Relative contribution of muscle strength, lean mass, and lower extremity motor function in explaining between-person variance in mobility in older adults.

BACKGROUND: Approximately 35% of individuals > 70 years have mobility limitations. Historically, it was posited lean mass and muscle strength were major contributors to mobility limitations, but recent findings indicate lean mass and muscle strength only moderately explain mobility limitations. One likely reason is that lean mass and muscle strength do not necessarily incorporate measures globally reflective of motor function (defined as the ability to learn, or to demonstrate, the skillful and efficient assumption, maintenance, modification, and control of voluntary postures and movement patterns). In this study we determined the relative contribution of lean mass, muscle strength, and the four square step test, as an index of lower extremity motor function, in explaining between-participant variance in mobility tasks. METHODS: In community-dwelling older adults (N = 89; 67% women; mean 74.9 ± 6.7 years), we quantified grip and leg extension strength, total and regional lean mass, and time to complete the four square step test. Mobility was assessed via 6-min walk gait speed, stair climb power, 5x-chair rise time, and time to complete a complex functional task. Multifactorial linear regression modeling was used to determine the relative contribution (via semi-partial r2) for indices of lean mass, indices of muscle strength, and the four square step test. RESULTS: When aggregated by sex, the four square step test explained 17-34% of the variance for all mobility tasks (p <  0.01). Muscle strength explained ~ 12% and ~ 7% of the variance in 6-min walk gait speed and 5x-chair rise time, respectively (p <  0.02). Lean mass explained 32% and ~ 4% of the variance in stair climb power and complex functional task time, respectively (p <  0.02). When disaggregated by sex, lean mass was a stronger predictor of mobility in men. CONCLUSION: The four square step test is uniquely associated with multiple measures of mobility in older adults, suggesting lower extremity motor function is an important factor for mobility performance. TRIAL REGISTRATION: NCT02505529 -2015/07/22.

Multiple measures of muscle function influence Sorensen Test performance in individuals with recurrent low back pain.

BACKGROUND: Sorensen Test time-to-task-failure (TTF) predicts several low back pain (LBP) clinical outcomes, including recurrence. Because the test is described as a measure of trunk extensor (TE) muscle endurance, LBP rehabilitation programs often emphasize endurance training, but the direct role of TE muscle function on Sorensen Test-TTF remains unclear. OBJECTIVE: To assess the discriminative and associative properties of multiple markers of isolated TE performance with regard to Sorensen Test-TTF in individuals with recurrent LBP. METHOD: Secondary analysis of baseline measures from participants in a registered (NCT02308189) trial (10 men; 20 women) was performed. Participants were classified by Sorensen Test-TTF as high, moderate or low risk for subsequent LBP episodes, and compared to determine if classification could discriminate differences in TE function. Correlations between Sorensen Test-TTF and isolated TE performance, anthropometrics and disability were investigated. RESULTS: Individuals at risk of subsequent LBP episodes had greater perceived disability and fat mass/TE strength ratios (P⩽ 0.05) than those not at risk. Modest, significant (r= 0.36-0.42, P⩽ 0.05) associations were found between Sorensen Test-TTF, TE endurance and fat mass/TE strength. Exploratory analyses suggested possible sex-specific differences related to Sorensen Test-TTF. CONCLUSIONS: Isolated TE muscle endurance is only one of several factors with similar influence on Sorensen Test-TFF, thus LBP rehabilitation strategies should consider other factors, including TE strength, anthropometrics and perceived disability.

Effect of Anodal Transcranial Direct Current Stimulation of the Motor Cortex on Elbow Flexor Muscle Strength in the Very Old.

