Biomedical subjects

Mark L Latash

Publications and source records attributed to Mark L Latash.

At least 19 recordsLinked to original sources

PubMed2006

Anticipatory adjustments of multi-finger synergies in preparation for self-triggered perturbations.

We studied changes in multi-finger synergies associated with predictable and unpredictable force perturbations applied to a finger during a multi-finger constant total force production task. The main hypothesis was that indices of multi-finger synergies can show anticipatory changes in preparation for a predictable perturbation. Subjects sat in a chair and pressed on force sensors with the four fingers of the right hand. The task was to produce a constant level of total force. The fingers acted against loads that produced upward directed forces. The loads (applied either to the index or to the ring finger) could be disengaged either by the subject or by the experimenter. An index of finger co-variation, DeltaV was computed across sets of 12 trials at each time sample and for all tasks separately. During steady-state force production, all subjects showed positive DeltaV values corresponding to strong negative covariation among finger forces interpreted as a force-stabilizing synergy. Prior to self-triggered unloading, subjects showed an anticipatory drop in DeltaV that started 100-125 ms prior to the unloading time. Such early changes were absent in trials with experimenter-triggered unloading. After an unloading, subjects changed forces of both perturbed and unperturbed fingers and reached a new sharing pattern of the total force. In experimenter-triggered conditions, changes in the forces of unperturbed fingers could be seen as early as 120 ms following an unloading. The index DeltaV dropped following a perturbation and then recovered; the recovery occurred faster in self-triggered conditions. We conclude that humans can use feed-forward changes in multi-finger synergies (anticipatory synergy adjustments) in anticipation of a predictable perturbation. These changes may help avoid prolonged weakening of a multi-digit force-stabilizing synergy. We discuss a possibility that anticipatory postural adjustments may represent a particular case of the phenomenon of anticipatory synergy adjustments and suggest a hierarchical control scheme that incorporates a possibility of independent control over the output of a multi-element system and covariation patterns among outputs of its elements.

Adult↗

PubMed2006

Muscle synergies during voluntary body sway: combining across-trials and within-a-trial analyses.

We investigated co-varied changes in muscle activity during voluntary sway tasks that required a quick shift of the center of pressure (COP). We hypothesized that multi-muscle synergies (defined as task-specific covariation of elemental variables, muscle modes) stabilize a COP location in the anterior-posterior direction prior to a voluntary COP shift and that during the shift the synergies would weaken. Standing subjects performed two tasks, a cyclic COP shift over a range corresponding to 80% of the maximal amplitude of voluntary COP shift at 1 Hz and a unidirectional quick COP shift over the same nominal amplitude. The cyclic sway task was used to define muscle modes (M-modes, leg and trunk muscle groups with parallel scaling of muscle activation level within a group) and the relations between small changes in the magnitudes of M-modes [in the principal component analysis (PCA), the M-mode magnitudes are equivalent to PC scores] and COP shifts. A novel approach was used involving PCA applied to indices of muscle integrated activity measured both within a trial and across trials. The unidirectional sway task was performed in a self-paced (SP) manner and under a typical simple reaction time (RT) instruction. M-modes were also defined along trials at those tasks; they have been shown to be similar across tasks. Integrated indices of muscle activity in the SP-sway and RT-sway tasks were transformed into the M-modes. Variance in the M-mode space was partitioned into two components, one that did not affect the average value of COP shift (V (UCM)) and the other that did (V (ORT)). An index (DeltaV) corresponding to the normalized difference between V (UCM) and V (ORT) was computed. During steady-state posture, DeltaV was positive corresponding to most M-mode variance lying in a sub-space corresponding to a stable COP location across trials. Positive DeltaV values have been interpreted as reflecting a multi-M-mode synergy stabilizing the COP location. The magnitude of DeltaV was larger in SP trials than in RT trials. During voluntary COP shifts, the DeltaV magnitude dropped to zero or even became negative. We conclude that M-mode synergies stabilize COP location during quiet standing, while these synergies weaken or disappear during fast voluntary COP shifts. Under RT conditions, the COP stabilizing synergies were weaker supposedly to facilitate a quick COP shift without time for preparation. The suggested method of M-mode identification may potentially be applied to analysis of postural synergies in persons with impaired postural control such as elderly persons, persons with atypical development, or in the course of rehabilitation after an injury.

