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.
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).
We explored the role of the basal ganglia in two components of multi-finger synergies by testing a group of patients with early-stage Parkinsons disease and a group of healthy controls. Synergies were defined as co-varied adjustments of commands to individual fingers that reduced variance of the total force and moment of force. The framework of the uncontrolled manifold hypothesis was used to quantify such co-variation patterns, while average performance across repetitive trials (sharing patterns) was analyzed using the analytical inverse optimization (ANIO) approach. The subjects performed four-finger pressing tasks that involved the accurate production of combinations of the total force and total moment of force and also repetitive trials at two selected combinations of the total force and moment. The ANIO approach revealed significantly larger deviations of the experimental data planes from an optimal plane for the patients compared to the control subjects. The synergy indices computed for total force stabilization were significantly higher in the control subjects compared to the patients; this was not true for synergy indices computed for moment of force stabilization. The differences in the synergy indices were due to the larger amount of variance that affected total force in the patients, while the amount of variance that did not affect total force was comparable between the groups. We conclude that the basal ganglia play an important role in both components of synergies reflecting optimization of the sharing patterns and stability of performance with respect to functionally important variables.
The purpose of this study was to investigate the effects of changes in the proprioceptive signals induced by muscle vibration on multi-finger interaction and coordination. We hypothesized that unintended force production by non-instructed fingers (enslaving) would increase with muscle vibration while synergy indices during steady-state force production would drop. The framework of the uncontrolled manifold hypothesis was used to quantify indices of multi-finger synergies stabilizing total force during steady-state force production and anticipatory changes in these indices (anticipatory synergy adjustments, ASAs) in preparation to a quick force pulse production with and without hand-muscle vibration at 80 Hz. The dominant hands of twelve healthy right-handed subjects were tested under three conditions: no vibration, vibration of the palmar surface of the hand, and vibration of the forearm applied over the flexor muscles. There were no significant effects of vibration on maximal voluntary force. The magnitude of enslaving was larger during vibration of the hand compared to the other two conditions. During steady-state force production, strong synergies stabilizing total force were seen in all three conditions; however, indices of force-stabilizing synergies were lower during vibration of the hand. Prior to the force pulse initiation, the synergy index started to drop earlier and over a larger magnitude without vibration compared to either vibration condition. Effects of vibration on enslaving and synergy index may be due to diffuse reflex effects of the induced afferent activity on alpha-motoneuronal pools innervating the extrinsic flexor compartments. We conclude that multi-finger synergies are not based on signals from muscle receptors. The smaller synergy indices and ASAs may reflect supraspinal effects of the vibration-induced afferent activity, in particular its interactions with trans-thalamic loops.
The aim of this study was to test the mechanical advantage (MA) hypothesis in multifinger torque production tasks in humans: fingers with longer moment arms produce greater force magnitudes during torque production tasks. There were eight experimental conditions: two prehension types determined by different mechanical constraints (i.e., fixed- and free-object prehension) with two torque directions (supination and pronation) and two torque magnitudes (0.24 and 0.48 N·m). The subjects were asked to produce prescribed torques during the fixed-object prehension or to maintain constant position of the free hand-held object against external torques. The index of MA was calculated for agonist and antagonist fingers, which produce torques in the same and opposite directions to the target torques, respectively. Within agonist fingers, the fingers with longer moment arms produced greater grasping forces while within antagonist fingers, the fingers with shorter moment arms produced greater forces. The MA index was greater in the fixed-object condition as compared with the free-object condition. The MA index was greater in the pronation condition than in the supination condition. This study supports the idea that the CNS utilizes the MA of agonist fingers, but not of antagonist fingers, during torque production in both fixed- and free-object conditions.
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 (D (ANGLE)) 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.