BACKGROUND AND PURPOSE: Muscle weakness predisposes older adults to a fourfold increase in functional limitations and has previously been associated with reduced motor cortex excitability in aging adults. The purpose of this study was to determine whether a single session of anodal transcranial direct current stimulation (tDCS) of the motor cortex would increase elbow flexion muscle strength and electromyographic (EMG) amplitude in very old individuals. METHODS: Eleven very old individuals-85.8 (4.3) years-performed 3 maximal isometric elbow flexion contractions before and after 20 minutes of sham or anodal tDCS on different days. Order of stimulation was randomized, and the study participants and investigators were blinded to condition. In addition, voluntary activation capacity of the elbow flexors was determined by comparing voluntary and electrically evoked forces. RESULTS: Anodal tDCS did not alter muscle strength or EMG activity in comparison to sham stimulation. Elbow flexion voluntary activation capacity was very high among the study participants: 99.3% (1.8%). CONCLUSION: Contrary to our hypothesis, we observed no effect of anodal tDCS and no impairment in elbow flexor voluntary activation capacity in the very old. Whether anodal tDCS would exert a positive effect and support our initial hypothesis in another muscle group that does exhibit impairments in voluntary activation in older adults is a question that is still to be addressed.

Changes in DXA-derived lean mass and MRI-derived cross-sectional area of the thigh are modestly associated.

Dual-energy X-ray absorptiometry (DXA) derived measures of lean mass demonstrate strong associations with magnetic resonance imaging (MRI) derived measures of muscle volume (MV) in cross-sectional studies, however, few studies have compared changes in response to an intervention. The purpose of this study was to determine the accuracy of DXA at detecting changes in lean mass, using MRI-derived MV as a reference standard. 10 male and 16 female subjects (29.2 ± 9.5 years) underwent DXA and MRI scans before and after a 10-week resistance training intervention. DXA thigh lean mass was compared to MRI mid-thigh MV, and percent change in size was compared between MRI and DXA. There was a strong correlation between measures cross-sectionally (r = 0.89) in agreement with previous investigations. However, there was a modest correlation of percentage change over time between methods (r = 0.49). Bland-Altman plots revealed that the amount of random error increased as the magnitude of the change from baseline increased. DXA measures of change in lean mass were modestly associated with MRI measures of change in MV. While there are several advantages to using DXA for the measurement of lean mass, the inability to accurately detect changes over time calls into question its use in clinical trials.

Blood Flow-restricted Exercise Does Not Induce a Cross-Transfer of Effect: A Randomized Controlled Trial.

PURPOSE: The goal of this trial was to determine whether low-load blood flow-restricted (BFR) exercise of appendicular muscles induces a cross-transfer of effect to the trunk extensor (TE) muscles, such that low-load TE exercise would enhance TE size and function to a greater extent than standard low-load exercise in people with recurrent low back pain (LBP). We also investigated the direct effects of BFR exercise in the appendicular muscles. METHODS: Thirty-two adults with recurrent, nonspecific LBP were randomized into two groups: Appendicular BFR exercise (BFR exercise) or control exercise (CON exercise). All participants trained (two times per week) for 10 wk, with a 12-wk follow-up. Participants performed three sets of leg extension (LE), plantar flexion (PF), and elbow flexion (EF) exercises followed by low-load TE exercise without BFR. Outcome measures included magnetic resonance imaging-derived muscle size (quadriceps and TE), strength (LE, PF, EF, and TE), and endurance (LE and TE). RESULTS: There was no evidence for a cross-transfer of effect to the TE. There was also no statistically significant enhancement of limb skeletal muscle size or function of BFR relative to CON exercise at any time point; though, moderate effect sizes for BFR exercise were observed for enhanced muscle size and strength in the leg extensors. CONCLUSIONS: Low-load BFR exercise of the appendicular muscles did not result in a cross-transfer of effect to the TE musculature. There was also no significant benefit of low-load BFR exercise on the appendicular muscle size and function, suggesting no benefit from low-load BFR exercise in adults with recurrent, nonspecific LBP.

Vibrotactile Feedback Alters Dynamics Of Static Postural Control In Persons With Parkinson's Disease But Not Older Adults At High Fall Risk.

