Jaebum Park

Changes in multifinger interaction and coordination in Parkinson's disease.

In this study, we tested several hypotheses related to changes in finger interaction and multifinger synergies during multifinger force production tasks in Parkinsons disease. Ten patients with Parkinsons disease, mostly early stage, and 11 healthy control subjects participated in the study. Synergies were defined as covaried adjustment of commands to fingers that stabilized the total force produced by the hand. Both Parkinsons disease patients and control subjects performed accurate isometric force production tasks with the fingers of both the dominant and nondominant hands. The Parkinsons disease patients showed significantly lower maximal finger forces and higher unintended force production (enslaving). These observations suggest that changes in supraspinal control have a major effect on finger individuation. The synergy indexes in the patients were weaker in both steady-state and cyclic force production tasks compared with the controls. These indexes also were stronger in the left (nondominant) hand in support of the dynamic-dominance hypothesis. Half of the patients could not perform the cyclic task at the highest frequency (2 Hz). Anticipatory adjustments of synergies prior to a quick force pulse production were delayed and reduced in the patients compared with the controls. Similar differences were observed between the asymptomatic hands of the patients with symptoms limited to one side of the body and matched hands of control subjects. Our study demonstrates that the elusive changes in motor coordination in Parkinsons disease can be quantified objectively, even in patients at a relatively early stage of the disease. The results suggest an important role of the basal ganglia in synergy formation and demonstrate a previously unknown component of impaired feedforward control in Parkinsons disease reflected in the reduced and delayed anticipatory synergy adjustments.

Optimality vs. variability: an example of multi-finger redundant tasks

Two approaches to motor redundancy, optimization and the principle of abundance, seem incompatible. The former predicts a single, optimal solution for each task, while the latter assumes that families of equivalent solutions are used. We explored the two approaches using a four-finger pressing task with the requirement to produce certain combination of total normal force and a linear combination of normal forces that approximated the total moment of force in static conditions. In the first set of trials, many force–moment combinations were used. Principal component (PC) analysis showed that over 90% of finger force variance was accounted for by the first two PCs. The analytical inverse optimization (ANIO) approach was applied to these data resulting in quadratic cost functions with linear terms. Optimal solutions formed a hyperplane (“optimal plane”) in the four-dimensional finger force space. In the second set of trials, only four force–moment combinations were used with multiple repetitions. Finger force variance within each force–moment combination in the second set was analyzed within the uncontrolled manifold (UCM) hypothesis. Most finger force variance was confined to a hyperplane (the UCM) compatible with the required force–moment values. We conclude that there is no absolute optimal behavior, and the ANIO yields the best fit to a family of optimal solutions that differ across trials. The difference in the force-producing capabilities of the fingers and in their moment arms may lead to deviations of the “optimal plane” from the subspace orthogonal to the UCM. We suggest that the ANIO and UCM approaches may be complementary in the analysis of motor variability in redundant systems.

Effects of olivo-ponto-cerebellar atrophy (OPCA) on finger interaction and coordination

OBJECTIVES We investigated changes in finger interaction and coordination in patients with olivo-ponto-cerebellar atrophy (OPCA) using the recently developed approach to motor synergies based on the principle of motor abundance. METHODS OPCA patients and control subjects performed sets of maximal and submaximal force production tasks by the fingers of each of the hands. Indices of multi-finger synergies were quantified within the framework of the uncontrolled manifold hypothesis. RESULTS The patients showed lower maximal forces, higher indices of finger interdependence (enslaving), and lower indices of multi-finger synergies stabilizing total force in four-finger tasks. In addition, the patients showed an impaired ability to adjust synergies in preparation to a quick action (small and delayed anticipatory synergy adjustments). The synergy indices showed significant correlations with the clinical scores (both UPDRS total motor scores and ataxia related sub-scores). The observed changes in the indices of finger interaction and coordination were qualitatively similar to those reported earlier for patients with Parkinsons disease; however, the magnitude of the changes was much higher in the OPCA group. CONCLUSIONS These findings fit the hypotheses on the role of the cerebellum in assembling motor synergies and in the feed-forward control of action. They suggest that the synergy index measured in artificial, constrained laboratory tasks may be predictive of more general changes in motor behavior. SIGNIFICANCE The results suggest that studies of multi-digit synergies may be particularly sensitive to subcortical disorders and may provide a much-needed tool for quantitative assessment of impaired coordination in such patients.