Adult↗

PubMed2006

Effects of friction at the digit-object interface on the digit forces in multi-finger prehension.

The effects of surface friction at the digit-object interface on digit forces were studied when subjects (n=8) statically held an object in a five-digit grasp. The friction conditions were SS (all surfaces are sandpaper), RR (all are rayon), SR (S for the thumb and R for the four fingers), and RS (the reverse of SR). The interaction effects of surface friction and external torque were also examined using five torques (-0.5, -0.25, 0, +0.25, +0.5 Nm). Forces and moments exerted by the digits on a handle were recorded. At zero torque conditions, in the SS and RR (symmetric) tasks the normal forces of the thumb and virtual finger (VF, an imagined finger with the mechanical effect equal to that of the four fingers) were larger for the RR than the SS conditions. In the SR and RS (asymmetric) tasks, the normal forces were between the RR and SS conditions. Tangential forces were smaller at the more slippery side than at the less slippery side. According to the mathematical optimization analysis decreasing the tangential forces at the more slippery sides decreases the cost function values. The difference between the thumb and VF tangential forces, DeltaF (t), generated a moment of the tangential forces (friction-induced moment). At non-zero torque conditions the friction-induced moment and the moment counterbalancing the external torque (equilibrium-necessitated moment) could be in same or in opposite directions. When the two moments were in the same direction, the contribution of the moment of tangential forces to the total moment was large, and the normal forces were relatively low. In contrast, when the two moments were in opposite directions, the contribution of the moment of tangential forces to the total moment markedly decreased, which was compensated by an increase in the moment of normal forces. The apparently complicated results were explained as the result of summation of the friction-related (elemental) and torque-related (synergy) components of the central commands to the individual digits.

Adaptation, Physiological↗

PubMed2005

Muscle synergies involved in preparation to a step made under the self-paced and reaction time instructions.

OBJECTIVE: To investigate multi-muscle synergies during preparation to making a step and their changes with speed of stepping and method of step initiation, self-paced vs. reaction-time. METHODS: Standing healthy persons performed stepping tasks under 3 instructions, 'comfortably, self-paced', 'very quick, self-paced', and 'as fast as possible to a visual signal'. Electromyographic (EMG) signals of 10 postural muscles were recorded and analyzed. The participants also performed standard tasks associated with releasing a load. These trials were used to identify muscle groupings (M-modes) associated with shifts of the center of pressure (COP) and relations between small changes in the M-modes and COP shifts. The framework of the uncontrolled manifold hypothesis was used to partition the EMG variance across stepping trials into two components that kept constant and changed the COP coordinates in the anterior-posterior (AP) and medio-lateral (ML) directions. RESULTS: Muscle synergies stabilizing COP trajectories in both AP and ML directions were observed in both the stepping and supporting legs. The synergies tended to weaken and disappear close to the moment of take-off. They showed only minor differences across the 3 tasks. In contrast, mechanical and EMG characteristics of postural adjustments to stepping were significantly larger and showed higher rates of change during quick steps and steps under the reaction-time instruction. CONCLUSIONS: COP shifts associated with preparation to making a step are stabilized by multi-muscle synergies. These synergies are robust across stepping speed and methods of step initiation, self-paced and reaction-time. SIGNIFICANCE: Studies of multi-muscle postural synergies are important for understanding postural disorders in neurological patients and changes in postural strategies with age.

Adult↗

PubMed2005

Prehension stability: experiments with expanding and contracting handle.

We studied adjustments in digit forces and moments during holding a vertically oriented handle under slow, externally imposed changes in the width of the grasp. Subjects (n = 8) grasped a customized motorized handle with five digits and held it statically in the air. The handle width either increased (expanded) or decreased (contracted) at a rate of 1.0, 1.5, or 2.0 mm/s, while the subjects were asked to ignore the handle width changes, and their attention was distracted. External torques of 0.0, 0.25, and 0.5 Nm were applied to the handle in two directions. Forces and moments at the digit tips were measured with six-component sensors. The analysis was performed at the virtual finger (VF) and individual finger (IF) levels (VF is an imagined finger that produces the same wrench, i.e., the force and moment, as several fingers combined). In all the tasks, the normal VF and thumb forces increased with the handle expansion and decreased with the handle contraction. Similar behavior was seen for the thumb tangential force. In contrast, the VF tangential force decreased with the handle expansion and increased with the handle contraction. The changes in the tangential forces assisted the perturbations in the tasks requiring exertion of the supination moments and annulled the perturbation in the pronation effort tasks. In the former tasks, the equilibrium was maintained by the changes of the moments of normal forces, whereas in the latter tasks, the equilibrium was maintained by the changes of the moments of the tangential forces. Analysis at the IF level has shown that the resultant force and moment exerted on the object could arise from dissimilar adjustments of individual fingers to the same handle width change. The complex adjustments of digit forces to handle width change may be viewed as coming from two sources. First, there are local spring-like adjustments of individual digit forces and moments caused by both mechanical properties of the digits and the action of spinal reflexes. These stiffness-like reactions mainly assist in perturbing the rotational equilibrium of the object rather than in maintaining it. Second, there are tilt-preventing adjustments defined by the common task constraints that unite the digits into a task-specific synergy. The "virtual springs theory" developed in robotics literature is insufficient for describing the phenomena observed in human grasping.

