Multi-task prioritization during the performance of a postural-manual and communication task.

Individuals are often required to complete two tasks simultaneously, such as walking while talking. Although the influence of performing a cognitive task during upright standing has been studied, less is known regarding how individuals prioritize the performance of multiple goal-directed tasks. The purpose of this study was to examine how young adults prioritize the performance of two goal-directed tasks while standing: generating speech (communicating) while performing a postural-manual task. Fifteen young adult female participants completed four conditions of the postural-manual task, quietly and while talking; a control condition of speech while standing was also completed. Task accuracy was defined as holding a block within an opening without hitting the perimeter and speaking clearly. Participants generally prioritized accurate communication over accurate completion of the postural-manual task, as evidenced by modifying their manual and postural strategies and decreasing manual accuracy.

The Effect of Holding an Object on Postural Stability and Suprapostural Task Performance.

Holding an object has been found to reduce postural sway during quiet standing. However, people normally stand to accomplish suprapostural goals, such as fitting a key into a lock. Postural control should therefore be assessed by examining postural outcomes in the context of suprapostural task performance. This study assessed whether holding an object increased standing postural stability and improved the performance of a concurrent precision manual task. A total of 15 young adults performed a precision manual task with their dominant hand while holding or not holding an object in their nondominant hand. Postural stability was assessed using measures of postural sway and time to boundary. Suprapostural task performance was assessed as an error count. Holding did not influence postural sway or suprapostural task performance. Discrepancies among previous studies coupled with the present findings suggest that the effects of holding an object on standing posture are highly sensitive to the experimental context. The authors provide several explanations for their findings and discuss the limitations of previous suggestions that holding an object may have clinical relevance for balance-compromised populations.

The efficacy of the Microsoft KinectTM to assess human bimanual coordination.

The Microsoft Kinect has been used in studies examining posture and gait. Despite the advantages of portability and low cost, this device has not been used to assess interlimb coordination. Fundamental insights into movement control, variability, health, and functional status can be gained by examining coordination patterns. In this study, we investigated the efficacy of the Microsoft Kinect to capture bimanual coordination relative to a research-grade motion capture system. Twenty-four healthy adults performed coordinated hand movements in two patterns (in-phase and antiphase) at eight movement frequencies (1.00-3.33 Hz). Continuous relative phase (CRP) and discrete relative phase (DRP) were used to quantify the means (mCRP and mDRP) and variability (sdCRP and sdDRP) of coordination patterns. Between-device agreement was assessed using Bland-Altman bias with 95 % limits of agreement, concordance correlation coefficients (absolute agreement), and Pearson correlation coefficients (relative agreement). Modest-to-excellent relative and absolute agreements were found for mCRP in all conditions. However, mDRP showed poor agreement for the in-phase pattern at low frequencies, due to large between-device differences in a subset of participants. By contrast, poor absolute agreement was observed for both sdCRP and sdDRP, while relative agreement ranged from poor to excellent. Overall, the Kinect captures the macroscopic patterns of bimanual coordination better than coordination variability.

A model to investigate the mechanisms underlying the emergence and development of independent sitting.

When infants first begin to sit independently, they are highly unstable and unable to maintain upright sitting posture for more than a few seconds. Over the course of 3 months, the sitting ability of infants drastically improves. To investigate the mechanisms controlling the development of sitting posture, a single-degree-of-freedom inverted pendulum model was developed. Passive muscle properties were modeled with a stiffness and damping term, while active neurological control was modeled with a time-delayed proportional-integral-derivative (PID) controller. The findings of the simulations suggest that infants primarily utilize passive muscle stiffness to remain upright when they first begin to sit. This passive control mechanism allows the infant to remain upright so that active feedback control mechanisms can develop. The emergence of active control mechanisms allows infants to integrate sensory information into their movements so that they can exhibit more adaptive sitting.

Task-dependent postural control throughout the lifespan.

Routine activities performed while standing and walking require the ability to appropriately and continuously modulate postural movements as a function of a concurrent task. Changes in task-dependent postural control contribute to the emergence, maturation, and decline of complex motor skills and stability throughout the lifespan.

Integrating Motor Variability Evaluation Into Movement System Assessment.