BACKGROUND: Aging and Parkinson's disease are often associated with impaired postural control. Providing extrinsic feedback via vibrotactile sensation could supplement intrinsic feedback to maintain postural control. RESEARCH QUESTION: We investigated the postural control response to vibrotactile feedback provided at the trunk during challenging stance conditions in older adults at high fall risk and individuals with Parkinson's disease compared to healthy older adults. METHODS: Nine older adults at high fall risk, 9 persons with Parkinson's disease and 10 healthy older adults performed 30s quiet standing on a force platform under five challenging stance conditions with eyes open/closed and standing on firm/foam surface with feet together, each with and without vibrotactile feedback. During vibrotactile feedback trials, feedback was provided when participants swayed >10% over the center of their base of support. Participants were instructed vibrations would be in response to their movement. Magnitude of postural sway was estimated using center of pressure path length, velocity, and sway area. Dynamics of individuals' postural control was evaluated using detrended fluctuation analysis. RESULTS: Results showed that vibrotactile feedback induced a change in postural control dynamics among persons with Parkinson's disease when standing with intact intrinsic visual input and altered intrinsic somatosensory input, but there was no change in sway magnitude. However, use of vibrotactile feedback did not significantly alter dynamics of postural control in older adults with high risk of falling or reduce the magnitude of sway. SIGNIFICANCE: Considering the effects of vibrotactile feedback were dependent on the population and stance condition, designing an optimal therapeutic regimen for balance training should be carefully considered and be specific to a target population. Furthermore, our results suggest that explicit instructions on how to respond to the vibrotactile feedback could affect training outcome.

Preliminary Evidence That Excitatory Transcranial Direct Current Stimulation Extends Time to Task Failure of a Sustained, Submaximal Muscular Contraction in Older Adults.

BACKGROUND: Decreased cortical excitability has been proposed as a potential mechanism underlying task failure during sustained muscular contractions, and cortical excitability may decrease with old age. We tested the hypothesis that transcranial direct current stimulation, which has been reported to raise cortical excitability, would prolong the time to task failure during a sustained muscular contraction in older adults. METHODS: Thirteen older adults (68.3±2.0 years; eight women and five men) performed isometric, elbow flexions to failure while receiving sham or anodal transcranial direct current stimulation. Order of stimulation was randomized, and the subjects and investigators were blinded to condition. Time to task failure was measured alongside selected psychological indices of perceived exertion and affect. RESULTS: Anodal transcranial direct current stimulation prolonged mean time to task failure by approximately 15% (16.9±2.2 vs 14.7±1.8 minutes) and slowed the rate of increase in rating of perceived exertion (0.29±0.03 vs 0.31±0.03) relative to the sham condition. CONCLUSIONS: These preliminary findings suggest that anodal transcranial direct current stimulation enhances time to task failure of a sustained, submaximal contraction in older adults by potentially increasing cortical excitability and/or influencing the perception of exertion. These results raise the question of whether interventions that acutely increase cortical excitability could enhance physical function and/or exercise-induced adaptations in older adults.

Effectiveness of blood flow restricted exercise compared with standard exercise in patients with recurrent low back pain: study protocol for a randomized controlled trial.

BACKGROUND: Low back pain is a highly prevalent condition in the United States and has a staggeringly negative impact on society in terms of expenses and disability. It has previously been suggested that rehabilitation strategies for persons with recurrent low back pain should be directed to the medial back muscles as these muscles provide functional support of the lumbar region. However, many individuals with low back pain cannot safely and effectively induce trunk muscle adaptation using traditional high-load resistance exercise, and no viable low-load protocols to induce trunk extensor muscle adaptation exist. Herein, we present the study protocol for a randomized controlled trial that will investigate the "cross-transfer" of effects of a novel exercise modality, blood flow restricted exercise, on cross-sectional area (primary outcome), strength and endurance (secondary outcomes) of trunk extensor muscles, as well as the pain, disability, and rate of recurrence of low back pain (tertiary outcomes). METHODS AND STUDY DESIGN: This is a single-blinded, single-site, randomized controlled trial. A minimum of 32 (and up to 40) subjects aged 18 to 50 years with recurrent low back pain and poor trunk extensor muscle endurance will be recruited, enrolled and randomized. After completion of baseline assessments, participants will be randomized in a 1:1 ratio to receive a 10-week resistance exercise training program with blood flow restriction (BFR exercise group) or without blood flow restriction (control exercise group). Repeat assessments will be taken immediately post intervention and at 12 weeks after the completion of the exercise program. Furthermore, once every 4 weeks during a 36-week follow-up period, participants will be asked to rate their perceived disability and back pain over the past 14 days. DISCUSSION: This study will examine the potential for blood flow restricted exercise applied to appendicular muscles to result in a "cross-transfer" of therapeutic effect to the lumbar musculature in individuals with low back pain. The results of this study will provide important insights into the effectiveness of this novel exercise modality, which could potentially provide the foundation for a cost-effective and easy-to-implement rehabilitation strategy to induce muscle adaptation in the absence of high mechanical and compressive loading on the spine. TRIAL REGISTRATION: This trial is registered with ClinicalTrials.gov (registration number: NCT02308189, date of registration: 2 December 2014).

