Impairment of posturo-kinetic co-ordination during initiation of forward oriented stepping movements in parkinsonian patients

A neuromechanical model characterizing the motor planning and posture control in the voluntary lean in Parkinson's disease

Parkinson's disease targets patients' cognitive and motor abilities, including postural control. Many studies have been carried out to introduce mathematical models for a better understanding of postural control in such patients and the relation between the model parameters and the clinical assessments. So far, these studies have addressed this connection merely in static tests, such as quiet stance. The aim of this study is to develop a model for voluntary lean, and as such, identify the model parameters for both PD patients and healthy subjects from experimental data. The proposed model comprises planning and control sections. The model parameters for the planning section were extracted from the time response characteristics. Parameters for the control section were identified based on the spatial characteristics of the center-of-pressure (COP) response using an optimization process. 24 PD patients along with 24 matched healthy subjects participated in the study. The results showed a significant difference between the two groups in terms of temporal parameters for the planning section. This difference emphasizes bradykinesia as an essential symptom of PD. Also, differences were found for the postural control section. In all directions, the proportional gain of the feedback controller was significantly larger in PD patients; however, the gain of the feedforward controller was significantly smaller in PD patients. Furthermore, the control gains were strongly correlated with the clinical scales (Functional Reach Test and Unified Parkinson's Disease Rating Scale) in certain directions. In conclusion, the new model helps to better understand and quantify some PD symptoms in voluntary lean tasks.

Gait initiation in Parkinson's disease: comparison of timing and displacement during anticipatory postural adjustments as a function of motor severity and apathy in a large cohort

Introduction

Gait initiation is preceded by three anticipatory postural adjustment (APA) phases. In Parkinson's disease (PD) generated force, displacement and timing during APA differ from healthy controls. APA might be influenced by disease status, weight or emotion. It is unknown how motor severity, disease duration or presence of apathy influences APA timing and displacement.

Methods

We included 99 people with PD and 50 healthy controls (HC) to perform five gait initiation trials following an auditory cue. Force plates measured timing and center of pressure (CoP) displacement during APA phases.

Results

Time to gait initiation (tGI) was higher in the PD group (p < 0.001, t = 2.74, 95%CI (0.008, 0.066)). The first two APA phases (APA1 and APA2a) lasted longer in PD (respectively p < 0.001, t = 3.87, 95%CI (0.091, 0.28) and p < 0.001, t = 4.1, 95%CI (0.031, 0.091)). Mean CoP displacement, variability in timing and displacement did not differ. A multiple regression model was used to determine if clinical variables were related to gait initiation parameters. tGI was predicted by age (p < 0.001) and weight (p = 0.005). The duration of APA1 was predicted by weight (p = 0.006) and APA2a by age (p < 0.001). Variability in duration of the locomotor phase (LOC) was predicted by age (p < 0.001).

Conclusion

tGI and initial APA phases are longer in PD than in HC. There are no significant differences in variability of timing or displacement between the two groups. Gait initiation parameters are independent of disease duration, motor severity, medication usage or apathy in PD. Our findings suggest that cueing does not speed up gait initiation but reduces variability.

Startle-induced rapid release of a gait initiation sequence in Parkinson's disease with freezing of gait

OBJECTIVE

Freezing of gait (FOG) in Parkinson's disease (PD) is characterized by the inability to initiate stepping, despite the intention to do so. This study used a startling acoustic stimulus paradigm to examine if the capacity to select, prepare and initiate gait under simple and choice reaction time conditions are impaired in people with PD and FOG.

METHODS

Thirty individuals (10 PD with FOG, 10 PD without FOG, and 10 controls) performed an instructed-delay gait initiation task under simple and choice reaction time conditions. In a subset of trials, a startle stimulus (124 dB) was presented 500 ms before the time of the imperative go-cue. Anticipatory postural adjustments preceding and accompanying gait initiation were quantified.

RESULTS

The presentation of a startling acoustic stimulus resulted in the rapid initiation of an anticipatory postural adjustment sequence during both the simple and choice reaction time tasks in all groups.

CONCLUSIONS

The neural capacity to prepare the spatial and temporal components of gait initiation remains intact in PD individuals with and without FOG.

SIGNIFICANCE

The retained capacity to prepare anticipatory postural adjustments in advance may explain why external sensory cues are effective in the facilitation of gait initiation in people with PD with FOG.

Vertical-horizontal illusory effects with gaze restrictions do not change length estimations using the lower limb

Gaze direction and use of visual feedback can affect illusory influences over perceptions and manual length size estimates of the vertical-horizontal (V-H) illusion, in which the vertical, bisecting segment of an inverted T (IT) appears longer than the horizontal, bisected segment. We questioned whether V-H illusory influences would also exist for the lower limb. Participants stepped forward in an attempt to make the toe-to-toe distance of their dominant foot equal to a short or long bisecting segment length of a vertically projected IT. Performances under three gaze conditions included: maintaining gaze on the IT intersection throughout a trial for target fixation (TF); viewing the intersection for 4 s then looking down and performing the step for movement fixation (MF); and viewing the intersection for 4 s then maintaining gaze on the remembered location of the intersection and performing the step for remembered target fixation (RTF). Variables included step displacement, peak velocity (PV), and normalized ground reaction force amplitude (GRFampN), as well as time to peak and peak amplitude of the center of pressure (COPtime and COPamp, respectively). Main effects of gaze on PV, GRFampN, COPtime, and COPamp revealed lower values for MF compared to TF and RTF, which did not exist for step displacement. No significant correlations existed between step displacement and other variables across participants. Together, we found evidence to suggest differences between movement planning and movement completion. Exploitation of deceptive visual cues can guide step planning and early step execution, but do not guide final step estimations.

