Tai Chi counteracts age-related somatosensation and postural control declines among older adults.

Objective: To investigate the effect of a 16-week Tai Chi practice on strength, tactile sensation, kinesthesia, and static postural control among older adults of different age groups. Methods: This is a quasi-experimental study. Thirteen participants aged 60-69 years (60-69yr), 11 aged 70-79 years (70-79yr), and 13 aged 80-89 years (80-89yr) completed 16 weeks of 24-form Tai Chi practice. Their ankle and hip peak torque, tactile sensation, ankle and knee kinesthesia, and the root mean square of the center of pressure (Cop-RMS) were measured before (week 0) and after (week 17) practice. Results: 80-89yr showed less ankle plantar/dorsiflexion and hip abduction peak torques (p = 0.003, p < 0.001, p = 0.001), and a greater ankle plantar/dorsiflexion kinesthesia (p < 0.001, p = 0.002) than 60-69yr and 70-79yr. Greater ankle plantar/dorsiflexion and hip abduction torques (p = 0.011, p < 0.001, p = 0.045), improved arch and heel tactile sensation (p = 0.040, p = 0.009), and lower knee flexion/extension kinesthesia (p < 0.001, p = 0.044) were observed at week 17. The significant group*practice interaction for the fifth metatarsal head tactile sensation (p = 0.027), ankle plantar/dorsiflexion kinesthesia (p < 0.001, p = 0.004), and the CoP-RMS in the mediolateral direction (p = 0.047) only in 80-89yr revealed greater improvement at week 17. Conclusion: Tai Chi practice increased strength, tactile sensation, kinesthesia, and static postural control among older adults. Tai Chi practice improved tactile, kinesthesia sensations, and static postural control among older adults over 80, who presented with worse strength and kinesthesia than their younger counterparts. Tai Chi practice offers a safe exercise option for those aged over 80 to encourage improvements in sensorimotor control.

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.

Passive static stretching alters the characteristics of the force-velocity curvature differently for fast and slow muscle groups-A practical application of Hill's equation.

Studies showed fast muscle fibers have a greater constant b value of Hill's equation than that of slow muscle fibers, and the changing ratio of b/Vmax indicates the altered characteristics of muscles under certain conditions such as static stretching. This study was to investigate the effect of acute passive static stretching on the curvature of force-velocity curve in people with different muscle fiber types. A two-step work was conducted in current study through using Hill's equation: 1) calculated b values for each subject at different conditions (non-stretched and stretched) to determine muscle groups, and 2) examined the effect of static stretching on different muscle groups. Sixty-five college students performed isokinetic leg extensions at 5 speeds to test peak torque, following either a non-stretching or two passive static quadriceps stretching exercises. The peak torque and corresponding velocity were used to calculate the b constant. Data reduction consisted of calculating a Z score for each non-stretched and stretched b values. Individuals, whose non-stretched b constant was above or below one standard deviation of the Z score, were designated as the less curved (fast) and more curved (slow) groups, respectively. A paired t-test was used to analyze the pre and post intervention effect on b values for each group (p < 0.05). This study found passive static stretching significantly altered the b constant of the fast group, but no effect on slow group. Therefore, we suggest static stretching should be avoided immediately before fast or explosive activities in individuals using predominantly fast muscle fibers.

Eye movement influences on coupled and decoupled eye-hand coordination tasks.

Visually guided reaching precision and accuracy depend on the level of coupling between movements of the eyes and hand. In the present study, participants performed central fixations and either saccadic or smooth pursuit eye movements during fast and accurate reaching tasks involving eye-hand coupling and decoupling to better understand type of eye movement influence over upper limb control. Some eye-hand coupling and decoupling tasks also included hand reversals, where the hand moves away from the target to direct a cursor toward the target to account for various levels of hand-cursor and eye-cursor coupling. Regardless of eye-movement type, eye-hand-cursor coupling produced an endpoint accuracy advantage over decoupling. Use of hand reversal decreased peak speed and increased response time of the hand, whether considering fixation or a given eye movement. Use of smooth pursuit slowed hand movements relative to saccades, yet improved endpoint accuracy. Compared to central fixations, using smooth pursuit also slowed hand movements, while using saccades decreased, thus improved, hand reaction times. Data suggest an advantage, when using smooth pursuit to track the hand movement for the greatest endpoint accuracy, an advantage when using saccades for the fastest movements, and an eye-hand coupling advantage when using saccades for the shortest reactions. Researchers should provide clear eye-movement instructions for participants and/or monitor the eyes when assessing similar upper limb control to account for possible differences in eye movements used. Moreover, the type of eye movement chosen for participants should correspond to the primary goal of the task.