Adaptation, Physiological↗

PubMed2005

Maintaining rotational equilibrium during object manipulation: linear behavior of a highly non-linear system.

We address issues of simultaneous control of the grasping force and the total moment of forces applied to a handheld object during its manipulation. Six young healthy male subjects grasped an instrumented handle and performed its cyclic motion in the vertical direction. The handle allowed for setting different clockwise (negative) or counterclockwise torques. Three movement frequencies: 1, 1.5 and 2 Hz, and five different torques: -1/3, -1/6, 0, 1/6 and 1/3 Nm, were used. The rotational equilibrium was maintained by two means: (1) Concerted changes of the moments produced by the normal and tangential forces, specifically anti-phase changes of the moments during the tasks with zero external torque and in-phase changes during the non-zero-torque tasks, and (2) Redistribution of the normal forces among individual fingers such that the agonist fingers--the fingers that resist external torque--increased the force in phase with the acceleration, while the forces of the antagonist fingers--those that assist the external torque--especially, the fingers with the large moment arms, the index and little fingers, stayed unchanged. The observed effects agree with the principle of superposition--according to which some complex actions, for example, prehension, can be decomposed into elemental actions controlled independently--and the mechanical advantage hypothesis according to which in moment production the fingers are activated in proportion to their moment arms with respect to the axis of rotation. We would like to emphasize the linearity of the observed relations, which was not prescribed by the task mechanics and seems to be produced by specific neural control mechanisms.

Adult↗

PubMed2005

Muscle synergies involved in shifting the center of pressure while making a first step.

We used the framework of the uncontrolled manifold (UCM) hypothesis to analyze multi-muscle synergies involved in making a step by a standing person. We hypothesized that leg and trunk muscles are organized into stable groups (muscle modes, M-modes) related to shifts of the center of pressure (COP) in the anterior-posterior and medio-lateral directions. Another hypothesis was that the magnitudes of the modes co-vary across repetitive trials to stabilize a certain magnitude of the COP shift in both directions. M-modes were defined using principal component analysis applied to indices of changes in the electromyographic (EMG) activity prior to releasing variable loads that were held by the subject using a pulley system. For the task of releasing the load behind the body three M-modes associated with a backward COP shift were defined. Four M-modes were defined for the task of releasing the load at the body side associated with a lateral COP shift. Multiple regression analysis was used to relate changes in the M-mode magnitudes to COP shifts. EMG changes prior to making a step were quantified over five 100 ms time windows before the lift-off of the stepping leg. Two components of the variance in the M-mode space computed across repetitions of a stepping task were quantified-a component that did not affect the average COP shift in a particular direction (variance within the UCM, V (UCM)), and a component that affected the COP shift (variance orthogonal to the UCM, V (ORT)). V (UCM) was significantly higher than V (ORT) for both directions of the COP shifts. This relation was observed for the M-modes in the stepping leg as well as in the support leg. The stepping leg showed a different time evolution of the ratio V (UCM)/V (ORT) such that the difference between the two variance components disappeared closer to the time of the lift-off. The findings corroborate both main hypotheses. The study supports a view that control of whole-body actions involves grouping the muscles, using fewer elemental variables to scale the muscle activity, and forming synergies in the space of the elemental variables that stabilize time profiles of important performance variables.

Adult↗

PubMed2005

Two kinematic synergies in voluntary whole-body movements during standing.