Postural Adjustments and Biomechanics During Gait Initiation and Obstacle Negotiation: A Comparison Between Akinetic-Rigid and Hyperkinetic Parkinson's Disease

Background: Individuals with Parkinson's disease (PD) exhibit different combinations of motor symptoms. The most frequent subtypes are akinetic-rigid (AK-R) and hyperkinetic (HYP). Motor symptoms, such as rigidity and bradykinesia, can directly affect postural adjustments and performance in daily tasks, like gait initiation and obstacles negotiation, increasing the risk of falls and functional dependence. Objective: To compare postural adjustments and biomechanical parameters during the gait initiation and obstacle negotiation of people with AK-R and HYP PD and correlate with functional mobility and risk of falls. Methods: Cross-sectional study. Thirty-three volunteers with PD were divided into two groups according to clinical motor manifestations: AK-R (n = 16) and HYP (n = 17). We assessed the anticipatory (APA), compensatory (CPA) postural adjustments analyzing kinematic, kinetic and, electromyographic parameters during the gait initiation and obstacle negotiation tests. We applied independent T-tests and Pearson correlation tests for comparisons and correlations, respectively (α = 0.05). Results: In the APA phase of the gait initiation test, compared to the functional HYP group, the AK-R group showed shorter time for single support (p = 0.01), longer time for double support (p = 0.01) accompanied by a smaller first step (size, p = 0.05; height, p = 0.04), and reduced muscle activation of obliquus internus (p = 0.02). Similarly, during the first step in the obstacle negotiation test, the AK-R group showed less step height (p = 0.01) and hip excursion (p = 0.02), accompanied by a reduced mediolateral displacement of the center of pressure (p = 0.02) during APA, and activation of the gluteus medius (p = 0.02) and the anterior tibialis (p = 0.04) during CPA in comparison with HYP group. Conclusion: The findings suggest that people with AK-R present impaired postural adjustments during gait initiation and obstacles negotiation compared to hyperkinetic PD. Based on defined motor symptoms, the proposition presented here revealed consistent postural adjustments during complex tasks and, therefore, may offer new insights onto PD motor evaluation and neurorehabilitation.

Effects of treadmill training on gait of elders with Parkinson's disease: a literature review

Parkinson's disease is the second most common neurodegenerative disorder in old age. Aging process for elders with Parkinson's disease can induce gait disturbances with more functional disabilities than for elders without the disease. Treadmill training as a therapy has resulted in notable effects on the gait of patients with Parkinson's disease and may be a resource for geriatric neurological rehabilitation. This review aimed to study the effects on gait after treadmill training in elderly patients with Parkinson's disease. The search was performed in the databases PubMed®, LILACS, PEDro and EMBASE, with the following keywords: "Parkinson's disease", "elderly", "treadmill training" and "gait evaluation". The quality of the studies included was assessed by PEDro Scale. Eleven studies met the inclusion and exclusion criteria. Eight studies were randomized, and only one did a follow-up. One can observe in this review that treadmill training with or without weight support (at least 20 minutes, two to three times a week, with progressive increase of loads, for minimum of 6 weeks) in elderly patients with the Parkinson's disease was effective to improve gait. In addition, both were considered safe (since some studies described the use of belts, even in unsupported training) and can be associated with therapies complementary to gait, such as repetitive transcranial magnetic stimulation, visual cues or anodal transcranial direct current stimulation. Treadmill training in elderly patients with Parkinson's disease is an intervention that improves gait outcomes, but further studies are required for better proofs.

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.

Cortical Oscillations during Gait: Wouldn't Walking be so Automatic?

Gait is often considered as an automatic movement but cortical control seems necessary to adapt gait pattern with environmental constraints. In order to study cortical activity during real locomotion, electroencephalography (EEG) appears to be particularly appropriate. It is now possible to record changes in cortical neural synchronization/desynchronization during gait. Studying gait initiation is also of particular interest because it implies motor and cognitive cortical control to adequately perform a step. Time-frequency analysis enables to study induced changes in EEG activity in different frequency bands. Such analysis reflects cortical activity implied in stabilized gait control but also in more challenging tasks (obstacle crossing, changes in speed, dual tasks…). These spectral patterns are directly influenced by the walking context but, when analyzing gait with a more demanding attentional task, cortical areas other than the sensorimotor cortex (prefrontal, posterior parietal cortex, etc.) seem specifically implied. While the muscular activity of legs and cortical activity are coupled, the precise role of the motor cortex to control the level of muscular contraction according to the gait task remains debated. The decoding of this brain activity is a necessary step to build valid brain-computer interfaces able to generate gait artificially.

Pre-Movement Cortico-Muscular Dynamics Underlying Improved Parkinson Gait Initiation after Instructed Arm Swing

BACKGROUND

The supplementary motor area (SMA) is implicated in both motor initiation and stereotypic multi-limb movements such as walking with arm swing. Gait in Parkinson's disease exhibits starting difficulties and reduced arm swing, consistent with reduced SMA activity.

OBJECTIVE

We tested whether enhanced arm swing could improve Parkinson gait initiation and assessed whether increased SMA activity during preparation might facilitate such improvement.

METHODS

Effects of instructed arm swing on cortical activity, muscle activity and kinematics were assessed by ambulant EEG, EMG, accelerometers and video in 17 Parkinson patients and 19 controls. At baseline, all participants repeatedly started walking after a simple auditory cue. Next, patients started walking at this cue, which now meant starting with enhanced arm swing. EEG changes over the putative SMA and leg motor cortex were assessed by event related spectral perturbation (ERSP) analysis of recordings at Fz and Cz.

RESULTS

Over the putative SMA location (Fz), natural PD gait initiation showed enhanced alpha/theta synchronization around the auditory cue, and reduced alpha/beta desynchronization during gait preparation and movement onset, compared to controls. Leg muscle activity in patients was reduced during preparation and movement onset, while the latter was delayed compared to controls. When starting with enhanced arm swing, these group differences virtually disappeared.

CONCLUSION

Instructed arm swing improves Parkinson gait initiation. ERSP normalization around the cue indicates that the attributed information may serve as a semi-internal cue, recruiting an internalized motor program to overcome initiation difficulties.