Common assessment tools for determining therapeutic success in rehabilitation typically focus on task-based outcomes. Task-based outcomes provide some understanding of the individual's functional ability and motor recovery; however, these clinical outcomes may have limited translation to a patient's functional ability in the real world. Limitations arise because (1) the focus on task-based outcome assessment often disregards the complexity of motor behavior, including motor variability, and (2) mobility in highly variable real-world environments requires movement adaptability that is made possible by motor variability. This Perspective argues that incorporating motor variability measures that reflect movement adaptability into routine clinical assessment would enable therapists to better evaluate progress toward optimal and safe real-world mobility. The challenges and opportunities associated with incorporating variability-based assessment of pathological movements are also discussed. This Perspective also indicates that the field of rehabilitation needs to leverage technology to advance the understanding of motor variability and its impact on an individual's ability to optimize movement. IMPACT: This Perspective contends that traditional therapeutic assessments do not adequately evaluate the ability of individuals to adapt their movements to the challenges faced when negotiating the dynamic environments encountered during daily life. Assessment of motor variability derived during movement execution can address this issue and provide better insight into a patient's movement stability and maneuverability in the real world. Creating such a shift in motor system assessment would advance understanding of rehabilitative approaches to motor system recovery and intervention.

The control of posture in newly standing infants is task dependent.

The postural sway patterns of newly standing infants were compared under two conditions: standing while holding a toy and standing while not holding a toy. Infants exhibited a lower magnitude of postural sway and more complex sway patterns when holding the toy. These changes suggest that infants adapt postural sway in a manner that facilitates visually fixating on and stabilizing the toy in their hand. When simply standing, infants exhibited postural sway patterns that appeared to be more exploratory in nature. Exploratory sway patterns may allow infants to learn the affordances of their new standing posture. These results demonstrate that newly standing infants are capable of task-dependent postural control.

Development of the coordination between posture and manual control.

Studies have suggested that proper postural control is essential for the development of reaching. However, little research has examined the development of the coordination between posture and manual control throughout childhood. We investigated the coordination between posture and manual control in children (7- and 10-year-olds) and adults during a precision fitting task as task constraints became more difficult. Participants fit a block through an opening as arm kinematics, trunk kinematics, and center of pressure data were collected. During the fitting task, the precision, postural, and visual constraints of the task were manipulated. Young children adopted a strategy where they first move their trunk toward the opening and then stabilize their trunk (freeze degrees of freedom) as the precision manual task is being performed. In contrast, adults and older children make compensatory trunk movements as the task is being performed. The 10-year-olds were similar to adults under the less constrained task conditions, but they resembled the 7-year-olds under the more challenging tasks. The ability to either suppress or allow postural fluctuations based on the constraints of a suprapostural task begins to develop at around 10 years of age. This ability, once developed, allows children to learn specific segmental movements required to complete a task within an environmental context.

Co-occurrences of fall-related factors in adults aged 60 to 85 years in the United States National Health and Nutrition Examination Survey.

A broad set of factors are associated with falling (e.g., age, sex, physical activity, vision, health), but their co-occurrence is understudied. Our objectives were to quantify the number and pattern of co-occurring fall-related factors. Data were obtained from the U.S. National Health and Nutrition Examination Survey (N = 1,957, 60-85 years). Twenty fall-related factors were included (based on previous research), covering a wide range including cognitive, motor, sensory, health, and physical activity measures. The number and pattern of co-occurring fall-related factors were quantified with logistic regression and cluster analyses, respectively. Most participants (59%) had ≥4 fall-risk factors, and each additional risk factor increased the odds of reporting difficulty with falling by 1.28. The identified clusters included: (1) healthy, (2) cognitive and sensory impaired, and (3) health impaired. The mean number of co-occurring fall-related factors was 3.7, 3.8, and 7.2, for clusters 1, 2, and 3, respectively (p<0.001). These observations indicate that co-occurrence of multiple fall-risk factors was common in this national sample of U.S. older adults and the factors tended to aggregate into distinct clusters. The findings support the protective effect of physical activity on fall-risk, the association between gait speed and falls, and the detrimental effect of health-related factors on difficulty with falls (e.g., arthritis, prescription medications). Cluster analyses revealed a complex interplay between sex and BMI that may alter the role of BMI in the etiology of falls. Cluster analyses also revealed a large detrimental effect of health-related factors in cluster 3; it is important to extend current fall interventions (typically focused on balance, flexibility, strength, cognitive, fear factors) to include health-related interventions that target factors such as BMI and arthritis.