Eye-hand decoupling decreases visually guided reaching independently of posture but reduces sway while standing: Evidence for supra-postural control.

We investigated whether visually guided reaching differs for sitting and standing postures while the eyes and hand are coupled to move in the same direction or decoupled to move in opposite directions. We also investigated how coupled and decoupled reaching tasks influenced standing postural control. Eighteen healthy young adults (M = 21 years) moved a cursor using finger movements along a vertical touchscreen while sitting or standing. In an eye-hand coupling (EH) task, participants moved their finger/cursor from a central target to a peripheral target located either up, down, left, or right. In an eye-hand decoupling (EHD) task, participant's finger movement moved the cursor in the opposite direction. Sway measures during the standing condition and kinematic variables for the cursor offered insight into whole-body control. Performances in EH revealed smaller errors and faster movements than EHD regardless of postural condition. Similar hand movements existed between sitting and standing when accounting for task, while greater variability in absolute endpoint errors existed for standing than sitting when task was ignored. Less postural sway existed for EHD than EH when standing. These data provide evidence that when participants decoupled the eyes and hand movement direction while standing, they attenuated sway to support control of this complex, cognitively demanding, visuomotor task.

Gaze Direction Changes the Vertical-Horizontal Illusory Effects on Manual Length Estimations.

We examined potentially deceptive influences of the vertical-horizontal (V-H) illusion on manual length estimations. When viewing V-H illusory configurations, people perceive that the bisecting segment length exceeds the bisected segment length when segments are actually equal. Participants used downward or rightward pointing movements to manually estimate the length of a short bisecting segment of the V-H illusion in upright or rotated configurations. Participants directed their gaze freely, on the configuration, or on the movement space. Manual length estimations for upright and rotated configurations depended on gaze direction, revealing bisection influences only for restricted viewing. People produced illusory influences on perceptuomotor control only when gaze was directed toward V-H configurations or their movement. Exploitation of deceptive visual cues can direct upper limb control for sensorimotor coordination.

Effects of exoskeleton use on movement kinematics during performance of common work tasks: A case study.

BACKGROUND: An exoskeleton may assist performance of basic work-related tasks. Its application should not alter user kinematics, which compromise user safety. OBJECTIVE: This case study was used to assess whether people wearing a lower-body K-SRDTM exoskeleton could complete common work tasks without altering kinematics that may increase injury risk. METHODS: Three males performed three tasks: kneeling and standing (kneel), lifting and lowering a weighted box floor-to-waist (lift), and stair-climbing with a weighted box (climb), all repeated with and without exoskeleton use (EXO, NONE). RESULTS: Kinematics with EXO often mimicked NONE. Hip and knee flexion with EXO often exceeded NONE without increasing heart rate for kneel. During lift with EXO, participants avoided greater lateral trunk flexion associated with injuries and used the preferred semi-squat technique. Participants produced more foot clearance with EXO than NONE during climb. Other outcomes of heart rate, perceived exertion, fatigue, and usability were mixed. CONCLUSIONS: EXO augmentation does not need to alter movement kinematics during performances of kneel, lift, and climb tasks. EXO kinematic alterations did not appear to compromise user safety in terms of lateral trunk bending. It may encourage good technique, such as greater foot clearance to avoid tripping, for some tasks, and changes in lifting strategies to avoid extreme flexion and protect passive tissues.

Visually-guided saccades attenuate postural sway under non-fatigued, fatigued, and stretched states.

Muscular fatigue, which reduces force output and position sense, often leads to increased sway and potential balance impairments. In contrast, visually-guided saccadic eye movements (saccades) can attenuate sway more than fixating gaze on an external target. The goals of this study were to determine whether the use of saccades could reduce the increased postural sway in a fatigued state and to better understand the contributions to fatigue-induced increased sway. We compared the effects of gazing at a fixation point (FP) and performing saccades (SAC) on various spatial and temporal measures of the center of pressure (CoP) while participants stood as still as possible on a force plate. Participants used either a narrow or wide base of support and performed three trials for each eye movement condition (SAC, FP) in three states (non-fatigued-NF, stretched-S, and fatigued-F). Calf raises to exhaustion induced ankle fatigue. Extreme plantar- and dorsi-flexion induced stretch. SAC significantly decreased sway and increased time-series complexity (sample entropy) compared to FP. F increased sway and decreased time-series complexity compared to NF and S states, which were similar. Reduced force production, which accompanies muscle fatigue and stretching, did not account for increased sway associated with acute bouts of ankle muscle fatigue. Increased position sense often associated with muscle stretching likely compensated for any reduced force output for S, while the decreased position sense associated with F probably explained the increased sway in this state. Performing saccadic eye movements during quiet stance can help reduce sway under various states.

