COP was not. A possible explanation for these results may be the forward trunk lean and the center of mass proximity of the base of support boundary, which induced smaller and faster APA, increased swing foot/support foot weight transfer, and increased load transfer to the first step.

Age effects on the control of dynamic balance during step adjustments under temporal constraints

This study investigated the age effects on the control of dynamic balance during step adjustments under temporal constraints. Fifteen young adults and 14 older adults avoided a virtual white planar obstacle by lengthening or shortening their steps under free or constrained conditions. In the anterior-posterior direction, older adults demonstrated significantly decreased center of mass velocity at the swing foot contact under temporal constraints. Additionally, the distances between the 'extrapolated center of mass' position and base of support at the swing foot contact were greater in older adults than young adults. In the mediolateral direction, center of mass displacement was significantly increased in older adults compared with young adults. Consequently, older adults showed a significantly increased step width at the swing foot contact in the constraint condition. Overall, these data suggest that older adults demonstrate a conservative strategy to maintain anterior-posterior stability. By contrast, although older adults are able to modulate their step width to maintain mediolateral dynamic balance, age-related changes in mediolateral balance control under temporal constraints may increase the risk of falls in the lateral direction during obstacle negotiation.

The use of turning tasks in clinical gait analysis for children with cerebral palsy

BACKGROUND

Turning while walking is a crucial component of locomotion that is performed using an outside (step) or inside (spin) limb strategy. The aims of this paper were to determine how children with cerebral palsy perform turning maneuvers and if specific kinematic and kinetic adaptations occur compared to their typically developing peers.

METHODS

Motion capture data from twenty-two children with cerebral palsy and fifty-four typically developing children were collected during straight and 90° turning gait trials. Experimental data were used to compute spatio-temporal parameters, margin of stability, ground reaction force impulse, as well as joint kinematics and kinetics.

FINDINGS

Both child groups preferred turning using the spin strategy. The group of children with cerebral palsy exhibited the following adaptations during turning gait compared to the typically developing group: stride length was decreased across all phases of the turn with largest effect size for the depart phase (2.02), stride width was reduced during the turn phase, but with a smaller effect size (0.71), and the average margin of stability during the approach phase of turning was reduced (effect size of 0.98). Few overall group differences were found for joint kinematic and kinetic measures; however, in many cases, the intra-subject differences between straight walking and turning gait were larger for the majority of children with cerebral palsy than for the typically developing children.

INTERPRETATION

In children with cerebral palsy, turning gait may be a better discriminant of pathology than straight walking and could be used to improve the management of gait abnormalities.

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.

Directional specificity of postural threat on anticipatory postural adjustments during lateral leg raising

This study explored the directional specificity of fear of falling (FoF) effects on the stabilizing function of anticipatory postural adjustments (APA). Participants (N = 71) performed a series of lateral leg raises from an elevated surface in three conditions: in the "Control condition", participants stood at the middle of the surface; in the two test conditions, participants were positioned at the lateral edge of the surface so that the shift of the whole-body centre-of-mass during APA for leg raising was directed towards the edge ("Approach condition") or was directed away from the edge ("Avoidance condition"). Results showed that the amplitude of APA was lower in the "Approach condition" than in the "Control condition" (p < .01); this reduction was compensated for by an increase in APA duration (p < .05), so that both postural stability and motor performance (in terms of peak leg velocity, final leg posture and movement duration) remained unchanged. These changes in APA parameters were not present in the "Avoidance condition". Participants further self-reported a greater FoF (p < .001) and a lower ability to avoid a fall (p < .001) in the "Approach condition" (but not in the "Avoidance condition") than in the "Control condition". The results of this study show that the effects of FoF do not solely depend on initial environmental conditions, but also on the direction of APA relative to the location of the postural threat. These results support the so-called Motivational Direction Hypothesis, according to which approach and avoidance behaviours are primed by emotional state.

Acute effects of muscle fatigue on anticipatory and reactive postural control in older individuals: a systematic review of the evidence

BACKGROUND

Falls are the leading cause of traumatic brain injury and fractures and the No. 1 cause of emergency department visits by older adults. Although declines in muscle strength and sensory function contribute to increased falls in older adults, skeletal muscle fatigue is often overlooked as an additional contributor to fall risk. In an effort to increase awareness of the detrimental effects of skeletal muscle fatigue on postural control, we sought to systematically review research studies examining this issue.

PURPOSE

The specific purpose of this review was to provide a detailed assessment of how anticipatory and reactive postural control tasks are influenced by acute muscle fatigue in healthy older individuals.