Physically coupling two objects in a bimanual task alters kinematics but not end-state comfort.

People often grasp objects with an awkward grip to ensure a comfortable hand posture at the end of the movement. This end-state comfort effect is a predominant constraint during unimanual movements. However, during bimanual movements the tendency for both hands to satisfy end-state comfort is affected by factors such as end-orientation congruency and task context. Although bimanual end-state comfort has been examined when the hands manipulate two independent objects, no research has examined end-state comfort when the hands are required to manipulate two physically-coupled objects. In the present experiment, kinematics and grasp behavior during a unimanual and bimanual reaching and placing tasks were examined, when the hands manipulate two physically-connected objects. Forty-five participants were assigned to one of three groups; unimanual, bimanual no-spring (the objects were not physically connected), and bimanual spring (the objects were connected by a spring), and instructed to grasp and place objects in various end-orientations, depending on condition. Physically connecting the objects did not affect end-state comfort prevalence. However, it resulted in decreased interlimb coupling. This finding supports the notion of a flexible constraint hierarchy, in which action goals guide the selection of lower level action features (i.e., hand grip used for grasping), and the particular movements used to accomplish that goal (i.e., interlimb coupling) are controlled throughout the movement.

Preschoolers search for hidden objects.

The issue of whether young children use spatio-temporal information (e.g., movement of objects through time and space) and/or contact-mechanical information (e.g., interaction between objects) to search for a hidden object was investigated. To determine whether one cue can have priority over the other, a dynamic event that put these cues into conflict was created, with only spatio-temporal information being valid. The 3-year-olds used in the study were found to use the valid spatio-temporal cue exclusively and seemed to ignore the contact-mechanical cue. Both search behavior and eye tracking during the event support the view of a sophisticated sensitivity to the validity of a cue in indicating a target's hidden location.

Developmental changes in the dynamical structure of postural sway during a precision fitting task.

Recent research using measures to assess the time-dependent structure of postural fluctuations has provided new insights into the stability and adaptability of human postural control in adults. To date, little research has examined how postural dynamics reflecting the stability and adaptability of postural control may change as a function of development, especially during supra-postural tasks. The goal of this study was to examine the dynamics of postural fluctuations during a manual-fitting task in which precision, visual and postural task constraints were altered in children and adults. Three age groups were tested: 7-, 10-year olds and college aged adults. Recurrence quantification analysis (RQA) was used to assess the regularity (percent determinism) and complexity (entropy) of the center of pressure (CoP) in the anterior-posterior (AP) and medial-lateral (ML) directions. The CoP patterns exhibited by adults were more deterministic and more complex (higher entropy) than those of the 7-year-old children under the different experimental manipulations. No differences between the adults and the 10-year-old children were observed. The increase in determinism with a corresponding increase in entropy exhibited by the adults and older-children during a manual fitting task may be a prospective mechanism over which postural movements follow a more predictable path allowing for stable and flexible task performance. Our results also support the notion that complex postural fluctuations (as measured by RQA entropy) are functional and typically increase as the precision requirements of a manual task increase.

Conflicting cues in a dynamic search task are reflected in children's eye movements and search errors.

Three-year-olds were given a search task with conflicting cues about the target's location. A ball rolled behind a transparent screen and stopped behind one of four opaque doors mounted into the screen. A wall that protruded above one door provided a visible cue of blockage in the ball's path, while the transparent screen allowed visual tracking of the ball's progress to its last disappearance. On some trials these cues agreed and on others they conflicted. One group saw the ball appear to pass through the wall, violating its solidity, and another group saw the ball stop early, behind a door before the visual wall. Children's eye movements were recorded with an Applied Science Laboratories eye tracker during these real object events. On congruent trials, children tended to track the ball to the visible barrier and select that door. During conflict trials, children's eye movements and reaching errors reflected the type of conflict they experienced. Our data support Scholl and Leslie's (1999) hypotheses that spatio-temporal and contact mechanical knowledge are based on two separate, distinct mechanisms.

Influence of embedding parameters and noise in center of pressure recurrence quantification analysis.