Behavioral inflexibility and motor dedifferentiation in persons with Parkinson's disease: bilateral coordination deficits during a unimanual reaching task.

We evaluated kinematics of people with Parkinson's disease (PD) and age-matched controls during cued and uncued reaching movements. Maximum hand velocity, its variability and shoulder-to-shoulder coupling, quantified by phase locking value (PLV), were compared between PD (n=14) and Control (n=4). The PD group achieved significantly lower maximum hand velocities during the reaching movement in comparison to the Control group (p=0.05), whereas the Control group exhibited significantly greater variability (i.e., larger SDs) of maximum hand velocities across the blocks than the PD group (p=0.01). Persons with PD exhibited higher PLVs than the healthy elderly individuals when performing reaching movements with their dominant side (p=0.05), while the PLVs did not differ between groups when the movements were performed with their non-dominant hand. The present study suggests that persons with PD have a reduced ability to: (1) modulate maximum reaching velocity; and (2) alter coordination across the shoulders to different reaching actions. In persons with PD, the velocity-oriented (dominant) limb becomes slowed and less flexible, to the point that its movement dynamics are effectively similar to that of the position-oriented (non-dominant) limb.

Execution of Activities of Daily Living in Persons with Parkinson Disease.

INTRODUCTION: Muscular weakness and the motor difficulties associated with Parkinson disease (PD) often impair the performance of activities of daily living (ADL). However, little is known about the magnitude and distribution of relative muscular effort of persons with PD during ADL. The purpose of this investigation was to determine the relative magnitude of lower extremity moment production that persons with PD use to perform common ADL. METHODS: Fifteen participants with mild-to-moderate PD and 14 age/sex-matched controls volunteered. Participants performed a series of ADL tasks, as follows: gait initiation (GI), gait, and stair ascending tasks. Participants were then asked to perform maximal-effort isokinetic tests of hip and knee extension and ankle plantarflexion at speeds of 90° per second and 120° per second. Relative effort was quantified as a percentage of the maximal isokinetic value produced by a joint during performance of the ADL. Relative effort and peak isokinetic joint moments were analyzed using a mixed-model ANOVA with repeated measures. All other comparisons were evaluated using independent t-tests. RESULTS: Persons with PD produced smaller ankle plantarflexion moment at both 90° per second and 120° per second (P < 0.05). Relative effort during GI (271% vs 189%, P < 0.05) and gait (270% vs 161%, P < 0.05) was significantly greater at the ankle in persons with PD. Contribution of the ankle to the support moment was lower in PD during stair ascending (24% vs 34%) and GI (63% vs 57%) compared with that in controls. CONCLUSIONS: The reduced ankle moments during ADL are indicative of deficits in muscular capabilities in those with PD. Moreover, PD caused a redistribution of joint torques, such that PD participants used their hip extensors more and ankle plantarflexors less.

Discriminating features of gait performance in progressive supranuclear palsy.

INTRODUCTION: Progressive supranuclear palsy (PSP) is the most common form of atypical Parkinsonism; however it is underdiagnosed and often misdiagnosed as Parkinson's disease (PD). METHODS: We investigated gait initiation (GI) and gait performance in a total of 36 participants (12 PSP, 12 PD and 12 healthy age- and gender-matched controls) to gain further insight into specific motor deficits that characterize dynamic postural control and gait in PSP. Anticipatory postural adjustments (APAs), quantified by center of pressure (COP) displacement and speed prior to an initial heel off, and the maximum distance (COPCOM) between COP and center of mass (COM) during all three GI phases were calculated to evaluate dynamic postural control. Steady-state gait performance was also evaluated and compared across the groups. RESULTS: APAs in PSP were significantly altered such that the posterior COP shift is profoundly diminished when compared to PD (p < 0.05). Moreover, proper velocity control during GI in PSP was affected, particularly in the mediolateral direction, when compared to PD (p < 0.05). The diminished COPCOM distance is further indicative of more severe dynamic postural instability in PSP than in PD (p < 0.05). Significant differences in spatiotemporal parameters, inter-step variability, and asymmetry during gait in PSP, in comparison with PD were also identified (all p's < 0.05). CONCLUSION: The present study reveals that the compensatory GI strategy in PSP is distinct from PD and paradoxically induces lateral instability. Further, gait performance in PSP is slower and more variable which could be the consequence of lateral instability and fear of falling.