METHODS

An extensive search was performed using the CINAHL, Scopus, PubMed, SPORTDiscus, and AgeLine databases for the period from inception of each database to June 2013. This systematic review used standardized search criteria and quality assessments via the American Academy for Cerebral Palsy and Developmental Medicine Methodology to Develop Systematic Reviews of Treatment Interventions (2008 version, revision 1.2, AACPDM, Milwaukee, Wisconsin).

RESULTS

A total of 334 citations were found. Six studies were selected for inclusion, whereas 328 studies were excluded from the analytical review. The majority of articles (5 of 6) utilized reactive postural control paradigms. All studies incorporated extrinsic measures of muscle fatigue, such as declines in maximal voluntary contraction or available active range of motion. The most common biomechanical postural control task outcomes were spatial measures, temporal measures, and end-points of lower extremity joint kinetics.

CONCLUSION

On the basis of systematic review of relevant literature, it appears that muscle fatigue induces clear deteriorations in reactive postural control. A paucity of high-quality studies examining anticipatory postural control supports the need for further research in this area. These results should serve to heighten awareness regarding the potential negative effects of acute muscle fatigue on postural control and support the examination of muscle endurance training as a fall risk intervention in future studies.

Footwear and Foam Surface Alter Gait Initiation of Typical Subjects

Gait initiation is the task commonly used to investigate the anticipatory postural adjustments necessary to begin a new gait cycle from the standing position. In this study, we analyzed whether and how foot-floor interface characteristics influence the gait initiation process. For this purpose, 25 undergraduate students were evaluated while performing a gait initiation task in three experimental conditions: barefoot on a hard surface (barefoot condition), barefoot on a soft surface (foam condition), and shod on a hard surface (shod condition). Two force plates were used to acquire ground reaction forces and moments for each foot separately. A statistical parametric mapping (SPM) analysis was performed in COP time series. We compared the anterior-posterior (AP) and medial-lateral (ML) resultant center of pressure (COP) paths and average velocities, the force peaks under the right and left foot, and the COP integral x force impulse for three different phases: the anticipatory postural adjustment (APA) phase (Phase 1), the swing-foot unloading phase (Phase 2), and the support-foot unloading phase (Phase 3). In Phase 1, significantly smaller ML COP paths and velocities were found for the shod condition compared to the barefoot and foam conditions. Significantly smaller ML COP paths were also found in Phase 2 for the shod condition compared to the barefoot and foam conditions. In Phase 3, increased AP COP velocities were found for the shod condition compared to the barefoot and foam conditions. SPM analysis revealed significant differences for vector COP time series in the shod condition compared to the barefoot and foam conditions. The foam condition limited the impulse-generating capacity of COP shift and produced smaller ML force peaks, resulting in limitations to body-weight transfer from the swing to the support foot. The results suggest that footwear and a soft surface affect COP and impose certain features of gait initiation, especially in the ML direction of Phase 1.

Effects of temporal constraints on medio-lateral stability when negotiating obstacles

If an obstacle suddenly appears during walking, either the crossing step can be lengthened or the precrossing step shortened to avoid the obstacle. We investigated the effects of temporal constraints on dynamic stability during step adjustments. Twelve healthy young adults avoided a virtual white planar obstacle by lengthening or shortening their steps under free or constrained conditions. When constrained, participants had only one step to avoid the obstacle. The results indicated that center of mass (COM) displacement in the mediolateral (ML) direction and the COM velocity toward the swing-leg side during the crossing step were significantly increased in the long-constraint compared with the long-free condition. Consequently, the extrapolated COM (XcoM) position at the swing foot contact was also located further toward the swing-leg side. However, the distances between the XcoM and base of support (BOS) at the swing foot contact in the ML direction was unchanged because of greater lateral foot placement. In the anteriorposterior (AP) direction, temporal constraints led to greater AP COM displacement. The XcoM-BOS distance in the AP direction was unchanged in the long-constraint condition because of greater step length. However, the value became negative in the short-constraint condition, violating the conditions for dynamic stability, because step length adjustments were obstructed by the spatial constraints of the obstacles. These results suggest that temporal constraints affect postural stability in the AP and ML directions during step adjustments. AP and ML stability at swing foot contact are maintained through adjustments of step length and lateral foot placement, respectively.