Gait bradykinesia: difficulty in switching posture/gait measured by the anatomical y-axis vector of the sole in Parkinson's disease

This study in Parkinson's disease examined how spatiotemporal parameters in gait bradykinesia link to difficulty in terminating posture and initiating gait locomotion. 41 idiopathic Parkinson's disease patients and 15 age-matched healthy subjects participated in this study. After the patients fixated on a visual-fixation-target, gait was triggered by visual or vocal cue-stimulus. The LED instructed subjects to quickly achieve their own comfortable walking speed on a level floor. The posterior-anterior force of the y-axis vectors of sole relating to soleus and tibialis-anterior EMGs were examined. Step-gain was defined as the duration of the swing-phase relative that of the contralateral stance-phase. Dynamic-ratio was defined as the duration the fore-foot phase relative to that of the ipsilateral stance-phase as forward-oriented movement in each step. The pause in tonic soleus EMG was defined as the off-latency of posture (termination) and the onset of a tibialis-anterior EMG-burst as the on-latency of gait. In Parkinson's disease, soleus off-latencies were prolonged, whereas tibialis-anterior on-latencies were less prolonged. Unsynchronized off/on-latency differences correlated with spatiotemporal parameters of dynamic-ratios, step-gains, gait-initiation, and gait speed in gait bradykinesia. Delayed EMG off-latencies correlated with prolonged motor-latencies in gait bradykinesia as delayed initial backward body-shift. A delayed and deficient initial backward body-shift of y-axis vector was linked to each difficulty in terminating posture and initiating gait, changing to random gait akinesia. Gait bradykinesia in Parkinson's disease stemmed from unsynchronized off/on-latency EMG activities, linking to each difficulty in terminating posture and initiating gait synergic movement through an initial backward body-shift.

Abnormal Muscle Activity and Variability Before, During, and After the Occurrence of Freezing in Parkinson's Disease

Freezing of gait (FOG) is often experienced in advanced stages of Parkinson's disease (PD) and can lead to an increased risk of falls. Although spatiotemporal characteristics of FOG are well-described, their underlying neuromuscular mechanisms remain poorly understood. Several studies have demonstrated an abnormal activation of distal muscles of the lower limb and coordination impairments during gait in people with PD (pwPD). However, few have investigated how various characteristics of electromyograms (EMGs) change before, during and after a freezing episode (FE). Our objective was to quantify changes in proximal and distal leg muscle activity associated with FEs. In this study, 12 pwPD, confirmed as freezers, performed a repetitive stepping-in-place task used to elicit FE. Surface EMGs were collected from proximal [rectus femoris and biceps femoris (BF)] and distal [tibialis anterior (TA) and gastrocnemius medialis (GM)] muscles. Data epochs of 500 ms were extracted from EMG time series at four different periods: baseline, 2 s before a FE, during a FE, and 2 s after a FE. For each epoch, EMG amplitude [root-mean-square (RMS)], variability [coefficient of variation (CoV)], and inter-muscle functional connectivity (mutual information) were quantified. Results from the analysis of 21 FEs show a significant main effect of Period for EMG amplitude in bilateral TA and in the least affected GM (p < 0.01), with decreased activation before freezing that remained low during and after the FE. On the other hand, a main effect of Period was also found in bilateral BF muscles (p < 0.01) but with increased activation before freezing that was generally sustained during and after FE. Main effects of Period were also found for all measures of variability, except for the least affected GM, showing reduced variability during the FE that returned to baseline in all muscles except both TA. Moreover, an increase in functional connectivity between the least affected distal muscles was seen before the FE. Our findings confirm that many characteristics of EMG patterns of both distal and proximal leg muscles change throughout periods of a FE, suggesting both impairment and adaptive strategies from proximal muscles.

Functional motor control deficits in older FMR1 premutation carriers

Individuals with fragile X mental retardation 1 (FMR1) gene premutations are at increased risk for fragile X-associated tremor/ataxia syndrome (FXTAS) during aging. However, it is unknown whether older FMR1 premutation carriers, with or without FXTAS, exhibit functional motor control deficits compared with healthy individuals. The purpose of this study, therefore, was to determine whether older FMR1 premutation carriers exhibit impaired ability to perform functional motor tasks. Eight FMR1 premutation carriers (age: 58.88 ± 8.36 years) and eight age- and sex-matched healthy individuals (60.13 ± 9.25 years) performed (1) a steady isometric force control task with the index finger at 20% of their maximum voluntary contraction (MVC) and; (2) a single-step task. During the finger abduction task, firing rate of multiple motor units of the first dorsal interosseous (FDI) muscle was recorded. Compared with healthy controls, FMR1 premutation carriers exhibited (1) greater force variability (coefficient of variation of force) during isometric force (1.48 ± 1.02 vs. 0.63 ± 0.37%; P = 0.04); (2) reduced firing rate of multiple motor units during steady force, and; (3) reduced velocity of their weight transfer during stepping (156.62 ± 26.24 vs. 191.86 ± 18.83 cm/s; P = 0.01). These findings suggest that older FMR1 premutation carriers exhibit functional motor control deficits that reflect either subclinical issues associated with premutations independent of FXTAS, or prodromal markers of the development of FXTAS.

Gait bradykinesia in Parkinson's disease: a change in the motor program which controls the synergy of gait

This study examined how gait bradykinesia is changed by the motor programming in Parkinson's disease. Thirty-five idiopathic Parkinson's disease patients and nine age-matched healthy subjects participated in this study. After the patients fixated on a visual-fixation target (conditioning-stimulus), the voluntary-gait was triggered by a visual on-stimulus. While the subject walked on a level floor, soleus, tibialis anterior EMG latencies, and the y-axis-vector of the sole-floor reaction force were examined. Three paradigms were used to distinguish between the off-/on-latencies. The gap-task: the visual-fixation target was turned off; 200 ms before the on-stimulus was engaged (resulting in a 200 ms-gap). EMG latency was not influenced by the visual-fixation target. The overlap-task: the on-stimulus was turned on during the visual-fixation target presentation (200 ms-overlap). The no-gap-task: the fixation target was turned off and the on-stimulus was turned on simultaneously. The onset of EMG pause following the tonic soleus EMG was defined as the off-latency of posture (termination). The onset of the tibialis anterior EMG burst was defined as the on-latency of gait (initiation). In the gap-task, the on-latency was unchanged in all of the subjects. In Parkinson's disease, the visual-fixation target prolonged both the off-/on-latencies in the overlap-task. In all tasks, the off-latency was prolonged and the off-/on-latencies were unsynchronized, which changed the synergic movement to a slow, short-step-gait. The synergy of gait was regulated by two independent sensory-motor programs of the off- and on-latency levels. In Parkinson's disease, the delayed gait initiation was due to the difficulty in terminating the sensory-motor program which controls the subject's fixation. The dynamic gait bradykinesia was involved in the difficulty (long off-latency) in terminating the motor program of the prior posture/movement.