Age-related changes in optimality and motor variability: an example of multifinger redundant tasks

We used two methods, analytical inverse optimization (ANIO) and uncontrolled manifold (UCM) analysis of synergies, to explore age-related changes in finger coordination during accurate force and moment of force production tasks. The two methods address two aspects of the control of redundant systems: Finding an optimal solution (an optimal sharing pattern) and using variable solutions across trials (covarying finger forces) that are equally able to solve the task. Young and elderly subjects produced accurate combinations of total force and moment by pressing with the four fingers of the dominant hand on individual force sensors. In session-1, single trials covered a broad range of force–moment combinations. Principal component (PC) analysis showed that the first two PCs explained about 90% and 75% of finger force variance for the young and elderly groups, respectively. The magnitudes of the loading coefficients in the PCs suggested that the young subjects used mechanical advantage to produce moment while elderly subjects did not (confirmed by analysis of moments produced by individual digits). A co-contraction index was computed reflecting the magnitude of moment produced by fingers acting against the required direction of the total moment. This index was significantly higher in the young group. The ANIO approach yielded a quadratic cost function with linear terms. In the elderly group, the contribution of the forces produced by the middle and ring fingers to the cost function value was much smaller than in the young group. The angle between the plane of experimental observations and the plane of optimal solutions (D-angle), was very small (about 1.5°) in the young group and significantly larger (about 5°) in the elderly group. In session-2, four force–moment combinations were used with multiple trials at each. Covariation among finger forces (multifinger synergies) stabilizing total force, total moment, and both was seen in both groups with larger synergy indices in the young group. Multiple regression analysis has shown that, at higher force magnitudes, the synergy indices defined with the UCM method were significantly related to the percent of variance accounted by the first two PCs and to the D-angle computed using the ANIO method. We interpret the results as pointing at a transition with age from synergic control to element-based control (back-to-elements hypothesis). Optimization and analysis of synergies are complementary approaches that focus on two aspects of multidigit coordination, sharing and covariation, respectively.

Effects of Aquatic Backward Locomotion Exercise and Progressive Resistance Exercise on Lumbar Extension Strength in Patients Who Have Undergone Lumbar Diskectomy

OBJECTIVE To compare the effects of aquatic backward locomotion exercise and progressive resistance exercise with a machine on lumbar extension strength in patients who have undergone diskectomy for a lumbar disk herniation. DESIGN Prospective comparative study. SETTING Department of Kinesiology at a state university. PARTICIPANTS Male patients (N=30) with disk herniation at spinal levels L3 to S1 completed this study as subjects. INTERVENTION After the diskectomy for a lumbar disk herniation, all patients had 6 weeks of rest time. At the end of the rest period, the aquatic backward locomotion exercise and progressive resistance exercise groups, respectively, started first 6 weeks of underwater training and lumbar extension training twice per week. After completion of the first 6-week training, subjects participated in a second 6-week training. After the whole 12-week training, subjects had no training for 6 weeks (detraining) and a follow-up 6-week training (retraining). The control (CON) group did not undergo any training. MAIN OUTCOME MEASURES For each test, maximum voluntary isometric lumbar extension strength was measured in 7 trunk positions (72 degrees , 60 degrees , 48 degrees , 36 degrees , 24 degrees , 12 degrees , and 0 degrees of the trunk angle). RESULTS The progressive resistance exercise and aquatic backward locomotion exercise groups showed increases in lumbar extension strength after the first 6-week training, although they were not statistically different from the CON group. After a second 6-week training, the progressive resistance exercise and aquatic backward locomotion exercise groups showed statistically significant increases in their strength levels as compared with the CON group. After the detraining period, the strength levels of the progressive resistance exercise and aquatic backward locomotion exercise groups did not statistically differ from the CON group. After the retraining period, the progressive resistance exercise and aquatic backward locomotion exercise groups showed increases in their strength levels, which were different from that of the CON group. CONCLUSIONS The results obtained suggested that the aquatic backward locomotion exercise is as beneficial as progressive resistance exercise for improving lumbar extension strength in patients after lumbar diskectomy surgery.

Optimality versus variability: effect of fatigue in multi-finger redundant tasks

We used two methods to address two aspects of multi-finger synergies and their changes after fatigue of the index finger. Analytical inverse optimization (ANIO) was used to identify cost functions and corresponding spaces of optimal solutions over a broad range of task parameters. Analysis within the uncontrolled manifold (UCM) hypothesis was used to quantify co-variation of finger forces across repetitive trials that helped reduce variability of (stabilized) performance variables produced by all the fingers together. Subjects produced steady-state levels of total force and moment of force simultaneously as accurately as possible by pressing with the four fingers of the right hand. Both before and during fatigue, the subjects performed single trials for many force–moment combinations covering a broad range; the data were used for the ANIO analysis. Multiple trials were performed at two force–moment combinations; these data were used for analysis within the UCM hypothesis. Fatigue was induced by 1-min maximal voluntary contraction exercise by the index finger. Principal component (PC) analysis showed that the first two PCs explained over 90% of the total variance both before and during fatigue. Hence, experimental observations formed a plane in the four-dimensional finger force space both before and during fatigue conditions. Based on this finding, quadratic cost functions with linear terms were estimated from the experimental data. The dihedral angle between the plane of optimal solutions and the plane of experimental observations (DANGLE) was very small (a few degrees); it increased during fatigue. There was an increase in fatigue of the coefficient at the quadratic term for the index finger force balanced by a drop in the coefficients for the ring and middle fingers. Within each finger pair (index–middle and ring–little), the contribution of the “central” fingers to moment production increased during fatigue. An index of antagonist moment production dropped with fatigue. Fatigue led to higher co-variation indices during pronation tasks (index finger is an agonist) but opposite effects during supination tasks. The results suggest that adaptive changes in co-variation indices that help stabilize performance may depend on the role of the fatigued element, agonist or antagonist.