BioKin: an ambulatory platform for gait kinematic and feature assessment

A platform to move gait analysis, which is normally restricted to a clinical environment in a well-equipped gait laboratory, into an ambulatory system, potentially in non-clinical settings is introduced. This novel system can provide functional measurements to guide therapeutic interventions for people requiring rehabilitation with limited access to such gait laboratories. BioKin system consists of three layers: a low-cost wearable wireless motion capture sensor, data collection and storage engine, and the motion analysis and visualisation platform. Moreover, a novel limb orientation estimation algorithm is implemented in the motion analysis platform. The performance of the orientation estimation algorithm is validated against the orientation results from a commercial optical motion analysis system and an instrumented treadmill. The study results demonstrate a root-mean-square error less than 4° and a correlation coefficient more than 0.95 when compared with the industry standard system. These results indicate that the proposed motion analysis platform is a potential addition to existing gait laboratories in order to facilitate gait analysis in remote locations.

Biomechanical organization of gait initiation depends on the timing of affective processing

Gait initiation (GI) from a quiet bipedal posture has been shown to be influenced by the emotional state of the actor. The literature suggests that the biomechanical organization of forward GI is facilitated when pleasant pictures are shown, as compared to unpleasant pictures. However, there are inconsistencies in the literature, which could be due to the neural dynamics of affective processing. This study aimed to test this hypothesis, using a paradigm whereby participants initiated a step as soon as they saw an affective picture (i.e., onset), or as soon as the picture disappeared from the screen (i.e., offset). Pictures were a priori categorized as pleasant or unpleasant, and could also vary in their arousing properties. We analyzed center-of-pressure and center-of-gravity dynamics as a function of emotional content. We found that gait was initiated faster with pleasant images at onset, and faster with unpleasant images at offset. Also, with offset GI the peak velocity of the COG was reduced, and subjects took smaller steps, with unpleasant images relative to pleasant images. The results are discussed in terms of current knowledge regarding temporal processing of emotions, and its effects on GI.

Kinematic parameters of throwing performance in patients with schizophrenia using a markerless motion capture system

Motor dysfunction is consistently reported but understudied in schizophrenia. It has been hypothesized that this abnormality may reflect a neuro-developmental disorder underlying this illness. The main goal of this study was to analyze movement patterns used by participants with schizophrenia and healthy controls during overarm throwing performance, using a markerless motion capture system. Thirteen schizophrenia patients and 16 healthy control patients performed the overarm throwing task in a markerless motion capture system. Participants were also examined for the presence of motor neurological soft signs (mNSS) using the Brief Motor Scale. Schizophrenia patients demonstrated a less developed movement pattern with low individualization of components compared to healthy controls. The schizophrenia group also displayed a higher incidence of mNSS. The presence of a less mature movement pattern can be an indicator of neuro-immaturity and a marker for atypical neurological development in schizophrenia. Our findings support the understanding of motor dysfunction as an intrinsic part of the disorder of schizophrenia.

Postural adjustments to support surface perturbations during reaching depend upon body-target reference frame

We investigated whether target position relative to the body modifies the postural adjustments produced when reaching movements are perturbed by unexpected displacements of the support surface. Eleven healthy participants reached to a target located at their midline, acromion height and at 130% their outstretched arm length. They stood on two force plates mounted on a moveable platform, capable of delivering horizontal forward ramp-and-hold perturbations. Three types of trial were given: reach only (R), perturbations only (P) and reaching movements during which a perturbation was given at a random delay after reach onset (RP). The target could be mounted either on a frame suspended from the ceiling such that it remained world-fixed (exocentric target, RP/X) or at an equivalent position on the moving platform so that it moved with the body (egocentric target, RP/E). Arm and body 3D kinematics and muscle activity from the right tibialis anterior (rTA) and soleus (rSOL) muscles were recorded. Normalised rTA activity was significantly lower in RP than in P trials. Furthermore, long-latency rTA muscle activity was lower in RP/E than in RP/X conditions when perturbations were given during either the arm deceleration phase of reaching. The rSOL muscle activity was lowest for the RP/E (arm deceleration) condition. When balance is perturbed during reaching, the manner in which the target moves relative to the body determines the muscle activity produced in the lower-limb muscles. Furthermore, a target that moves with the body requires a different regulation of muscle activity compared with one that moves independently of the body.