Effects of phase proprioceptive training on balance in patients with chronic stroke

[Purpose] This study aimed to investigate the effect of phase proprioceptive training on balance in patients with chronic stroke. [Subjects and Methods] Participants included 30 patients with stroke who were randomly assigned to the proprioceptive training group (n=15) or control group (n=15). Participants in the proprioceptive training group underwent proprioceptive training and received general physical therapy each for a total of 20 thirty-minute sessions, five times per week, during a period of four weeks; the control group received general physical therapy for a total of 20 sixty-minute sessions, five times per week, during a period of four weeks. [Results] All participants were evaluated with the Berg Balance Scale, Timed Up and Go (TUG) test, and Activities-specific Balance Confidence (ABC) Scale instrument before and after intervention. After training, the differences in BBS, TUG, and ABC scores in the proprioceptive group were significantly greater than those in the control group. [Conclusion] In conclusion, proprioceptive training was effective on balance ability. Therefore, proprioceptive training may be efficient when combining general physical therapy with phase proprioceptive training for patients with impairments of balance. Further research is needed to investigate proprioceptive training methods.

Recovery from Multiple APAs Delays Gait Initiation in Parkinson's Disease

Background: Freezing of gait in Parkinson’s disease (PD) has been linked with deficits in inhibitory control, but causal mechanisms are not established. Freezing at gait initiation (start hesitation) is often accompanied by multiple anticipatory postural adjustments (APAs). If inhibition deficits contribute to freezing by interfering with ability to inhibit initial weight shifts in the wrong direction, then PD subjects should experience more episodes of multiple APAs than healthy controls (HCs) do. If inhibition deficits contribute to freezing by interfering with ability to release a previously inhibited step following multiple APAs, then step onset following multiple APAs should be delayed more in people with PD than in HCs.

Methods: Older adults with PD and HC subjects rapidly initiated stepping in response to a light cue in blocks of simple (SRT) and choice (CRT) conditions. We recorded kinematics and ground reaction forces, and we administered the Stroop task to assess inhibitory control.

Results: Multiple APAs were more common in CRT than SRT conditions but were equally common in HC and PD subjects. Step onsets were delayed in both conditions and further delayed in trials with multiple APAs, except for HC subjects in SRT trials. Poor Stroop performance correlated with many multiple APAs, late step onset, and rearward position of center of mass (COM) at cue presentation. Forward motion of the COM during the APA was higher in trials with multiple APAs than in trials with single APAs, especially in CRT trials and in PD subjects without self-reported freezing.

Conclusion: Start hesitation is not caused by multiple APAs per se, but may be associated with difficulty recovering from multiple APAs, due to difficulty releasing a previously inhibited step.

Subliminal gait initiation deficits in rapid eye movement sleep behavior disorder: A harbinger of freezing of gait?

BACKGROUND

Muscle activity during rapid eye movement sleep is markedly increased in people with rapid eye movement sleep behavior disorder and people with Parkinson's disease (PD) who have freezing of gait. This study examined whether individuals with rapid eye movement sleep behavior disorder who do not have a diagnosis of PD show abnormalities in gait initiation that resemble the impairments observed in PD and whether there is a relationship between these deficits and the level of rapid eye movement sleep without atonia.

METHODS

Gait initiation and polysomnography studies were conducted in 4 groups of 10 participants: rapid eye movement sleep behavior disorder, PD with and without freezing of gait, and controls.

RESULTS

Significant reductions were seen in the posterior shift of the center of pressure during the propulsive phase of gait initiation in the groups with rapid eye movement sleep behavior disorder and PD with freezing of gait when compared with controls and PD nonfreezers. These reductions negatively correlated with the amount of rapid eye movement sleep without atonia. The duration of the initial dorsiflexor muscle burst during gait initiation was significantly reduced in both PD groups and the rapid eye movement sleep behavior disorder cohort.

CONCLUSIONS

These results provide evidence that people with rapid eye movement sleep behavior disorder, prior to a diagnosis of a degenerative neurologic disorder, show alterations in the coupling of posture and gait similar to those seen in PD. The correlation between increased rapid eye movement sleep without atonia and deficits in forward propulsion during the push-off phase of gait initiation suggests that abnormities in the regulation of muscle tone during rapid eye movement sleep may be related to the pathogenesis of freezing of gait. © 2016 International Parkinson and Movement Disorder Society.

Anticipatory postural adjustments are unaffected by age and are not absent in patients with the freezing of gait phenomenon

In bipedal gait, the initiation of the first step is preceded by a complex sequence of movements which shift the centre of mass of the body towards the stance foot to allow for a step of the swing foot. These anticipatory postural adjustments (APAs) have been investigated in order to elucidate movement strategies in healthy and diseased persons. We studied the influence of several external parameters (age, type of step initiation) on APAs and investigated whether Parkinsonian patients may have different APAs. As a result, we found that externally elicited steps were preceded by faster and larger APAs than self-timed steps. Parkinsonian patients without the freezing of gait (FOG) phenomenon showed overall slightly reduced APAs but did not clearly differ from patients with FOG. Multiple APAs were seen in up to 25 % of the steps of the patients and in a much lower percentage of the steps of control subjects. The results indicate that APAs are significantly influenced by the timing of a step, i.e. are larger in externally elicited steps. The patients showed an overall preserved APA pattern but slowed movements and amplitude, indicating that increased bradykinesia due to progressive illness is a plausible explanation for these findings. The freezing phenomenon is not explained by a general absence or massive reduction in APA measures.

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.