We used a particular computational approach, the uncontrolled manifold hypothesis, to investigate joint angle covariation patterns during whole-body actions performed by standing persons. We hypothesized that two kinematic synergies accounted for the leg/trunk joint covariation across cycles during a rhythmic whole-body motion to stabilize two performance variables, the trunk orientation in the external space and the horizontal position of the center of mass (COM). Subjects stood on a force plate and performed whole-body rhythmic movements for 45 s under visual feedback on one of the four variables, the position of the center of pressure or the angle in one of the three joints (ankle, knee, or hip). The Fitts-like paradigm was used with two target amplitudes and six indices of difficulty (ID) for each of the four variables. This was done to explore the robustness of kinematic postural synergies. A speed-accuracy trade-off was observed in all feedback conditions such that the movement time scaled with ID and the scaling differed between the two movement amplitudes. Principal-component (PC) analysis showed the existence of a single PC in the joint space that accounted for over 95% of the joint angle variance. Analysis within the uncontrolled manifold hypothesis has shown that data distributions in the joint angle space were compatible with stabilization of both trunk orientation and COM location. We conclude that trunk orientation and the COM location are stabilized by co-varied changes of the major joint angles during whole-body movements. Despite the strong effects of movement amplitude and ID on performance, the structure of the joint variance showed only minor dependence on these task parameters. The two kinematic synergies (co-varied changes in the joint angles that stabilized the COM location and trunk orientation) have proven to be robust over a variety of tasks.

Adaptation, Physiological↗

PubMed2005

Internal forces during object manipulation.

Internal force is a set of contact forces that does not disturb object equilibrium. The elements of the internal force vector cancel each other and, hence, do not contribute to the resultant (manipulation) force acting on the object. The mathematical independence of the internal and manipulation forces allows for their independent (decoupled) control realized in robotic manipulators. To examine whether in humans internal force is coupled with the manipulation force and what grasping strategy the performers utilize, the subjects (n=6) were instructed to make cyclic arm movements with a customized handle. Six combinations of handle orientation and movement direction were tested. These involved: parallel manipulations (1) VV task (vertical orientation and vertical movement) and (2) HH task (horizontal orientation and horizontal movement); orthogonal manipulations (3) VH task (vertical orientation and horizontal movement) and (4) HV task (horizontal orientation and vertical movement); and diagonal manipulations (5) DV task (diagonal orientation and vertical movement) and (6) DH task (diagonal orientation and horizontal movement). Handle weight (from 3.8 to 13.8 N), and movement frequency (from 1 to 3 Hz) were systematically changed. The analysis was performed at the thumb-virtual finger level (VF, an imaginary finger that produces a wrench equal to the sum of wrenches produced by all the fingers). At this level, the forces of interest could be reduced to the internal force and internal moment. During the parallel manipulations, the internal (grip) force was coupled with the manipulation force (producing object acceleration) and the thumb-VF forces increased or decreased in phase: the thumb and VF worked in synchrony to grasp the object more strongly or more weakly. During the orthogonal manipulations, the thumb-VF forces changed out of phase: the plots of the internal force vs. object acceleration resembled an inverted letter V. The HV task was the only task where the relative phase (coupling) between the normal forces of the thumb and VF depended on oscillation frequency. During the diagonal manipulations, the coupling was different in the DV and DH tasks. A novel observation of substantial internal moments is described: the moments produced by the normal finger forces were counterbalanced by the moments produced by the tangential forces such that the resultant moments were close to zero. Implications of the findings for the notion of grasping synergies are discussed.

Adult↗

PubMed2005

Reversals of anticipatory postural adjustments during voluntary sway in humans.

We describe reversals of anticipatory postural adjustments (APAs) with the phase of a voluntary cyclic whole-body sway movement. Subjects (n=9) held a standard load in extended arms and released it by a bilateral shoulder abduction motion in a self-paced manner at different phases of the sway. The load release task was also performed during quiet stance in three positions: in the middle of the sway range and close to its extreme forward and backward positions. Larger APAs were seen during the sway task as compared to quiet stance. Although the direction of postural perturbation associated with the load release was always the same, the direction of the APAs in the leg muscles reversed when the subjects were close to the extreme forward position as compared to the APAs in other phases and during quiet stance. The trunk muscles showed smaller APA modulation at the extreme positions but larger modulation when passing through the middle position, depending on the direction of sway, forward or backward. The phenomenon of APA reversals emphasizes the important role of safety in the generation of postural adjustments associated with voluntary movements. Based on these findings, APAs could be defined as changes in the activity of postural muscles associated with a predictable perturbation that act to provide maximal safety of the postural task component.