Different damping responses explain vertical endpoint error differences between visual conditions.

Upright people making goal-directed movements in dark environments often vertically undershoot remembered target locations when compared to performances in illuminated environments. In this study, we wanted to determine whether influences of the gravitational pull and/or type of muscle activation could explain differences in vertical endpoint precision between movements to visually remembered target locations with and without allocentric cues available. We also used a simple damping model for movement trajectories to describe potential differences in behavior between visual conditions. Subjects performed straight arm pointing movements to REAL target locations or remembered target locations in darkness (DARK) or normal room lighting (LIGHT). Performances were made from UPRIGHT and INVERTED (upside down) body orientations. Starting arm position (UP by the ear; DOWN on the thigh) also varied so that eccentric or concentric muscle contractions for arm flexion or extension movements occurred primarily along the earth-fixed vertical either with or against the gravitational pull. Effects of visual condition (LIGHT, DARK), body orientation (UPRIGHT, INVERTED), starting arm position (UP, DOWN), and target level (Near, Middle, Far) on elevation endpoint errors revealed that subject's errors in the DARK were more negative than those in the LIGHT. Errors correlated well with movement displacement to reveal the common vertical undershooting bias in darkness exacerbated by inverting the body or requiring greater movement excursions. Although influences of gravitational pull and muscle activation type could not explain differences between visual conditions, modeling revealed critically damped behavior in the DARK and under-damped behavior in the LIGHT to indicate muscle energy dissipation without vision.

Select exercise modalities may reverse movement dysfunction because of peripheral neuropathy.

Peripheral neuropathy (PN) is a highly prevalent and potentially debilitating disease linked to mobility and postural control impairments and movement dysfunction in goal-directed movements. Although different exercises have produced functional improvements in PN, recent evidence indicates that exercises like Tai Chi can alter the damaged sensory system and facilitate recovery of mobility and balance, potentially reducing the reliance on other people.

Effect of step training and rhythmic auditory stimulation on functional performance in Parkinson patients.

BACKGROUND: Rhythmic auditory stimulation (RAS) can influence movement during straight line walking and direction transition in individuals with Parkinson disease (PD). OBJECTIVE: The authors studied whether multidirectional step training with RAS would generalize to functional gait conditions used in daily activities and balance. METHODS: In a matched-pairs design, 8 patients practiced externally paced (EP) stepping (RAS group), and 8 patients practiced internally paced (IP) stepping (no RAS group) for 6 weeks. Participants were evaluated on the first and last days of practice, and 1 week, 4 weeks, and 8 weeks after practice termination. Evaluations included a primary measurement--the Dynamic Gait Index (DGI)--and secondary measurements--the Unified Parkinson's Disease Rating Scale (UPDRS), Tinetti-gait and balance tests, Timed-Up-and-Go (TUG), and Freezing of Gait Questionnaire (FOGQ). RESULTS: The RAS group significantly improved performance on the DGI and several secondary measures, and they maintained improvements for the DGI, Tinetti, FOGQ, and balance and gait items of the UPDRS above pretraining values at least 4 weeks after practice termination. The no RAS group revealed several improvements with training but could not maintain these improvements for as long as the other group. CONCLUSIONS: Individuals with PD can generalize motor improvements achieved during multidirectional step training to contexts of functional gait and balance. Training with RAS is advantageous for enhancing functional gait improvements and the maintenance of functional gait and balance improvements over 8 weeks.

Age-related and sensory declines offer insight to whole body control during a goal-directed movement.

We examined the effects of aging and lower limb sensory deficits (LLSD) on whole body control. Performances of a reaching task involving a step were measured in subjects with LLSD and young and older controls. Having LLSD was accompanied with greater reach errors and variability in lateral step deviations. Aging effects explained the smaller step deviations and longer movement times. These results suggest that older adults with LLSD have performance declines associated with deficits of the disease and aging that often differ, at least for this goal-directed discrete task. Furthermore, longer MT for older controls and shorter MT for LLSD subjects were associated with greater movement coupling. Longer movement periods likely offered older controls time to use sensory feedback to maintain good endpoint accuracy. Evidently, somatosensations from the limbs used during performance of whole body movements are required for the most accurate goal-directed control.