Validity and reliability of an IMU-based method to detect APAs prior to gait initiation

Anticipatory postural adjustments (APAs) prior to gait initiation have been largely studied in traditional, laboratory settings using force plates under the feet to characterize the displacement of the center of pressure. However clinical trials and clinical practice would benefit from a portable, inexpensive method for characterizing APAs. Therefore, the main objectives of this study were (1) to develop a novel, automatic IMU-based method to detect and characterize APAs during gait initiation and (2) to measure its test-retest reliability. Experiment I was carried out in the laboratory to determine the validity of the IMU-based method in 10 subjects with PD (OFF medication) and 12 control subjects. Experiment II was carried out in the clinic, to determine test-retest reliability of the IMU-based method in a different set of 17 early-to-moderate, treated subjects with PD (tested ON medication) and 17 age-matched control subjects. Results showed that gait initiation characteristics (both APAs and 1st step) detected with our novel method were significantly correlated to the characteristics calculated with a force plate and motion analysis system. The size of APAs measured with either inertial sensors or force plate was significantly smaller in subjects with PD than in control subjects (p<0.05). Test-retest reliability for the gait initiation characteristics measured with inertial sensors was moderate-to-excellent (0.56<ICC<0.82) for both groups. Our findings support the feasibility of automatically characterizing postural preparation and gait initiation with body-worn inertial sensors that would be practical for unsupervised clinical and home settings.

What startles tell us about control of posture and gait

Recently, there has been an increase in studies evaluating startle reflexes and StartReact, many in tasks involving postural control and gait. These studies have provided important new insights. First, several experiments indicate a superimposition of startle reflex activity on the postural response during unexpected balance perturbations. Overlap in the expression of startle reflexes and postural responses emphasizes the possibility of, at least partly, a common substrate for these two types of behavior. Second, it is recognized that the range of behaviors, susceptible to StartReact, has expanded considerably. Originally this work was concentrated on simple voluntary ballistic movements, but gait initiation, online step adjustments and postural responses can be initiated earlier by a startling stimulus as well, indicating advanced motor preparation of posture and gait. Third, recent experiments on StartReact using TMS and patients with corticospinal lesions suggest that this motor preparation involves a close interaction between cortical and subcortical structures. In this review, we provide a comprehensive overview on startle reflexes, StartReact, and their interaction with posture and gait.

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.

Tai Chi Exercise can Improve the Obstacle Negotiating Ability of People with Parkinson's Disease: A Preliminary Study

[Purpose] The purpose of this study was to examine the effects of Tai Chi (TC)-based exercise on dynamic postural control during obstacle negotiation by subjects with mild or moderate Parkinson’s disease (PD). [Subjects] Twelve subjects (mean age, 65.3±6.1 years) diagnosed with idiopathic PD were enrolled for this study. [Methods] All the subjects were tested a week before and 12 weeks after the initiation of the TC exercise. In the test, they were instructed to negotiate an obstacle from the position of quiet stance at a normal speed. They were trained with TC exercise that emphasized multidirectional shift in weight bearing from bilateral to unilateral support, challenging the postural stability, three times per week for 12 weeks. Center of pressure (COP) trajectory variables before and after TC exercise were measured using two force plates. [Results] A comparison of the results between pre- and post-intervention showed a statistically significant improvement in anteroposterior and mediolateral displacement of COP. [Conclusion] Twelve weeks of TC exercise may be an effective and safe form of stand-alone behavioral intervention for improving the dynamic postural stability of patients with PD.

Why we should study gait initiation in Parkinson's disease

The gait initiation process is of particular interest in Parkinson's disease because it combines motor and cognitive components of movement preparation (referred to as anticipatory postural adjustments) and movement execution (the step by itself). Moreover, gait initiation in Parkinson's disease is often affected by motor blocks (a subtype of the "freezing of gait" phenomenon). Gait initiation disturbances in Parkinson's disease include delayed release of anticipatory postural adjustments, hypokinetic anticipatory postural adjustments (reduced scaling) and bradykinetic anticipatory postural adjustments (abnormal timing). The most extreme form is freezing of gait with sometimes the absence of anticipatory postural adjustments. Other phenomena can be also described in some freezing patients (such as multiple anticipatory postural adjustments, described clinically as "knee trembling"). The fact that emotion, attention, external triggers and dopaminergic drugs can all modify this motor program suggests the existence of a complex pathophysiological mechanism that involves not only locomotor networks but also cortical areas and the basal ganglia system. Abnormal coupling between standing posture and anticipatory postural adjustments and between the latter and step execution appears to be a crucial part of the pathophysiological mechanism. Although external cueing appears to be of interest, few studies have provided evidence of the efficacy of various rehabilitation methods in routine care.

Self-triggered assistive stimulus training improves step initiation in persons with Parkinson's disease

BACKGROUND

Prior studies demonstrated that hesitation-prone persons with Parkinson's disease (PDs) acutely improve step initiation using a novel self-triggered stimulus that enhances lateral weight shift prior to step onset. PDs showed reduced anticipatory postural adjustment (APA) durations, earlier step onsets, and faster 1st step speed immediately following stimulus exposure.

OBJECTIVE

This study investigated the effects of long-term stimulus exposure.

METHODS

Two groups of hesitation-prone subjects with Parkinson's disease (PD) participated in a 6-week step-initiation training program involving one of two stimulus conditions: 1) Drop. The stance-side support surface was lowered quickly (1.5 cm); 2) Vibration. A short vibration (100 ms) was applied beneath the stance-side support surface. Stimuli were self-triggered by a 5% reduction in vertical force under the stance foot during the APA. Testing was at baseline, immediately post-training, and 6 weeks post-training. Measurements included timing and magnitude of ground reaction forces, and step speed and length.

RESULTS

Both groups improved their APA force modulation after training. Contrary to previous results, neither group showed reduced APA durations or earlier step onset times. The vibration group showed 55% increase in step speed and a 39% increase in step length which were retained 6 weeks post-training. The drop group showed no stepping-performance improvements.

CONCLUSIONS

The acute sensitivity to the quickness-enhancing effects of stimulus exposure demonstrated in previous studies was supplanted by improved force modulation following prolonged stimulus exposure. The results suggest a potential approach to reduce the severity of start hesitation in PDs, but further study is needed to understand the relationship between short- and long-term effects of stimulus exposure.