Recurrence quantification analysis (RQA) can extract the dynamics of postural control from center of pressure (CoP) data by quantifying the system's repeatability, complexity, and local dynamic stability through several variables. Computation of these variables requires the selection of suitable embedding parameters for state space reconstruction (i.e. time delay and embedding dimension); however, it is unclear how the parameters influence RQA variables when examining noisy CoP data. This study evaluated the sensitivity of RQA variables to embedding parameter values and noise level, and assessed methods of selecting embedding parameters for CoP data. Five healthy male subjects maintained quiet stance for 30s while the anterior-posterior CoP was measured. The effect of noise was evaluated by adding uniform white noise of increasing amplitude to the raw CoP signal. The magnitude of all RQA variables decreased with increasing noise amplitude for all subjects. A sensitivity analysis was performed by systematically altering the embedding parameters for the raw data with and without a selected level of added noise. The key result was that, for all subjects, the RQA variables were sensitive to the embedding parameter values and the level of noise in the CoP data. Finally, the performance of false nearest neighbors and average displacement algorithms for choosing embedding parameters was evaluated. Both methods gave clear and consistent results for all subjects with either raw or noisy data. The results suggest that careful selection of embedding parameters is essential when using RQA to examine postural control based on noisy CoP data.

Walking while talking: Young adults flexibly allocate resources between speech and gait.

BACKGROUND: Walking while talking is an ideal multitask behavior to assess how young healthy adults manage concurrent tasks as it is well-practiced, cognitively demanding, and has real consequences for impaired performance in either task. Since the association between cognitive tasks and gait appears stronger when the gait task is more challenging, gait challenge was systematically manipulated in this study. OBJECTIVE: To understand how young adults accomplish the multitask behavior of walking while talking as the gait challenge was systematically manipulated. METHODS: Sixteen young adults (21 ±â€¯1.6 years, 9 males) performed three gait tasks with and without speech: unobstructed gait (easy), obstacle crossing (moderate), obstacle crossing and tray carrying (difficult). Participants also provided a speech sample while seated for a baseline indicator of speech. The speech task was to speak extemporaneously about a topic (e.g. first car). Gait speed and the duration of silent pauses during speaking were determined. Silent pauses reflect cognitive processes involved in speech production and language planning. RESULTS: When speaking and walking without obstacles, gait speed decreased (relative to walking without speaking) but silent pause duration did not change (relative to seated speech). These changes are consistent with the idea that, in the easy gait task, participants placed greater value on speech pauses than on gait speed, likely due to the negative social consequences of impaired speech. In the moderate and difficult gait tasks both parameters changed: gait speed decreased and silent pauses increased. CONCLUSION: Walking while talking is a cognitively demanding task for healthy young adults, despite being a well-practiced habitual activity. These findings are consistent with the integrated model of task prioritization from Yogev-Seligmann et al., [1].

Association between stride time fractality and gait adaptability during unperturbed and asymmetric walking.

Human locomotion is an inherently complex activity that requires the coordination and control of neurophysiological and biomechanical degrees of freedom across various spatiotemporal scales. Locomotor patterns must constantly be altered in the face of changing environmental or task demands, such as heterogeneous terrains or obstacles. Variability in stride times occurring at short time scales (e.g., 5-10 strides) is statistically correlated to larger fluctuations occurring over longer time scales (e.g., 50-100 strides). This relationship, known as fractal dynamics, is thought to represent the adaptive capacity of the locomotor system. However, this has not been tested empirically. Thus, the purpose of this study was to determine if stride time fractality during steady state walking associated with the ability of individuals to adapt their gait patterns when locomotor speed and symmetry are altered. Fifteen healthy adults walked on a split-belt treadmill at preferred speed, half of preferred speed, and with one leg at preferred speed and the other at half speed (2:1 ratio asymmetric walking). The asymmetric belt speed condition induced gait asymmetries that required adaptation of locomotor patterns. The slow speed manipulation was chosen in order to determine the impact of gait speed on stride time fractal dynamics. Detrended fluctuation analysis was used to quantify the correlation structure, i.e., fractality, of stride times. Cross-correlation analysis was used to measure the deviation from intended anti-phasing between legs as a measure of gait adaptation. Results revealed no association between unperturbed walking fractal dynamics and gait adaptability performance. However, there was a quadratic relationship between perturbed, asymmetric walking fractal dynamics and adaptive performance during split-belt walking, whereby individuals who exhibited fractal scaling exponents that deviated from 1/f performed the poorest. Compared to steady state preferred walking speed, fractal dynamics increased closer to 1/f when participants were exposed to asymmetric walking. These findings suggest there may not be a relationship between unperturbed preferred or slow speed walking fractal dynamics and gait adaptability. However, the emergent relationship between asymmetric walking fractal dynamics and limb phase adaptation may represent a functional reorganization of the locomotor system (i.e., improved interactivity between degrees of freedom within the system) to be better suited to attenuate externally generated perturbations at various spatiotemporal scales.