The forces behind the words: Development of the Kinetic Pen

This paper describes the creation of a Kinetic Pen capable of measuring the six-component force and torque that each of four individual contacts applies to the pen during writing. This was done by staggering the mounting of the four sensors along the long axis of the pen and having an extended arm run from the sensor to the grip site, preventing a clustering of the sensors where the digit tips meet while grasping. The implications of this tool allow handwriting studies to be expanded from two-dimensional pen-tip kinematics to three-dimensional dynamics at each contact point between the hand and pen.

Dopaminergic modulation of motor coordinaton in Parkinson's disease

BACKGROUND We applied the idea of synergies and the framework of the uncontrolled manifold hypothesis to explore the effects of dopamine replacement therapy on finger interaction and coordination in patients with early-stage Parkinsons disease (PD). METHODS Eight patients performed single-finger and multi-finger force production tasks with both the dominant and non-dominant hand before (off-drug) and after (on-drug) taking their dopaminergic medications. Synergy indices were defined as co-varied adjustments of commands to fingers that stabilized the total force produced by the hand. RESULTS PD patients showed significantly lower maximal finger forces off-drug compared to the on-drug condition, while indices of finger individuation (enslaving) were unchanged. The synergy indices were weaker during steady-state force production off-drug compared to on-drug. Anticipatory adjustments of synergies prior to the quick force pulse initiation were delayed and reduced off-drug as compared to the on-drug condition. These drug effects were observed in both the symptomatic and asymptomatic hands of the patients whose symptoms were limited to one side of the body. CONCLUSIONS The study demonstrates dopaminergic modulation of motor coordination in PD and supports that the analysis of different components of multi-finger synergies offers a set of indices sensitive to the effects of dopamine replacement therapy in early-stage PD. The results suggest an important role of the basal ganglia in synergy formation and in feed-forward synergy adjustments. Future studies using these methods may yield more objective, quantitative biomarker(s) of motor coordination impairments in PD, and better understanding of subcortical involvement in the neural control of natural actions.

Prehension synergies and hand function in early-stage Parkinson's disease.

We explored the multi-digit synergies and hand performance in object manipulations and pressing tasks in patients with early-stage Parkinson’s disease (PD) and healthy controls. Synergies were defined as inter-trials co-variation patterns among forces/moments produced by individual digits that stabilized a resultant mechanical variable. The subjects performed three main tasks: pressing (steady-state force production followed by a force pulse into the target), prehension (manipulation of a handheld instrumented handle imitating the action of taking a sip from a glass), and functional object manipulation (moving a glass with water as quickly and accurately as possible along a chain of targets). The PD patients were slower compared to controls in all three tasks. Patients showed smaller synergy indices in the pressing and prehension tasks. In the prehension tasks, patients showed elevated grip force at steady states with smaller grip force modulation during the handle motion. PD patients showed smaller feed-forward synergy adjustments in preparation to the quick action in the pressing and (to a smaller degree) prehension tasks. Synergy indices correlated with the time index of performance in the functional glass-with-water task, whereas none of the indices correlated with the Unified PD Rating Scale part III—motor scores. We interpret the results as pointing at an important role of subcortical structures in motor synergies and their feed-forward adjustments to action.

Handwriting: Hand–pen contact force synergies in circle drawing tasks

This study investigated synergistic actions of hand-pen contact forces during circle drawing tasks in three-dimensional (3D) space. Twenty-four right-handed participants drew thirty concentric circles in the counterclockwise (CCW) and clockwise (CW) directions. Three-dimensional forces acting on an instrumented pen as well as 3D linear and angular positions of the pen were recorded. These contact forces were then transformed into the 3D radial, tangential, and normal force components specific to circle drawing. Uncontrolled manifold (UCM) analysis was employed to calculate the magnitude of the hand-pen contact force synergy. Three hypotheses were tested. First, hand-pen contact force synergies during circle drawing are dependent on the angular position of the pen tip. Second, hand-pen contact force synergies are dependent on force components in circle drawing. Third, hand-pen contact force synergies are greater in CCW direction than CW direction. The results showed that the strength of the hand-pen contact force synergy increased during the initial phase of circle drawing and decreased during the final phase. The synergy strength was greater for the radial and tangential components as compared to the normal component. Also, the circle drawing in CW direction was associated with greater hand-pen contact force synergy than the CCW direction. The results of this study suggest that the central nervous system (CNS) prioritizes hand-pen contact force synergies for the force components (i.e., radial and tangential) that are critical for circle drawing. The CNS modulates hand-pen contact force synergies for preparation and conclusion of circle drawing, respectively.