The interaction of postural and voluntary strategies for stability in Parkinson's disease

This study assessed the effects of stability constraints of a voluntary task on postural responses to an external perturbation in subjects with Parkinson's disease (PD) and healthy elderly participants. Eleven PD subjects and twelve control subjects were perturbed with backward surface translations while standing and performing two versions of a voluntary task: holding a tray with a cylinder placed with the flat side down [low constraint (LC)] or with the rolling, round side down [high constraint (HC)]. Participants performed alternating blocks of LC and HC trials. PD participants accomplished the voluntary task as well as control subjects, showing slower tray velocity in the HC condition compared with the LC condition. However, the latency of postural responses was longer in the HC condition only for control subjects. Control subjects presented different patterns of hip-shoulder coordination as a function of task constraint, whereas PD subjects had a relatively invariant pattern. Initiating the experiment with the HC task led to 1) decreased postural stability in PD subjects only and 2) reduced peak hip flexion in control subjects only. These results suggest that PD impairs the capacity to adapt postural responses to constraints imposed by a voluntary task.

Anticipatory postural adjustments during step initiation: elicitation by auditory stimulation of differing intensities

Step initiation is associated with anticipatory postural adjustments (APAs) that vary according to the speed of the first step. When step initiation is elicited by a "go" signal (i.e. in a reaction time task), the presentation of an unpredictable, intense, acoustic startling stimulus (engaging a subcortical mechanism) simultaneously with or just before the imperative "go" signal is able to trigger early-phase APAs. The aim of the present study was to better understand the mechanisms underlying APAs during step initiation. We hypothesized that the early release of APAs by low-intensity, non-startling stimuli delivered long before an imperative "go" signal indicates the involvement of several different mechanisms in triggering APAs (and not just acoustic reflexes triggering brainstem structures). Fifteen healthy subjects were asked to respond to an imperative visual "go" signal by initiating a step with their right leg. A brief, binaural 40, 80 or 115 dB auditory stimulus was given 1.4 s before the "go" signal. Participants were instructed not to respond to the auditory stimulus. The centre of pressure trajectory and the electromyographic activity of the orbicularis oculi, sternocleidomastoid and tibialis anterior muscles were recorded. All three intensities of the auditory stimulus were able to evoke low-amplitude, short APAs without subsequent step execution. The louder the stimulus, the more frequent the elicitation. Depending on the intensity of the stimulus, APAs prior to step initiation can be triggered without the evocation of a startle response or an acoustic blink. Greater reaction times for these APAs were observed for non-startling stimuli. This observation suggested the involvement of pathways that did not involve the brainstem as a "prime mover".

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.

Online control of anticipated postural adjustments in step initiation: evidence from behavioral and computational approaches

Anticipatory postural adjustments (APAs) prior to step execution are thought to be immutable once released. Here we challenge this assumption by testing whether APAs can be modified online if a body perturbation occurs during execution. Two directions of perturbation (resisting and assisting) relative to the body weight transfer were used during the execution of APAs. We found that APAs are modified online (increase in both ground pressure and muscle activity) to compensate for resisting perturbations. The outcomes of a biomechanical model confirmed that the early changes in the APAs resulted from an active control of the APAs and were not merely mechanical consequences of the perturbation. However, no modification of the initial feedforward command was observed for assisting perturbations. The motor command changes for the resisting perturbation may originate from the mismatch between passively originated forces and those actively specified by the central command when acting in the opposite direction. The absence of a mismatch in the assisting perturbation might explain why the central nervous system was not prompted to modify the APAs in this condition.

Long-term evaluation of gait initiation in six Parkinson's disease patients with bilateral subthalamic stimulation

Defined as the transient state between standing and walking, gait initiation is negatively affected in Parkinson's disease (PD), which often results in significant disability. Although deep brain stimulation (DBS) is the most common surgical procedure for PD, the long-term effects of DBS on gait initiation are not well studied. The present study evaluated the long-term effects of subthalamic nucleus (STN) DBS on the preparation phase of gait initiation using principal component (PC) analysis. Six patients with PD who had undergone STN DBS and 24 healthy control subjects were evaluated. PD subjects were assessed 11.3±10.3 (P1) and 78.9±10.6 (P2) months after surgery. PD subjects were tested with STN DBS in two conditions: without medication and with medication. PC analysis was applied separately for the vertical, anterior-posterior and medial-lateral components of ground reaction force (GRF) recorded during gait initiation. Three PC scores were chosen by the scree test for each GRF component and all these PC scores were used for calculating a standard distance between healthy controls and PD subjects. The Friedman test showed a significant difference in standard distance among conditions (P=0.004), with the post-hoc test recognizing differences among P1 conditions and P2 medication-on condition. The eigenvector loading factors pointed to major differences between PD conditions surrounding the maximum amplitude of vertical and anterior-posterior GRF. For the studied sample, all distances increased in the follow-up evaluation (P2) with and without medications, indicating a worsening in gait initiation after seven years.

Posture and locomotion coupling: a target for rehabilitation interventions in persons with Parkinson's disease

Disorders of posture, balance, and gait are debilitating motor manifestations of advancing Parkinson's disease requiring rehabilitation intervention. These problems often reflect difficulties with coupling or sequencing posture and locomotion during complex whole body movements linked with falls. Considerable progress has been made with demonstrating the effectiveness of exercise interventions for individuals with Parkinson's disease. However, gaps remain in the evidence base for specific interventions and the optimal content of exercise interventions. Using a conceptual theoretical framework and experimental findings, this perspective and review advances the viewpoint that rehabilitation interventions focused on separate or isolated components of posture, balance, or gait may limit the effectiveness of current clinical practices. It is argued that treatment effectiveness may be improved by directly targeting posture and locomotion coupling problems as causal factors contributing to balance and gait dysfunction. This approach may help advance current clinical practice and improve outcomes in rehabilitation for persons with Parkinson's disease.". . .postural activity should be regarded as a function in its own right and not merely as a component of movement. . ."James Purdon Martin.

Variable initial swing side and prolonged double limb support represent abnormalities of the first three steps of gait initiation in patients with Parkinson's disease with freezing of gait

This study investigated abnormalities of the first three steps of gait initiation in patients with Parkinson’s disease without freezing of gait (PD − FOG) and investigated which abnormalities are related to FOG. Seven PD − FOG and seven age-matched healthy controls performed self-generated or cue-triggered gait initiation. Data for PD patients with FOG (PD + FOG) were cited from a previous study using a procedure similar to that used in the present study. Gait initiation was abnormal, and external cue normalized some abnormalities in PD − FOG. The initial swing side was fairly consistent among the trials in both PD − FOG and in healthy controls, although the initial swing side was inconsistent in PD + FOG. The duration of the first double limb support (DLS) was the only parameter that depends on FOG severity and that was abnormal in PD + FOG but was not abnormal in PD − FOG. The variability of the initial swing side and prolonged first DLS are abnormalities specifically related to FOG.