Asymmetry of anticipatory postural adjustment during gait initiation

The purpose of this study was to investigate the asymmetry of anticipatory postural adjustment (APA) during gait initiation and to determine whether the process of choosing the initial swing leg affects APA during gait initiation. The participants initiated gait with the leg indicated by a start tone or initiated gait with the leg spontaneously chosen. The dependent variables of APA were not significantly different among the condition of initiating gait with the preferred leg indicated by the start tone, the condition of initiating gait with the non-preferred leg indicated by the start tone, and the condition of initiating gait with the leg spontaneously chosen. These findings fail to support the view that the process of choosing the initial swing leg affects APA during gait initiation. The lateral displacement of the center of pressure in the period in which shifting the center of pressure to the initial swing phase before initiating gait with the left leg indicated by the external cue was significantly larger than that when initiating gait with the right leg indicated by the external cue, and significantly larger than that when initiating gait with the leg spontaneously chosen. Weight shift to the initial swing side during APA during gait initiation was found to be asymmetrical when choosing the leg in response to an external cue.

Postural mechanisms to control body displacements in the performance of lateral gaze shifts

Medialateral postural control mechanisms (bodyweight distribution and center of pressure location) have been studied in static conditions. Our objective was to determine how these mechanisms are adjusted to perform voluntary movements, in our case 80° lateral gaze shifts at 0.125 Hz and 0.25 Hz. In healthy, young adults, we expected body marker (neck, lower back) and center of pressure displacements to be significantly greater in gaze shift conditions than in the stationary gaze condition. To explain these changes in center of pressure displacement, the amplitude contribution of both mechanisms was expected to increase significantly. All these results were found accordingly. Unexpectedly, the active contribution of the bodyweight distribution mechanism was negatively related to body marker displacements in the gaze shift conditions (ns in stationary condition). Moreover, changes in the contribution of the mechanisms were statistically weaker in effect size than changes in body displacement. However, the participants were not unstable because they performed the visual tasks as requested. We propose that the strength of medialateral postural control mechanisms may not only be strengthened to control challenging ML stance conditions but also slightly weakened to allow the performance of adequate body motions in ongoing tasks.

Influence of ankle loading on the relationship between temporal pressure and motor coordination during a whole-body paired task

We investigated whether ankle loading modifies the relationship between temporal pressure and motor coordination during a whole-body paired task. Eight young healthy adults standing in an erect posture performed multiple series of simultaneous rapid leg flexions paired with ipsilateral index finger extensions. They repeated the task ten times in three load conditions: unloaded, loaded (where additional 5-kg inertia was attached to the ankles), and post-loaded (immediately following the loaded condition). These conditions were conducted in two blocks of temporal pressure: self-initiated (SI) versus reaction time (RT). When participants were unloaded, the results showed that index finger extension preceded swing heel-off in RT, and conversely in SI. By contrast, when the participants were loaded, swing heel-off preceded index finger extension in both SI and RT, showing that loading modified the relationship between temporal pressure and movement synchronization in RT only. However, loading did not induce any increase in the error of synchronization. Furthermore, in both the unloaded and loaded conditions, the duration of "anticipatory postural adjustments" (APA) was shorter when the temporal pressure was increased. Interestingly, the shorter APA duration was compensated by an increase in APA amplitude. Thus, loading did not modify the relationship between temporal pressure and anticipatory postural dynamics. Post-loaded and unloaded conditions produced the same results. These results show that the central nervous system optimally adapts the relationship between temporal pressure and motor coordination to transitory changes in the mechanical properties of the lower limbs, here due to ankle loading.

Gait initiation in children with Rett syndrome

Rett syndrome is an X-linked neurodevelopmental condition mainly characterized by loss of spoken language and a regression of purposeful hand use, with the development of distinctive hand stereotypies, and gait abnormalities. Gait initiation is the transition from quiet stance to steady-state condition of walking. The associated motor program seems to be centrally mediated and includes preparatory adjustments prior to any apparent voluntary movement of the lower limbs. Anticipatory postural adjustments contribute to postural stability and to create the propulsive forces necessary to reach steady-state gait at a predefined velocity and may be indicative of the effectiveness of the feedforward control of gait. In this study, we examined anticipatory postural adjustments associated with gait initiation in eleven girls with Rett syndrome and ten healthy subjects. Muscle activity (tibialis anterior and soleus muscles), ground reaction forces and body kinematic were recorded. Children with Rett syndrome showed a distinctive impairment in temporal organization of all phases of the anticipatory postural adjustments. The lack of appropriate temporal scaling resulted in a diminished impulse to move forward, documented by an impairment in several parameters describing the efficiency of gait start: length and velocity of the first step, magnitude and orientation of centre of pressure-centre of mass vector at the instant of (swing-)toe off. These findings were related to an abnormal muscular activation pattern mainly characterized by a disruption of the synergistic activity of antagonistic pairs of postural muscles. This study showed that girls with Rett syndrome lack accurate tuning of feedforward control of gait.