Rethinking energy in parkinsonian motor symptoms: a potential role for neural metabolic deficits.

Parkinson's disease (PD) is characterized as a chronic and progressive neurodegenerative disorder that results in a variety of debilitating symptoms, including bradykinesia, resting tremor, rigidity, and postural instability. Research spanning several decades has emphasized basal ganglia dysfunction, predominantly resulting from dopaminergic (DA) cell loss, as the primarily cause of the aforementioned parkinsonian features. But, why those particular features manifest themselves remains an enigma. The goal of this paper is to develop a theoretical framework that parkinsonian motor features are behavioral consequence of a long-term adaptation to their inability (inflexibility or lack of capacity) to meet energetic demands, due to neural metabolic deficits arising from mitochondrial dysfunction associated with PD. Here, we discuss neurophysiological changes that are generally associated with PD, such as selective degeneration of DA neurons in the substantia nigra pars compacta (SNc), in conjunction with metabolic and mitochondrial dysfunction. We then characterize the cardinal motor symptoms of PD, bradykinesia, resting tremor, rigidity and gait disturbance, reviewing literature to demonstrate how these motor patterns are actually energy efficient from a metabolic perspective. We will also develop three testable hypotheses: (1) neural metabolic deficits precede the increased rate of neurodegeneration and onset of behavioral symptoms in PD; (2) motor behavior of persons with PD are more sensitive to changes in metabolic/bioenergetic state; and (3) improvement of metabolic function could lead to better motor performance in persons with PD. These hypotheses are designed to introduce a novel viewpoint that can elucidate the connections between metabolic, neural and motor function in PD.

Decreased dynamical complexity during quiet stance in children with autism spectrum disorders.

BACKGROUND: Postural control deficits in individuals with Autism Spectrum Disorders (ASD) are widely acknowledged; however, the underlying biomechanical features of these deficits remain unknown. Nonlinear analyses provide insight into the nature of how movement is controlled and have the potential to provide new insight into the postural control abnormalities associated with ASD. The purpose of this study was to further investigate postural control deficits in children with ASD through linear and nonlinear analyses of center of pressure (COP) data. METHODS: We evaluated COP data during quiet standing for 16 children with ASD and 17 age-matched typically developing (TD) children. The magnitude of COP fluctuations (COP ranges, velocity, and sway area) and complexity of postural control dynamics, quantified by multiscale entropy (MSE), were compared across groups. RESULTS: Children with ASD displayed larger fluctuations in their COP data, observed in COP ranges (95.5% mediolaterally and 46.9% anteroposteriorly, p<0.05 respectively) and COP sway area (885%, p<0.05). Children with ASD also displayed less complexity in their COP data, observed in the MSE complexity index (CI) (32.4% mediolaterally and 35.7% anteroposteriorly, p<0.05 respectively). CONCLUSIONS: The present study successfully revealed that children with ASD have more repetitive patterns in their COP data, indicating a less complex control of posture, on multiple time scales, during quiet stance. These findings suggest a more regular or restricted control of posture and may be an initial step in linking postural instability to stereotypic behavior and the neurobiology of ASD.

Tai Chi Exercise to Improve Non-Motor Symptoms of Parkinson's Disease.