An active balance board system with real-time control of stiffness and time-delay to assess mechanisms of postural stability.

Increased time-delay in the neuromuscular system caused by neurological disorders, concussions, or advancing age is an important factor contributing to balance loss (Chagdes et al., 2013, 2016a,b). We present the design and fabrication of an active balance board system that allows for a systematic study of stiffness and time-delay induced instabilities in standing posture. Although current commercial balance boards allow for variable stiffness, they do not allow for manipulation of time-delay. Having two controllable parameters can more accurately determine the cause of balance deficiencies, and allows us to induce instabilities even in healthy populations. An inverted pendulum model of human posture on such an active balance board predicts that reduced board rotational stiffness destabilizes upright posture through board tipping, and limit cycle oscillations about the upright position emerge as feedback time-delay is increased. We validate these two mechanisms of instability on the designed balance board, showing that rotational stiffness and board time-delay induced the predicted postural instabilities in healthy, young adults. Although current commercial balance boards utilize control of rotational stiffness, real-time control of both stiffness and time-delay on an active balance board is a novel and innovative manipulation to reveal balance deficiencies and potentially improve individualized balance training by targeting multiple dimensions contributing to standing balance.

Adaptations in interlimb and intralimb coordination to asymmetrical loading in human walking.

The purpose of this study was to examine both the intralimb (within a limb) and interlimb (between the right and left limbs) adaptations that occur in response to a unilaterally applied leg load as subjects walked at their preferred walking speed. It was hypothesized that this adaptation would alter interlimb coordination while intralimb coordination remained invariant. Subjects (n = 12) were required to walk on a treadmill at preferred walking speed. Bilateral 3-D kinematic data were collected while a load placed on the leg was increased. Gait adaptations to leg-loading were assessed through changes in coordination patterns between specific limb couplings. Continuous relative phase (CRP) was used to evaluate changes in limb coordination under each experimental load condition compared to a no load baseline condition. Both changes in magnitude of CRP (root-mean-square (RMS) analysis) as well as temporal changes in CRP across the stride cycle (cross-correlation) were assessed. Cross-Correlation values changed with load for all interlimb couplings assessed suggesting alterations in interlimb coordination across the stride cycle. CRP cross-correlation values were close to 1.0 in all the intralimb couplings examined, showing a relative invariance in intralimb coordination. Coordination changes in RMS were also observed for the interlimb couplings. RMS changes were also observed in the intralimb couplings on the loaded side. It appears that gait adaptations to a unilaterally applied leg load appear both at the intralimb and interlimb level. However, the majority of changes did appear at the interlimb level, where CRP as assessed through both cross-correlation and RMS measures changed. This study supports previous results that suggested a tighter coupling exists at the intralimb level, leaving the majority of gait adaptations to occur at the interlimb level. The observed adaptation in differences between interlimb and intralimb coordination may provide insight into gait adaptations in pathological gait.

Limit cycle oscillations in standing human posture.

Limit cycle oscillations (LCOs) are a hallmark of dynamic instability in time-delayed and nonlinear systems such as climate change models, biological oscillators, and robotics. Here we study the links between the human neuromuscular system and LCOs in standing posture. First, we demonstrate through a simple mathematical model that the observation of LCOs in posture is indicative of excessive neuromuscular time-delay. To test this hypothesis we study LCOs in the postural sway of individuals with multiple sclerosis and concussed athletes representing two different populations with chronically and acutely increased neuromuscular time-delays. Using a wavelet analysis method we demonstrate that 67% of individuals with multiple sclerosis and 44% of individuals with concussion exhibit intermittent LCOs; 8% of MS-controls, 0% of older adults, and 0% of concussion-controls displayed LCOs. Thus, LCOs are not only key to understanding postural instability but also may have important applications for the detection of neuromuscular deficiencies.