Reaching while standing in microgravity: a new postural solution to oversimplify movement control

Many studies showed that both arm movements and postural control are characterized by strong invariants. Besides, when a movement requires simultaneous control of the hand trajectory and balance maintenance, these two movement components are highly coordinated. It is well known that the focal and postural invariants are individually tightly linked to gravity, much less is known about the role of gravity in their coordination. It is not clear whether the effect of gravity on different movement components is such as to keep a strong movement-posture coordination even in different gravitational conditions or whether gravitational information is necessary for maintaining motor synergism. We thus set out to analyze the movements of eleven standing subjects reaching for a target in front of them beyond arm's length in normal conditions and in microgravity. The results showed that subjects quickly adapted to microgravity and were able to successfully accomplish the task. In contrast to the hand trajectory, the postural strategy was strongly affected by microgravity, so to become incompatible with normo-gravity balance constraints. The distinct effects of gravity on the focal and postural components determined a significant decrease in their reciprocal coordination. This finding suggests that movement-posture coupling is affected by gravity, and thus, it does not represent a unique hardwired and invariant mode of control. Additional kinematic and dynamic analyses suggest that the new motor strategy corresponds to a global oversimplification of movement control, fulfilling the mechanical and sensory constraints of the microgravity environment.

Postural preparation prior to stepping in patients with Parkinson's disease

People with Parkinson's disease (PD) frequently have difficulties with generating anticipatory postural adjustments (APAs) for forward propulsion and lateral weight transfer when initiating gait. This impairment has been attributed to deficits in motor planning and preparation. This study examined the preparation of APAs prior to an imperative cue to initiate forward stepping. A startling acoustic stimulus (SAS) was used to probe the state of preparation of the APA in eight PD (off medication) and seven matched control subjects. Subjects performed visually cued trials involving a pre-cue light instructing them to prepare to step, followed 3.5 s later by a go-cue light to rapidly initiate stepping. In random trials, a SAS (124 dB) was presented at −1,500, −1,000, −500, −250, −100, or 0 ms before the go-cue. Subjects also performed self-initiated steps. Ground reaction forces (GRFs), center of pressure (CoP) changes, and electromyographic (EMG) signals were recorded. The SAS triggered APAs in 94 ± 11% (PD) and 96 ± 8% (control) of trials at latencies 89 ± 4 ms (PD) and 97 ± 3 ms (control) earlier than Control trials. The temporal profile of APA preparation was similar between groups. However, peak EMG, GRF, and mediolateral CoP amplitudes were reduced in PD. SAS-evoked APAs at 0 ms matched Control trial APAs and were enhanced compared with self-initiated stepping. These results demonstrate that people with mild to moderate PD can plan and prepare the appropriate APA sequence prior to the expected cue to initiate gait; however, the prepared APAs are underscaled in magnitude.

Abnormalities of the first three steps of gait initiation in patients with Parkinson's disease with freezing of gait

The purpose of this study was to investigate abnormalities of the first three steps of gait initiation in patients with Parkinson's disease (PD) with freezing of gait (FOG). Ten PD patients with FOG and 10 age-matched healthy controls performed self-generated gait initiation. The center of pressure (COP), heel contact positions, and spatiotemporal parameters were estimated from the vertical pressures on the surface of the force platform. The initial swing side of gait initiation was consistent among the trials in healthy controls but not among the trials in PD patients. The COP and the heel contact position deviated to the initial swing side during the first step, and the COP passed medial to each heel contact position during the first two steps in PD patients. Medial deviation of the COP from the first heel contact position had significant correlation with FOG questionnaire item 5. These findings indicate that weight shifting between the legs is abnormal and that medial deviation of the COP from the first heel contact position sensitively reflects the severity of FOG during the first three steps of gait initiation in PD patients with FOG.

Perturbations of ground support alter posture and locomotion coupling during step initiation in Parkinson's disease

During the initiation of stepping, anticipatory postural adjustments (APAs) for lateral weight transfer and propulsion normally precede the onset of locomotion. In Parkinson's disease (PD), impaired step initiation typically involves altered APA ground force production with delayed step onset and deficits in stepping performance. If, as in stance and gait, sensory information about lower limb load is important for the control of stepping, then perturbations influencing loading conditions could affect the step initiation process. This study investigated the influence of changes in lower limb loading during step initiation in patients with PD and healthy control subjects. Participants performed rapid self-triggered step initiation with the impending single stance limb positioned over a pneumatically actuated platform. In perturbation trials, the stance limb ground support surface was either moved vertically downward (DROP) or upward (ELEVATE) by 1.5 cm shortly after the onset of the APA phase. Overall, PD patients demonstrated a longer APA duration, longer time to first step onset, and slower step speed than controls. In both groups, the DROP perturbation reinforced the intended APA kinetic changes for lateral weight transfer and resulted in a significant reduction in APA duration, increase in peak amplitude, and earlier time to first step onset compared with other conditions. During ELEVATE trials that opposed the intended weight transfer forces both groups rapidly adapted their stepping to preserve standing stability by decreasing step length and duration, and increasing step height and foot placement laterally. The findings suggested that sensory information associated with limb load and/or foot pressure modulates the spatial and temporal parameters of posture and locomotion components of step initiation in interaction with a centrally generated feedforward mode of neural control. Moreover, impaired step initiation in PD may at least acutely be enhanced by augmenting the coupling between posture and locomotion.

Short-term effects of posture-assisted step training on rapid step initiation in Parkinson's disease

BACKGROUND AND PURPOSE

Anticipatory postural adjustments (APAs) for lateral weight transfer and stability precede and accompany gait initiation. Individuals with Parkinson's disease (PD) show altered APA characteristics with delays in initiating stepping that may reflect impaired interactions between posture and locomotion. The purpose of this study was to determine the short-term effects of a single session of repetitive robotic assistance training with the APA on rapid step initiation in individuals with PD in the medications "on" state and healthy control individuals. Ground reaction forces and step kinematics were recorded.