BACKGROUND: A substantial number of individuals with Parkinson's disease exhibit debilitating non-motor symptoms that decrease quality of life. To date, few treatment options exist for the non-motor symptomatology related to Parkinson's disease. The goal of this pilot investigation was to determine the effects of Tai Chi exercise on the non-motor symptomology in Parkinson's disease. METHODS: Twenty-one individuals with Parkinson's disease were enrolled in a Tai Chi intervention (n=15) or a noncontact control group (n=6). Participants assigned to Tai Chi participated in 60-minute Tai Chi sessions three times per week, for 16 weeks. Pre and post measures included indices of cognitive-executive function including visuomotor tracking and attention, selective attention, working memory, inhibition, processing speed and task switching. Additionally, all participants were evaluated on the Parkinson's disease Questionnaire-39 and Tinetti's Falls Efficacy Scale. RESULTS: Results indicated that the Tai Chi training group had significantly better scores following the intervention than the control group on the Parkinson's disease Questionnaire-39 total score as well as the emotional well-being sub score. Trends for improvement were noted for the Tai Chi group on Digits Backwards, Tinetti's Falls Efficacy Scale, and the activities of daily living and communication sub scores of the Parkinson's disease Questionnaire-39. CONCLUSIONS: This research provides initial data that supports future studies to definitively establish efficacy of Tai Chi to improve non-motor features of Parkinson's disease.

Ambulation and Parkinson disease.

Parkinson disease is a progressive neurodegenerative disorder characterized by a variety of motor and nonmotor features. This article reviews the problems of postural instability and gait disturbance in persons with Parkinson disease through the discussion of (1) the neuropathology of parkinsonian motor deficits, (2) behavioral manifestations of gait and postural abnormalities observed in persons with Parkinson disease, and (3) pharmacologic, surgical, and physical therapy-based interventions to combat postural instability and gait disturbance. This article advances the treatment of postural instability and gait disturbance by condensing up-to-date knowledge and making it available to clinicians and rehabilitation professionals.

The effect of Tai Chi exercise on gait initiation and gait performance in persons with Parkinson's disease.

Gait dysfunction and postural instability are two debilitating symptoms in persons with Parkinson's disease (PD). Tai Chi exercise has recently gained attention as an attractive intervention for persons with PD because of its known potential to reduce falls and improve postural control, walking abilities, and safety at a low cost. The purpose of this report is to investigate the effect of Tai Chi exercise on dynamic postural control during gait initiation and gait performance in persons with idiopathic PD, and to determine whether these benefits could be replicated in two different environments, as complementary projects. In these two separate projects, a total of 45 participants with PD were randomly assigned to either a Tai Chi group or a control group. The Tai Chi groups in both projects completed a 16-week Tai Chi exercise session, while the control groups consisted of either a placebo (i.e., Qi-Gong) or non-exercise group. Tai Chi did not significantly improve Unified Parkinson's Disease Rating Scale Part III score, selected gait initiation parameters or gait performance in either project. Combined results from both projects suggest that 16 weeks of class-based Tai Chi were ineffective in improving either gait initiation, gait performance, or reducing parkinsonian disability in this subset of persons with PD. Thus the use of short-term Tai Chi exercise should require further study before being considered a valuable therapeutic intervention for these domains in PD.

Gait initiation impairments in both Essential Tremor and Parkinson's disease.

Gait initiation is a transitional task involving a voluntary shift from a static, stable position to a relatively less-stable state of locomotion. During gait initiation, anticipatory postural adjustments precede stepping in order to generate forward momentum while balance is maintained. While deficits in gait initiation are frequently reported for persons with Parkinson's disease, there is a paucity of information regarding gait initiation performance in persons with Essential Tremor. We investigated anticipatory postural adjustments and spatiotemporal characteristics of gait initiation in persons with Essential Tremor and compared them to persons with Parkinson's disease as well as age-matched neurologically healthy adults. Twenty-four persons with Essential Tremor, 31 persons with Parkinson's disease, and 38 age-matched controls participated. We compared anterior-posterior and mediolateral center of pressure movements and spatiotemporal stepping characteristics during gait initiation among the three groups using Mann-Whitney U-tests with Bonferroni corrections for multiple comparisons and one-way ANOVAs. Persons with Parkinson's disease demonstrated significantly reduced displacement and velocity of the center of pressure during early phases of anticipatory postural adjustments relative to controls. Displacement of the center of pressure was also reduced in persons with Essential Tremor, although at a later stage of the gait initiation process. Persons with Parkinson's disease and Essential Tremor demonstrated similar reductions in step length during gait initiation when compared to controls. Persons with Parkinson's disease and Essential Tremor exhibit different deficits in gait initiation when compared to healthy older adults. Therefore, this study provides further evidence differentiating motor control features in these movement disorders.