Pointing control using a moving base of support.

The purposes of this study were to determine whether gaze direction provides a control signal for movement direction for a pointing task requiring a step and to gain insight into why discrepancies previously identified in the literature for endpoint accuracy with gaze directed eccentrically exist. Straight arm pointing movements were performed to real and remembered target locations, either toward or 30 degrees eccentric to gaze direction. Pointing occurred in normal room lighting or darkness while subjects sat, stood still or side-stepped left or right. Trunk rotation contributed 22-65% to gaze orientations when it was not constrained. Error differences for different target locations explained discrepancies among previous experiments. Variable pointing errors were influenced by gaze direction, while mean systematic pointing errors and trunk orientations were influenced by step direction. These data support the use of a control strategy that relies on gaze direction and equilibrium inputs for whole-body goal-directed movements.

Allocentric cues do not always improve whole body reaching performance.

The aim of this investigation was to gain further insight into control strategies used for whole body reaching tasks. Subjects were requested to step and reach to remembered target locations in normal room lighting (LIGHT) and complete darkness (DARK) with their gaze directed toward or eccentric to the remembered target location. Targets were located centrally at three different heights. Eccentric anchors for gaze direction were located at target height and initial target distance, either 30 degrees to the right or 20 degrees to the left of target location. Control trials, where targets remained in place, and remembered target trials were randomly presented. We recorded movements of the hand, eye and head, while subjects stepped and reached to real or remembered target locations. Lateral, vertical and anterior-posterior (AP) hand errors and eye location, and gaze direction deviations were determined relative to control trials. Final hand location errors varied by target height, lighting condition and gaze eccentricity. Lower reaches in the DARK compared to the LIGHT condition were common, and when matched with a tendency to reach above the low target, help explain more accurate reaches for this target in darkness. Anchoring the gaze eccentrically reduced hand errors in the AP direction and increased errors in the lateral direction. These results could be explained by deviations in eye locations and gaze directions, which were deemed significant predictors of final reach errors, accounting for a 17-47% of final hand error variance. Results also confirmed a link between gaze deviations and hand and head displacements, suggesting that gaze direction is used as a common input for movement of the hand and body. Additional links between constant and variable eye deviations and hand errors were common for the AP direction but not for lateral or vertical directions. When combined with data regarding hand error predictions, we found that subjects' alterations in body movement in the AP direction were associated with AP adjustments in their reach, but final hand position adjustments were associated with gaze direction alterations for movements in the vertical and horizontal directions. These results support the hypothesis that gaze direction provides a control signal for hand and body movement and that this control signal is used for movement direction and not amplitude.

Gaze tracking accuracy in humans: two eyes are better than one.

Many people viewing a small distant object report the use of a dominant eye, which may change when viewing the same object in right or left viewing fields. This study was designed to determine if the accuracy in left and right gaze recordings from a video-based binocular eye tracking system with eye-head integration would change when observers looked at targets in different viewing fields. We also wanted to determine whether system accuracy was best using the dominant eye, the average of both eyes or an alignment switching strategy for various viewing fields. Bilateral eye and head movements were recorded to determine point of gaze (POG) for subjects viewing targets with the head neutral or rotated left or right with a stationary body (static) or while stepping toward a target and back (dynamic). Constant, absolute and variable POG and rotational errors were determined. Results showed that although errors were highest in the static condition for POG errors and in the dynamic condition for rotational errors, absolute rotational angle errors were similar for static and dynamic conditions, indicating that subject to object distance played an important role in determining POG errors. Furthermore, errors were not consistently lower for a given eye or for a given eye within a specific viewing field for the head rotations performed. Since low errors across the viewing fields were found by averaging left and right errors, it was concluded that this average and should be used in future studies involving relatively small eye-in-head rotations.

Comparing human reaches across three viewing conditions in a step and reach task.

The effects of gaze and step direction on a step and reach task were studied to gain insight to possible motor control strategies used in goal-directed whole-body movements. Head, foot and arm positions were monitored while subjects reached to nine targets in space. In dim light, subjects looked at and reached to actual targets or remembered targets, or reached to remembered targets initially located eccentric to gaze orientation. Final reaching errors were influenced by step and gaze orientations, but gaze direction variables were the largest contributors found to predict reach errors. While spatial memory of target location was initially encoded in eye-centered coordinates, memory of eccentric target location was not updated when subjects stepped and reached. Thus, control strategies were dependent on gaze direction in the dim light conditions.