METHODS

Subjects first performed baseline trials of unassisted self-paced rapid forward stepping. Next, a training acquisition series involved 50 trials with a lateral pull applied to the pelvis by a robotic system to assist with the early phase of the APA during stepping. To assess potential retention effects of training, unassisted stepping trials were evaluated immediately after acquisition trials (immediate retention) and one week later (one-week retention).

RESULTS

Overall, the subjects with PD had a longer APA duration (P < 0.03), and longer first step duration (P < 0.04) than the healthy control individuals. Compared with baseline, APA duration was shorter (P < 0.001) and step onset time became earlier (P < 0.001) for acquisition trials but these effects were not retained. Step duration, which became shorter (P < 0.001) during the late acquisition trials (P = 0.002), demonstrated immediate retention (P < 0.001) and one-week retention (P < 0.001).

CONCLUSION

Posture-assisted training, affecting the interaction between posture and locomotion, may have therapeutic potential for improving movement performance in individuals with PD.

The supplementary motor area contributes to the timing of the anticipatory postural adjustment during step initiation in participants with and without Parkinson's disease

The supplementary motor area (SMA) is thought to contribute to the generation of anticipatory postural adjustments (APAs, which act to stabilize supporting body segments prior to movement), but its precise role remains unclear. In addition, participants with Parkinson's disease (PD) exhibit impaired function of the SMA as well as decreased amplitudes and altered timing of the APA during step initiation, but the contribution of the SMA to these impairments also remains unclear. To determine how the SMA contributes to generating the APA and to the impaired APAs of participants with PD, we examined the voluntary steps of eight participants with PD and eight participants without PD, before and after disrupting the SMA and dorsolateral premotor cortex (dlPMC), in separate sessions, with 1-Hz repetitive transcranial magnetic stimulation (rTMS). Both groups exhibited decreased durations of their APAs after rTMS over the SMA but not over the dlPMC. Peak amplitudes of the APAs were unaffected by rTMS to either site. The symptom severity of the participants with PD positively correlated with the extent that rTMS over the SMA affected the durations of their APAs. The results suggest that the SMA contributes to the timing of the APA and that participants with PD exhibit impaired timing of their APAs, in part, due to progressive dysfunction of circuits associated with the SMA.

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 and balance disorders in Parkinson's disease: impaired active braking of the fall of centre of gravity

Gait and balance disorders are common in Parkinson's disease (PD), but its pathophysiology is still poorly understood. Step length, antero-posterior, and vertical velocities of the center of gravity (CG) during gait initiation were analyzed in 32 controls and 32 PD patients, with and without levodopa, using a force platform. Brain volumes and mesencephalic surface area were measured in PD patients. During the swing limb period, controls showed a fall in the CG, which was reversed before foot-contact indicating active braking of the CG fall. In PD patients, without levodopa, step length and velocity were significantly reduced and no braking occurred before foot-contact in 22 patients. With levodopa, step length and velocity increased in all patients and 7 patients improved their braking capacity. PD patients with normal braking (n = 17) had significantly lower gait and balance disorder scores and higher normalized-mesencephalic surface areas compared to patients with impaired braking (n = 15). The decreased step length and velocity, characteristic of PD, mainly result from degeneration of central dopaminergic systems. The markedly decreased braking capacity observed in half the PD patients contributes to their gait disorders and postural instability, perhaps as a result of nondopaminergic lesions, possibly at the mesencephalic level.

Anticipatory postural adjustments prior to step initiation are hypometric in untreated Parkinson's disease: an accelerometer-based approach

BACKGROUND AND PURPOSE

Anticipatory postural adjustments (APAs), prior to step initiation, are bradykinetic in advanced Parkinson's disease (PD) and may be one of the factors associated with 'start hesitation'. However, little is known about APAs in the early stage of PD. In this study, we determined whether body-worn accelerometers could be used to characterize step initiation deficits in subjects with early-to-moderate, untreated PD.

METHODS

Eleven PD and 12 healthy control subjects were asked to take two steps. Postural adjustments were compared from center of pressure (COP) and from acceleration of the trunk at the center of mass level (L5).

RESULTS

Our findings show that APAs measured from the peak COP displacement toward the swing leg and the peak trunk acceleration toward the stance leg were smaller in untreated PD compared with control subjects. The magnitude of APAs measured from peak COP displacements and accelerations were correlated.

CONCLUSION

These results suggest that quantitative analysis of step initiation from one accelerometer on the trunk could provide useful information for the characterization of patients in early stages of PD, when clinical evidence of start hesitation may not be detectable. Ambulatory monitoring of step initiation is also promising for monitoring patient progression in the home environment, and eventually providing feedback for preventing freezing of gait episodes.

Effects of Parkinson's disease on proprioceptive control of posture and reaching while standing

Although previous studies have shown pointing errors and abnormal multijoint coordination in seated subjects with Parkinson's disease (PD) who cannot view their arm, the extent to which subjects with PD have problems using proprioception to coordinate equilibrium maintenance and goal-oriented task execution has not been adequately investigated. If a common motor program controls voluntary arm pointing movements and the accompanying postural adjustments, then impairments of proprioceptive integration in subjects with PD should have similar effects on pointing and body center of mass (CoM) control with eyes closed. Ten standing subjects with PD (OFF-medication) and 10 age-matched control (CTR) subjects pointed to a target with their eyes closed and open. Although pointing accuracy was not significantly different between groups, body CoM displacements were reduced in subjects with PD, but not in CTR, when eyes were closed. In addition, with eyes closed, PD subjects showed reduced temporal coupling between pointing and CoM velocity profiles and reduced spatial coupling between pointing and CoM endpoints. This poor coupling with eyes closed could be related to the PD subjects' increased jerkiness of CoM displacements. The different effects of eye closure between CTR and PD subjects on the CoM displacements, but not pointing accuracy, are consistent with separate motor programs for the pointing and postural components of this task. Furthermore, the decoupling between the two movement components in subjects with PD when they could not use vision, suggests that the basal ganglia are involved in the integration of proprioceptive information for posture-movement coordination.