A recently developed method of analytical inverse optimization (ANIO) was used to compute cost functions based on sets of experimental observations in 4-finger pressing tasks with accurate total force and moment production. In different series, feedback on total force and moment was provided using the index finger force at its value, doubled, or halved. Finger force data across different force-moment combinations formed a plane. This allowed reconstructing cost functions as 2nd-order polynomials with linear terms. Changes in the coefficients of the cost function across the 3 series allowed the authors to offer a biomechanical interpretation related to constraints on finger forces with different lever arms. ANIO allows the authors to describe preferred regions within the space of solutions for redundant tasks in terms of cost functions.
We studied characteristics of digit action and their co-variation patterns across trials (prehension synergies) during static holding of an object while the external torque could change slowly and smoothly. The subjects held in the air an instrumented handle with an attachment that allowed a smooth change in the external torque over about 12 s; the load was always kept constant. Series of trials were performed under three conditions: The torque could be zero throughout the trial, or it could change slowly requiring a smooth change of the effort from a non-zero pronation value to zero (PR-0) or from a non-zero supination value to zero (SU-0). The handle was kept vertical at all times. Indices of variance and co-variation of elemental variables (forces and moments of force produced by individual digits) stabilizing such performance variables as total normal force, total tangential force, and total moment of force were computed at two levels of an assumed control hierarchy. At the upper level, the task is shared between the thumb and virtual finger (an imagined digit with the mechanical action equal to that of the four fingers), while at the lower level, the action of the virtual finger is shared among the actual four fingers. We analyzed the total moment of force as the sum of the moments of force produced by the thumb and virtual finger and also as the sum of the moments of force produced by the normal forces and tangential forces. The results showed that the adjustments in the total moment of force were produced primarily with changes in the moment produced by the virtual finger and by changes in the moment produced by the normal forces. The normal force of the thumb at the final state (which was the same across conditions) was larger in the two conditions with changes in the external torque. The safety margin was significantly higher in the PR-0 condition, and it dropped with the decrease in the external torque. A co-contraction index was computed to reflect the moment of force production by the fingers acting against the total moment produced by the virtual finger. It was higher for the SU-0 condition. Most variance indices dropped with a decrease in the external torque. The co-variation indices, however, remained unchanged over the trial duration. They showed signs of a trade-off between the two levels of the assumed hierarchy: larger indices at the higher level corresponded to smaller indices at the lower level. This study and the previous one (Sun et al. in Exp Brain Res 209:571-585, 2011) document several previously unknown features of prehensile tasks. The results show that characteristics of digit action and interaction in such tasks depend not only on the magnitudes of external constraints but also on a variety of other factors including time changes in the constraints and their history.
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.
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.
To date, limited examples of polyelectrolyte multilayers (PEMs) can be found that truly exploit the power of layer-by-layer nanoassembly to combine multiple functions into a complex multilayer. We demonstrate that PEMs can be designed as optimized coatings for implantable biosensors, exhibiting both diffusion control and protein resistance. PEM coatings comprising strong-weak and weak-weak pairs were evaluated, resulting in decreases in glucose diffusivity up to 5 orders of magnitude compared to water. Addition of poly(ethylene glycol) (PEG)-grafted terminal layers on the base diffusion-controlling multilayers substantially improved resistance to albumin adsorption relative to unmodified PEMs. For transport-controlling films comprising strong-weak polyelectrolyte pairs, the consistent diffusivity was observed even after exposure to protein-containing solutions, indicating minimal effects of biofouling. In contrast, the transport behavior of weak-weak polyelectrolyte pairs was susceptible to alteration by protein exposure, resulting in large variation in diffusivity, even when protein-resistant outer layers were employed.
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.
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.
Previous studies have shown that the interactions of human hand digits are influenced by internal constraints, i.e., biomechanical and central constraints. However, little is currently known about the influence of externally imposed mechanical constraints on multi-finger behavior. This study investigates maximal digit force production during fixed object and free object prehension in statics. The results from the fixed object prehension indicated that the closer the non-task finger was positioned to the task finger, the greater the force produced by the non-task finger, which supports the proximity hypothesis. Conversely, the non-task fingers with longer moment arms showed greater force production during free object prehension, which supports the mechanical advantage hypothesis. During the free object prehension, equal and opposite torques were produced by the digit normal forces and tangential forces, while this phenomenon was not observed in the fixed object prehension. The results also showed that the thumb normal force had a positive linear relationship with task-finger normal forces during fixed object prehension while the thumb normal force remained constant during free object prehension tasks. We concluded that the CNS employed different strategies when different sets of internal and external constraints are provided during multi-digit prehension tasks.