By counteracting gravity, triceps surae sets both kinematics and kinetics of gait

In the single-stance phase of gait, gravity acting on the center of mass (CoM) causes a disequilibrium torque, which generates propulsive force. Triceps surae activity resists gravity by restraining forward tibial rotation thereby tuning CoM momentum. We hypothesized that time and amplitude modulation of triceps surae activity determines the kinematics (step length and cadence) and kinetics of gait. Nineteen young subjects participated in two experiments. In the gait initiation (GI) protocol, subjects deliberately initiated walking at different velocities for the same step length. In the balance-recovery (BR) protocol, subjects executed steps of different length after being unexpectedly released from an inclined posture. Ground reaction force was recorded by a large force platform and electromyography of soleus, gastrocnemius medialis and lateralis, and tibialis anterior muscles was collected by wireless surface electrodes. In both protocols, the duration of triceps activity was highly correlated with single-stance duration (GI, R (2) = 0.68; BR, R (2) = 0.91). In turn, step length was highly correlated with single-stance duration (BR, R (2) = 0.70). Control of CoM momentum was obtained by decelerating the CoM fall via modulation of amplitude of triceps activity. By modulation of triceps activity, the central nervous system (CNS) varied the position of CoM with respect to the center of pressure (CoP). The CoM-CoP gap in the sagittal plane was determinant for setting the disequilibrium torque and thus walking velocity. Thus, by controlling the gap, CNS-modified walking velocity (GI, R (2) = 0.86; BR, R (2) = 0.92). This study is the first to highlight that by merely counteracting gravity, triceps activity sets the kinematics and kinetics of gait. It also provides evidence that the surge in triceps activity during fast walking is due to the increased requirement of braking the fall of CoM in late stance in order to perform a smoother step-to-step transition.

By counteracting gravity, triceps surae sets both kinematics and kinetics of gait

In the single-stance phase of gait, gravity acting on the center of mass (CoM) causes a disequilibrium torque, which generates propulsive force. Triceps surae activity resists gravity by restraining forward tibial rotation thereby tuning CoM momentum. We hypothesized that time and amplitude modulation of triceps surae activity determines the kinematics (step length and cadence) and kinetics of gait. Nineteen young subjects participated in two experiments. In the gait initiation (GI) protocol, subjects deliberately initiated walking at different velocities for the same step length. In the balance-recovery (BR) protocol, subjects executed steps of different length after being unexpectedly released from an inclined posture. Ground reaction force was recorded by a large force platform and electromyography of soleus, gastrocnemius medialis and lateralis, and tibialis anterior muscles was collected by wireless surface electrodes. In both protocols, the duration of triceps activity was highly correlated with single-stance duration (GI, R (2) = 0.68; BR, R (2) = 0.91). In turn, step length was highly correlated with single-stance duration (BR, R (2) = 0.70). Control of CoM momentum was obtained by decelerating the CoM fall via modulation of amplitude of triceps activity. By modulation of triceps activity, the central nervous system (CNS) varied the position of CoM with respect to the center of pressure (CoP). The CoM-CoP gap in the sagittal plane was determinant for setting the disequilibrium torque and thus walking velocity. Thus, by controlling the gap, CNS-modified walking velocity (GI, R (2) = 0.86; BR, R (2) = 0.92). This study is the first to highlight that by merely counteracting gravity, triceps activity sets the kinematics and kinetics of gait. It also provides evidence that the surge in triceps activity during fast walking is due to the increased requirement of braking the fall of CoM in late stance in order to perform a smoother step-to-step transition.

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

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

Kinetic changes in gait during low magnitude military load carriage

Indian infantry soldiers carry smaller magnitudes of loads for operational requirements. The ground reaction forces (GRFs) and impulse responses of 10 healthy male Indian infantry soldiers were collected while they walked carrying operational loads between 4.2 and 17.5 kg (6.5-27.2% of mean body weight (BW)) and a control condition of no external load (NL). The GRF and impulse components were normalised for BW, and data for each load condition were compared with NL in each side applying one-way analysis of variance followed by Dunnett's post hoc test. Right foot data were compared with corresponding left foot GRF data for all load conditions and NL. There were significant increases in vertical and anteroposterior GRFs with increase in load. Left and right feet GRF data in corresponding load conditions were significantly different in anteroposterior plane. No significant change was observed in the temporal components of support phase of gait. Changes in impulse parameter were observed in the anteroposterior and vertical planes while carrying load greater than 23 and 16.6% of BW for the right foot and left foot, respectively. Result indicates that smaller magnitudes of loads produced kinetic changes proportional to system weight, similar to heavier loads with the possibility of increased injury risk. Observed smaller asymmetric changes in gait may be considered as postural adjustment due to load. Unique physical characteristics of Indian soldiers and the probable design shortcomings of the existing backpack might have caused significant changes in GRF and peak impulse during smaller load carriage.