Evidence of an association among age-related changes in physical, psychomotor and autonomic function.

BACKGROUND: autonomic modulation of the heart, as measured by heart rate variability, is directly associated with cardiorespiratory fitness and inversely associated with all-cause mortality. The extent to which cardiorespiratory fitness and heart rate variability are related in older adults is difficult to ascertain due to difficulties in assessing physical fitness among older age groups. OBJECTIVE: to examine heart rate variability and measures of physical function, thereby allowing for the inclusion of a greater cross-section of older adults than can be tested using traditional fitness tests. METHODS: 39 older adults (mean age: 73.2+/-8.1 years; range=60-93 years) underwent evaluation of short-term (5 min) heart rate variability and performance of the American Alliance for Health Physical Education Recreation and Dance Functional Fitness Assessment for Older Adults. Pearson correlation, stepwise multiple regression, and factor analysis were used to describe associations among age, heart rate variability, and functional fitness test-items. RESULTS: significant associations were observed for age and the standard deviation of all normal RR intervals (r=-0.39, P<0.01), and the American Alliance for Health Physical Education Recreation and Dance cardiovascular endurance (r=0.45, P<0.01), strength (r=-0.53, P<0.001), agility (r=0.80, P<0.001), and coordination (r=0.57, P<0.001) items. Standard deviation of all normal RR intervals was negatively associated with the American Alliance for Health Physical Education Recreation and Dance agility (r=-0.37, P<0.01) and coordination (r=-0.49, P<0.001) items. Stepwise multiple regression included only the American Alliance for Health Physical Education Recreation and Dance coordination performance in predicting standard deviation of all normal RR intervals [standard deviation=63.98-2.5 (coordination), F=33.9, P<0.01]. Factor analysis revealed that age, agility, and coordination comprised one factor with a high degree of commonality. CONCLUSION: the association between heart rate variability and coordination suggests concurrent aging of autonomic and psychomotor function.

Drawing sequences of segments in 3D: kinetic influences on arm configuration.

Complex movements are generally thought to consist of a series of simpler elements. If this is so, how does the sensorimotor system assemble the pieces? This study recorded and evaluated sequences of arm movements to various targets placed in three-dimensional (3D) space. Subjects performed sequences consisting of single, double, or triple segments with the same first target but with different second targets. The data analysis focused on the first movement segment and evaluated hand path curvature, the hand's final approach to the first target, and the whole arm postures at the beginning and end. Although some idiosyncratic differences in approach were observed, only the final arm posture depended, in a consistent way, on which particular movement was to follow as the second segment. This provided evidence for "coarticulation" of the two segments, only at the level of arm posture, and simulations revealed that this anticipatory modification improved the energetic efficiency of the second segment. Data from movements through five consecutive triple segments (i.e., 5 triangles) were assessed to determine whether kinematic constraints, such as Donders' law, apply to repetitive drawing movements. Although such constraints could prevent the accumulation of changes in arm posture, this was not observed. Instead, in most cases, the elbow was a little bit higher at the end of each triangle than at the beginning. Taken together, the results suggest that coarticulation may facilitate the joining of two segments and the efficiency of the second movement, but does not extend over the drawing of several segments.

Using arm configuration to learn the effects of gyroscopes and other devices.

Previous studies have perturbed the association between motor commands and arm movements by applying forces to the arm during two-dimensional movements. These studies have revealed that, when the normal hand path is perturbed, subjects gradually adapt their motor commands to return to this path. The present study used the spin of a gyroscope to create a complex perturbation, as subjects reached to targets presented in three dimensions. Hand path did not change, but the whole-arm geometry ("arm configuration" in four dimensions) was altered. Over a series of several hundred reaches to various targets, subjects gradually returned the arm movement to its normal configuration. Furthermore, during the course of this learning, subjects used a strategy that involved manipulating arm posture. A similar strategy was observed when subjects made reaching movements with a rod attached to the upper arm to change its inertial characteristics. In both cases, the gradual return to the normal arm movement was accomplished without an increase in kinetic energy, suggesting that arm postures and movements (kinematics) and muscular forces (kinetics) may be mutually optimized. In contrast to previous studies, the present results highlight the role of arm configuration (rather than hand path) in learning and control.