Adult↗

PubMed2005

The emergence and disappearance of multi-digit synergies during force-production tasks.

We analyzed patterns of covariation among forces produced by the five digits of the human hand during tasks that required the production of a pattern of the total force consisting of ramp-up, constant force, and ramp-down segments with the time of the ramps ranging from 0 to 3000 ms. Patterns of the variance of the total force and the sum of the variances of individual digit forces were compared over sets of 12 trials at each task. The initiation of the ramp-up segment was associated with positive covariation of digit forces. Negative covariation among digit forces (force-stabilizing synergies) emerged after a critical time of 600-800 ms, which was only weakly dependent on the ramp time. These synergies persisted over the steady-state phase. A quantitative index of digit force covariation was introduced; it showed a drop about 100 ms before initiation of the ramp-down phase; we termed this phenomenon "anticipatory covariation" (ACV). The ramp-down phase was associated with rapid disappearance of the force-stabilizing synergy over a time period that ranged from 0 to 600 ms and scaled strongly with the duration of the force ramp. Thumb-virtual finger synergies showed qualitatively similar behavior to the multi-finger synergies (virtual finger is an imagined digit whose action is mechanically equivalent to the action of the four fingers). We conclude that abrupt changes in a time profile of total force are associated with transient destabilization of the total force. Changes in force-stabilizing synergies may occur in preparation to changes in the total force.

Adult↗

PubMed2005

A central back-coupling hypothesis on the organization of motor synergies: a physical metaphor and a neural model.

We offer a hypothesis on the organization of multi-effector motor synergies and illustrate it with the task of force production with a set of fingers. A physical metaphor, a leaking bucket, is analyzed to demonstrate that an inanimate structure can show apparent error compensation among its elements. A neural model is developed using tunable back-coupling loops as means of assuring error compensation in a task-specific way. The model demonstrates non-trivial features of multi-finger interaction such as delayed emergence of force stabilizing synergies and simultaneous stabilization of the total force and total moment produced by the fingers. The hypothesis suggests that neurophysiological structures involving short-latency feedback may play a central role in the formation of motor synergies.

Afferent Pathways↗

PubMed2005

Anticipatory covariation of finger forces during self-paced and reaction time force production.

We tested a hypothesis that humans can change patterns of finger force covariation in a task-specific manner in preparation to a change in the total force. Subjects performed quick targeted force pulses by pressing with four fingers on force sensors from a certain background force level to a target level. In self-paced trials, finger force modes (hypothetical commands to fingers) showed changes in covariation, computed across trials, more than 100 ms before changes in the total force. Half of the subjects showed large early changes in force mode covariation, while in the other half these changes were much smaller and were followed by a larger positive covariation of finger modes potentially destabilizing the total force profile. Such early covariation changes were absent under the simple reaction time instruction. We conclude that anticipatory covariation reflects control processes that can be expressed differently in different persons and modified depending on the available time for action preparation.

Adult↗

PubMed2005

Prehension synergies: trial-to-trial variability and principle of superposition during static prehension in three dimensions.

We performed three-dimensional analysis of the conjoint changes of digit forces during prehension (prehension synergies) and tested applicability of the principle of superposition to three-dimensional tasks. Subjects performed 25 trials at statically holding a handle instrumented with six-component force/moment sensors under seven external torque conditions; -0.70, -0.47, -0.23, 0.00, 0.23, 0.47, and 0.70 Nm about a horizontal axis in the plane passing through the centers of all five digit force sensors (the grasp plane). The total weight of the system was always 10.24 N. The trial-to-trial variability of the forces produced by the thumb and the virtual finger (an imagined finger producing the same mechanical effects as all 4 finger forces and moments combined) increased in all three dimensions with the external torque magnitude. The sets of force and moment variables associated with the moment production about the vertical axis in the grasp plane and the axis orthogonal to the grasp plane consisted of two noncorrelated subsets each; one subset of variables was related to the control of grasping forces (grasp control) and the other sassociated with the control of the orientation of the hand-held object (torque control). The variables associated with the moment production about the horizontal axis in the grasp plane did not include the grip force (the normal thumb and virtual finger forces) and showed more complex noncorrelated subsets. We conclude that the principle of superposition is valid for the prehension in three dimensions. The observed high correlations among forces and moments associated with the control of object orientation could be explained by chain effects, the sequences of cause-effect relations necessitated by mechanical constraints.

