James L. Patton

CENTER OF MASS VELOCITY-POSITION PREDICTIONS FOR BALANCE CONTROL

The purposes of this analysis were to predict the feasible movements during which balance can be maintained, based on environmental (contact force), anatomical (foot geometry), and physiological (muscle strength) constraints, and to identify the role of each constraint in limiting movement. An inverted pendulum model with a foot segment was used with an optimization algorithm to determine the set of feasible center of mass (CM) velocity-position combinations for movement termination. The upper boundary of the resulting feasible region ran from a velocity of 1.1 s-1 (normalized to body height) at 2.4 foot lengths behind the heel, to 0.45 s-1 over the heel, to zero over the toe, and the lower boundary from a velocity of 0.9 s-1 at 2.7 foot lengths behind the heel, to zero over the heel. Forward falls would be initiated if states exceeded the upper boundary, and backward falls would be initiated if the states fell below the lower boundary. Under normal conditions, the constraint on the size of the base of support (BOS) determined the upper and lower boundaries of the feasible region. However, friction and strength did limit the feasible region when friction levels were less than 0.82, when dorsiflexion was reduced more than 51%, or when plantar flexion strength was reduced more than 35%. These findings expand the long-held concept that balance is based on CM position limits (i.e. the horizontal CM position has to be confined within the BOS to guarantee stable standing) to a concept based on CM velocity-position limits.

Evaluation of robotic training forces that either enhance or reduce error in chronic hemiparetic stroke survivors.

This investigation is one in a series of studies that address the possibility of stroke rehabilitation using robotic devices to facilitate “adaptive training.” Healthy subjects, after training in the presence of systematically applied forces, typically exhibit a predictable “after-effect.” A critical question is whether this adaptive characteristic is preserved following stroke so that it might be exploited for restoring function. Another important question is whether subjects benefit more from training forces that enhance their errors than from forces that reduce their errors. We exposed hemiparetic stroke survivors and healthy age-matched controls to a pattern of disturbing forces that have been found by previous studies to induce a dramatic adaptation in healthy individuals. Eighteen stroke survivors made 834 movements in the presence of a robot-generated force field that pushed their hands proportional to its speed and perpendicular to its direction of motion — either clockwise or counterclockwise. We found that subjects could adapt, as evidenced by significant after-effects. After-effects were not correlated with the clinical scores that we used for measuring motor impairment. Further examination revealed that significant improvements occurred only when the training forces magnified the original errors, and not when the training forces reduced the errors or were zero. Within this constrained experimental task we found that error-enhancing therapy (as opposed to guiding the limb closer to the correct path) to be more effective than therapy that assisted the subject.

Robot-assisted adaptive training: custom force fields for teaching movement patterns

Based on recent studies of neuro-adaptive control, we tested a new iterative algorithm to generate custom training forces to trick subjects into altering their target-directed reaching movements to a prechosen movement as an after-effect of adaptation. The prechosen movement goal, a sinusoidal-shaped path from start to end point, was never explicitly conveyed to the subject. We hypothesized that the adaptation would cause an alteration in the feedforward command that would result in the prechosen movement. Our results showed that when forces were suddenly removed after a training period of 330 movements, trajectories were significantly shifted toward the prechosen movement. However, de-adaptation occurred (i.e., the after-effect washed out) in the 50-75 movements that followed the removal of the training forces. A second experiment suppressed vision of hand location and found a detectable reduction in the washout of after-effects, suggesting that visual feedback of error critically influences learning. A final experiment demonstrated that after-effects were also present in the neighborhood of training-44% of original directional shift was seen in adjacent, unpracticed movement directions to targets that were 60/spl deg/ different from the targets used for training. These results demonstrate the potential for these methods for teaching motor skills and for neuro-rehabilitation of brain-injured patients. This is a form of implicit learning, because unlike explicit training methods, subjects learn movements with minimal instructions, no knowledge of, and little attention to the trajectory.

Static versus dynamic predictions of protective stepping following waist-pull perturbations in young and older adults.

The purposes of this study were: (1) to determine the frequency of protective stepping for balance recovery in subjects of different ages and fall-status, and (2) to compare predicted stepping based on a dynamic model (Pai and Patton, 1997. Journal of Biomechanics 30, 347 354) involving displacement and velocity combinations of the center of mass (COM) versus a static model based on displacement alone against experimentally induced stepping. Responses to three different magnitudes of forward waist pulls were recorded for 13 young, 18 older-non-fallers and 18 older-fallers. The COM phase plane trajectories derived from motion analysis were compared with the model-predicted threshold values for stepping. We found that the older fallers had the highest percentage of stepping trials (52%), followed by older-non-fallers (17.3%), and young (2.7%) at the lowest perturbation level. Younger subjects stepped less often than the elderly at the middle level. Everyone consistently stepped at the highest level of perturbation. Overall, the dynamic model showed better predictive capacity (65%) than the static model (5%) for estimating the initiation of stepping. Furthermore, the threshold for step initiation derived from the dynamic model could consistently predict when a step must occur. However, it was limited, especially among older fallers at the low perturbation level, in that it considered some steps unnecessary that were presumably triggered by fear of falling or other factors.