Active control of approach-oriented posture is influenced by emotional reactions.

Emotions adaptively prepare the body to interact with the environment through execution of motor actions, but the extent to which emotional states modulate force control during directionally targeted movement tasks remains unknown. We sought to determine how emotions influence active control of approach-oriented posture. Participants (N = 43; 25 females) stood on a force plate and displaced their center of pressure (COP) anteriorly to a target at 50% of their maximum voluntary lean. After 7 s of real-time COP feedback, a picture representing 6 discrete categories (attack, mutilation, contamination, erotic couples, happy faces, and neutral objects) replaced the target and remained on the screen. Participants were instructed to maintain the target COP position throughout the trial. Deviation of the COP position relative to the target (root-mean-square error; RMSE) and error direction (constant error; CE) were evaluated during the feedback and picture portions of the trial. RMSE increased for all affective conditions following feedback occlusion. Following picture onset, lean error exhibited when viewing attack pictures was more anterior (i.e., greater CE) compared with the mutilation, contamination, and erotica conditions. Additionally, participants leaned more anteriorly (i.e., greater CE) during the happy faces condition compared with the mutilation and erotica conditions. Collectively, results indicate that the maintenance of an anterior COP position in the anterior direction is primarily modulated by the motivational direction of emotional stimuli.

Spatiotemporal variability during gait initiation in Parkinson's disease.

During gait initiation (GI), consistency of foot placement while stepping is important in making successful transitions from a state of stable static posture to an unstable state of dynamic locomotion. In populations characterized by gait dysfunction and postural instability, such as persons with Parkinson's disease (PD), the ability to generate a consistent stepping pattern during GI may be essential in the prevention of falls. However, little is known about GI variability in persons with PD as compared to their healthy elderly peers. Therefore, this study investigated spatiotemporal variability during the first two steps of GI in 46 persons with idiopathic PD and 49 healthy age-matched adults. Stepping characteristics, including the length, width, and time of the first two steps of GI as well as their coefficients of variation (CV) were compared between groups. Persons with PD initiated gait with significantly shorter steps (swing step length=.463 vs. .537 m, stance step length=.970 vs. 1.10 m) and higher variability in step length (swing step CV=8.82 vs. 5.45, stance step CV=6.76 vs. 3.61). Persons with PD also showed significantly higher variability in the time of the swing step (swing step CV=10.0 vs. 7.4). GI variability did not differ significantly between disease stages in persons with PD. Because greater variability in these measures during gait is related to an increased risk of falls, we propose that higher GI variability may play a considerable role in falls frequently observed during transitions from quiet standing in PD.

Tests of dorsolateral frontal function correlate with objective tests of postural stability in early to moderate stage Parkinson's disease.

A substantial number of individuals with Parkinson's disease who display impaired postural stability experience accelerated cognitive decline and an increased prevalence of dementia. To date, studies suggest that this relationship, believed to be due to involvement of nondopaminergic circuitry, occurs later in the disease process. Research has yet to adequately investigate this cognitive-posturomotor relationship especially when examining earlier disease states. To gain greater understanding of the relationship between postural stability and cognitive function/dysfunction we evaluated a more stringent, objective measure of postural stability (center of pressure displacement), and also more specific measures of cognition in twenty-two patients with early to moderate stage Parkinson's disease. The magnitude of the center of pressure displacement in this cohort was negatively correlated with performance on tests known to activate dorsolateral frontal regions. Additionally, the postural stability item of the UPDRS exhibited poor correlation with the more objective measure of center of pressure displacement and all specific measures of cognition. These results may serve as rationale for a more thorough evaluation of postural stability and cognition especially in individuals with mild Parkinson's disease. Greater understanding of the relationship between motor and cognitive processes in Parkinson's disease will be critical for understanding the disease process and its potential therapeutic possibilities.