Isolated and combined effects of asymmetric stance and pushing movement on the anticipatory and compensatory postural control

OBJECTIVE

To investigate effects of symmetric and asymmetric stance and pushing movement on anticipatory and compensatory postural adjustments (APAs and CPAs).

METHODS

Ten healthy volunteers stood symmetrically (feet parallel) or asymmetrically (one foot forward and the other backward) and pushed a handle with both hands or right or left hand. Bilateral EMG activity of the trunk and leg muscles and center of pressure (COP) displacements in the anterior-posterior (AP) and medial-lateral (ML) directions were recorded and analyzed during the APAs and CPAs.

RESULTS

Isolated asymmetry of stance was associated with larger muscle activity of the backward leg while isolated asymmetry of pushing movement induced larger trunk muscle activity on the contralateral side. A combined asymmetry of stance and pushing movement resulted in the increase or decrease of the thigh muscle activity and ML COP displacement depending on whether both asymmetries were induced on the same side of the body or on opposite sides.

CONCLUSIONS

Both isolated and combined asymmetries affect APAs and CPAs in pushing. Using combined asymmetry of stance and arm movement might be beneficial in performing pushing activity.

SIGNIFICANCE

The outcome of the study provides a basis for studying postural control in individuals with unilateral impairment while performing daily tasks involving pushing.

Human body parts tracking and kinematic features assessment based on RSSI and inertial sensor measurements

Acquisition of patient kinematics in different environments plays an important role in the detection of risk situations such as fall detection in elderly patients, in rehabilitation of patients with injuries, and in the design of treatment plans for patients with neurological diseases. Received Signal Strength Indicator (RSSI) measurements in a Body Area Network (BAN), capture the signal power on a radio link. The main aim of this paper is to demonstrate the potential of utilizing RSSI measurements in assessment of human kinematic features, and to give methods to determine these features. RSSI measurements can be used for tracking different body parts' displacements on scales of a few centimeters, for classifying motion and gait patterns instead of inertial sensors, and to serve as an additional reference to other sensors, in particular inertial sensors. Criteria and analytical methods for body part tracking, kinematic motion feature extraction, and a Kalman filter model for aggregation of RSSI and inertial sensor were derived. The methods were verified by a set of experiments performed in an indoor environment. In the future, the use of RSSI measurements can help in continuous assessment of various kinematic features of patients during their daily life activities and enhance medical diagnosis accuracy with lower costs.

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.

Three components of postural control associated with pushing in symmetrical and asymmetrical stance

A number of occupational and leisure activities that involve pushing are performed in symmetrical or asymmetrical stance. The goal of this study was to investigate early postural adjustments (EPAs), anticipatory postural adjustments (APAs), and compensatory postural adjustments (CPAs) during pushing performed while standing. Ten healthy volunteers stood in symmetrical stance (with feet parallel) or in asymmetrical stance (staggered stance with one foot forward) and were instructed to use both hands to push forward the handle of a pendulum attached to the ceiling. Bilateral EMG activity of the trunk and leg muscles and the center of pressure (COP) displacements in the anterior-posterior (AP) and medial-lateral (ML) directions were recorded and analyzed during the EPAs, APAs, and CPAs. The EMG activity and the COP displacement were different between the symmetrical and asymmetrical stance conditions. The COP displacements in the ML direction were significantly larger in staggered stance than in symmetrical stance. In staggered stance, the EPAs and APAs in the thigh muscles of the backward leg were significantly larger, and the CPAs were smaller than in the forward leg. There was no difference in the EMG activity of the trunk muscles between the stance conditions. The study outcome confirmed the existence of the three components of postural control (EPAs, APAs, and CPAs) in pushing. Moreover, standing asymmetrically was associated with asymmetrical patterns of EMG activity in the lower extremities reflecting the stance-related postural control during pushing. The study outcome provides a basis for studying postural control during other daily activities involving pushing.