Evaluation of a model that determines the stability limits of dynamic balance

A recent model of balance control has revealed two types of boundaries describing stability limits for center of mass (CM) dynamics: torque boundaries and state boundaries. The purpose of this study was to determine if these boundaries correctly characterize empirical data. We analyzed 2367 trials from 10 subjects who recovered their balance after they voluntarily pulled on a handle. We hypothesized that if model predictions were valid, both types of boundaries should encompass the empirical trajectories. We also hypothesized that each trajectorys nearest distance to the torque boundaries (the torque safety margin) would be correlated with the center of pressure (COP) safety margin, defined as the COPs nearest distance to the edge of the feet. The results supported the accuracy of the model-derived boundaries, with torque boundaries encompassing 100% and state boundaries encompassing 99.8% of the trials. Moreover, torque safety margins were highly correlated with COP safety margins, supporting the use of COP safety margins for estimating relative stability in dynamic tasks where balance is maintained. The distributions of the trajectories also suggested that a safety margin-oriented control strategy might be a robust alternative to the hypothesis that the central nervous system strives to optimize motion. The distinctions among different safety margins are discussed.

Custom-designed haptic training for restoring reaching ability to individuals with poststroke hemiparesis

We present an initial test of a technique for retraining reaching skills in patients with poststroke hemiparesis, in which errors are temporarily magnified to encourage learning and compensation. Individuals with poststroke hemiparesis held a horizontal plane robotic manipulandum that could exert a variety of forces while recording patients movements. We measured how well the patients recovered movement straightness in a single visit to the laboratory (approximately 3 h). Following training, we returned forces to zero for an additional 50 movements to discern if aftereffects lasted. We found that all subjects showed immediate benefit from the training, although 3 of the 10 subjects did not retain these benefits for the remainder of the experiment. We discuss how these approaches demonstrate great potential for rehabilitation tools that augment error to facilitate functional recovery.

Robots can teach people how to move their arm

Describes a new theoretical framework for robot-aided training of arm movements. This framework is based on recent studies of motor adaptation in human subjects and on general considerations about adaptive control of artificial and biological systems. The authors propose to take advantage of the adaptive processes through which subjects, when exposed to a perturbing field, develop an internal model of the field as a relation between experienced limb states and forces. The problem of teaching new movements is then reduced to the problem of designing force fields capable of inducing the desired movements as after-effects of the adaptation triggered by prolonged exposure to the fields. This approach is an alternative to more standard training methods based on the explicit specification of the desired movement to the learner. Unlike these methods, the adaptive process does not require explicit awareness of the desired movement as adaptation is uniquely concerned with restoring a preexisting kinematic pattern after a change in dynamical environment.

Relative stability improves with experience in a dynamic standing task

Abstract. This study tested the hypothesis that subjects improve their relative stability as they learn a dynamic pulling task. Healthy adult subjects practiced making brief horizontal pulls (<300xa0ms) on a handle to a range of target forces ranging from 20 to 80% of their estimated maximum for 5xa0days. They were instructed to always keep their feet flat and begin and end their motion in an upright posture. In order to do this, subjects had to develop the appropriate body momentum prior to the pull and then recover their balance following the pull. We analyzed relative stability during balance recovery, using two measures: spatial safety margin (minimum distance of the center of pressure, COP, to the edges of the feet) and temporal safety margin (minimum extrapolated time for the COP to reach the edges of the feet). We hypothesized that: (1) spatial and temporal safety margins would be uncorrelated; (2) safety-margin means would increase with practice; and (3) safety-margin standard deviations would decrease with practice. Two experiments were conducted: one where subjects practiced three force targets and positioned their initial COP in a small window, and one where subjects practiced two force targets with no initial COP constraint. Results showed that spatial and temporal safety margins were correlated but shared less than 6% variance, indicating that they reflected different aspects of control. Safety-margin averages increased with practice and standard deviations decreased with practice, indicating that the stability of balance control in the execution of this task became more robust. We suggest that the nervous system could use safety margins in both feedback and feedforward control of balance.

Error Augmentation Enhancing Arm Recovery in Individuals With Chronic Stroke A Randomized Crossover Design

Background. Neurorehabilitation studies suggest that manipulation of error signals during practice can stimulate improvement in coordination after stroke. Objective. To test visual display and robotic technology that delivers augmented error signals during training, in participants with stroke. Methods. A total of 26 participants with chronic hemiparesis were trained with haptic (via robot-rendered forces) and graphic (via a virtual environment) distortions to amplify upper-extremity (UE) tracking error. In a randomized crossover design, the intervention was compared with an equivalent amount of practice without error augmentation (EA). Interventions involved three 45-minute sessions per week for 2 weeks, then 1 week of no treatment, and then 2 additional weeks of the alternate treatment. A therapist provided a visual cursor using a tracking device, and participants were instructed to match it with their hand. Haptic and visual EA was used with blinding of participant, therapist, technician-operator, and evaluator. Clinical measures of impairment were obtained at the beginning and end of each 2-week treatment phase as well as at 1 week and at 45 days after the last treatment. Results. Outcomes showed a small, but significant benefit to EA training over simple repetitive practice, with a mean 2-week improvement in Fugl-Meyer UE motor score of 2.08 and Wolf Motor Function Test of timed tasks of 1.48 s. Conclusions. This interactive technology may improve UE motor recovery of stroke-related hemiparesis.

Haptic cooperation between people, and between people and machines

Haptic interaction between people and machines might benefit from an understanding of haptic communication between one person and another. We recently reported results showing that two people performing a physically shared dyadic task can outperform either person alone, even when the perception of each participant is that the other is a hindrance. Evidently a dyad quickly negotiates a more efficient motion strategy than is available to individuals. This negotiation must take place through a haptic channel of communication, and it is apparently at a level below the awareness of the participants. Here we report results on the motion strategy that emerged. By recording forces and motions we show that the dyads specialized temporally such that one member took on early parts of the motion and the other late parts. Tests in which one participants contribution was surreptitiously replaced by a motor did not elicit a similar cooperative response from the remaining human participant, showing that the language of haptic communication between people must be rather subtle