This study investigated the phenomena of finger enslaving, involuntary finger actions by non-intended fingers, and force deficit, smaller maximum force by all four fingers than the sum of individual finger maximum forces in individuals with cervical spinal cord injuries (SCI). A total of 16 subjects participated in this study: 8 with a cervical spinal cord injury and 8 controls. Each of the injured subjects had one paralyzed finger. The results showed that the efforts to produce force using any individual finger induced force production in all other fingers, suggesting finger force enslaving. The maximum force during the four-finger task was greater than the sum of the individual finger forces during single-finger tasks in the SCI group, which was reflected by positive force deficit, "force surplus". One may utilize these findings for rehabilitation of paralyzed fingers caused by cervical spinal injuries.
Continuous glucose monitoring is crucial to developing a successful artificial pancreas. However, biofouling and host response make in vivo sensor performance difficult to predict. We investigated changes in glucose diffusivity and sensor response of optical enzymatic glucose sensors due to biological exposure.
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.
A microliquid prism is a microchannel filled with two immiscible liquids, whose interface acts as a refractive surface. To steer a light beam that constructs optical images, the interface profile or the contact angle is modulated through electrowetting on a dielectric. Accurate, yet agile actuation of the liquid prism critically depends on the understanding of dynamics of the fluid interface. Here we fabricate liquid prisms, visualize the shape evolution of the interface, and theoretically model its dynamics. By comparing the magnitude of capillary forces to those of viscous, inertial and hydrostatic forces, we find that the meniscus motion within submillimetric channels is dominated by the capillary effect. The theoretical predictions for microscale meniscus dynamics are shown to agree well with the experimental measurements. We then discuss the formation of waves in millimetric liquid prisms, which may significantly limit fast and reliable operation of the optofluidic device.
The purpose of this study was to investigate central nervous system (CNS) strategies for controlling multifinger forces during a circle-drawing task. Subjects drew 30 concentric, discontinuous clockwise and counter clockwise circles, at self and experimenter-set paces. The three-dimensional trajectory of the pens center of mass and the three-dimensional forces and moments of force at each contact between the hand and the pen were recorded. Uncontrolled Manifold Analysis was used to quantify the synergies between pen-hand contact forces in radial, tangential and vertical directions. Results showed that synergies in the radial and tangential components were significantly stronger than in the vertical component. Synergies in the clockwise direction were significantly stronger than the counterclockwise direction in the radial and vertical components. Pace was found to be insignificant under any condition.
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.
The study investigates the effect of task uncertainty on motor synergies and movement time for a whole-body pointing task employing a Fitts like paradigm. Thirty-three healthy, young adults were asked to hold a 1.5-m long stick and point it as quickly and accurately as possible to the unmarked center of fixed targets on the ceiling at 150% of the subjects height from the ground. Each subject performed fifteen continuous repetitions for each target size (1cm, 2cm, 3cm, 5cm, 8cm, 13cm and 21cm diameters of circles). It was assumed that the task uncertainty increased as the target size increased. Motion capture was used to collect the data for joint angles in the sagittal plane and uncontrolled manifold (UCM) analysis was used in order to investigate synergistic actions of joints. Results from the study revealed that the movement time decreased as task uncertainty increased. The variability within the uncontrolled manifold (V(UCM)) systematically increased with task uncertainty, resulting in an increase in the index of inter-joint synergies (?V), although the pointing task errors (V(ORT)) were consistent across different target sizes. The results suggest that the central nervous system systematically modulates the inter-joint synergies with task uncertainty in the whole-body pointing task without affecting motor performance.
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