Adult↗

PubMed2005

Plastic changes in interhemispheric inhibition with practice of a two-hand force production task: a transcranial magnetic stimulation study.

We studied the effects of practice of an unusual two-hand finger force production task on electromyographic and force responses to transcranial magnetic stimulation (TMS). Prior to practice, force production by a hand resulted in decreased TMS-induced responses in the other hand. After practice, fingers that were explicitly required to produce force during practice showed a significant drop in these inhibitory effects, while other fingers did not. We conclude that interhemispheric inhibitory projections can show plastic changes that favor the execution of a practiced task.

Adult↗

PubMed2005

Motor variability within a multi-effector system: experimental and analytical studies of multi-finger production of quick force pulses.

The purpose of the study was to develop a model of force variability for a fast action performed by a multi-effector system and to verify it for multi-finger quick force production. The experiments involved quick isometric contractions to different target force levels using different finger combinations. Force variance calculated over sets of trials for a multi-finger force production task showed non-monotonic single-peak profiles of force variance with a peak at a time between the times of the maxima of the force rate and of the total force. When analyzed in the four-dimensional space of finger forces, the variance peak was mostly expressed in the direction of the force rate, and was absent in the directions orthogonal to it. The non-monotonic time profile of the force variance could be reproduced by a model of force production, which assumes that each finger force profile is based on a template function scaled in duration and magnitude with two parameters assigned prior to each trial with some variability. The model allows decomposition of the force variance into two fractions related to variability in setting the magnitude and duration scaling parameters. The former fraction changes monotonically with time, while the latter shows a transient peak in the middle of the action. The model was able to reproduce experimental variance time profiles across conditions with the total error of under 8%. The results demonstrate, in particular, that fast multi-finger actions may show transient changes in motor variability in certain directions of the finger force space, particularly in the direction of the first force derivative, without any task-specific coordinating action by the controller. These findings require a reconsideration of some of the conclusions drawn in recent studies on the structure of motor variability in redundant multi-effector systems.

Adult↗

PubMed2005

Joint angle variability in 3D bimanual pointing: uncontrolled manifold analysis.

The structure of joint angle variability and its changes with practice were investigated using the uncontrolled manifold (UCM) computational approach. Subjects performed fast and accurate bimanual pointing movements in 3D space, trying to match the tip of a pointer, held in the right hand, with the tip of one of three different targets, held in the left hand during a pre-test, several practice sessions and a post-test. The prediction of the UCM approach about the structuring of joint angle variance for selective stabilization of important task variables was tested with respect to selective stabilization of time series of the vectorial distance between the pointer and aimed target tips (bimanual control hypothesis) and with respect to selective stabilization of the endpoint trajectory of each arm (unimanual control hypothesis). The components of the total joint angle variance not affecting (V(COMP)) and affecting (V(UN)) the value of a selected task variable were computed for each 10% of the normalized movement time. The ratio of these two components R(V)=V(COMP)/V(UN) served as a quantitative index of selective stabilization. Both the bimanual and unimanual control hypotheses were supported, however the R(V) values for the bimanual hypothesis were significantly higher than those for the unimanual hypothesis applied to the left and right arm both prior to and after practice. This suggests that the CNS stabilizes the relative trajectory of one endpoint with respect to the other more than it stabilizes the trajectories of each of the endpoints in the external space. Practice-associated improvement in both movement speed and accuracy was accompanied by counter-intuitive lack of changes in R(V). Both V(COMP) and V(UN) variance components decreased such that their ratio remained constant prior to and after practice. We conclude that the UCM approach offers a unique and under-explored opportunity to track changes in the organization of multi-effector systems with practice and allows quantitative assessment of the degree of stabilization of selected performance variables.

Adult↗

PubMed2005

Viscoelastic response of the finger pad to incremental tangential displacements.

The finger pads of eight subjects were loaded by tangential displacement (x-perpendicular to the long axis of the finger) of a contacted surface when the distal and proximal interphalangeal joints (DIP and PIP, respectively) were alternately constrained. The finger pad responded in a linearly viscoelastic manner during loading, but exhibited highly nonlinear behavior upon unloading. The observed tangential force (F(T)) relaxations were nonlinear and could be modeled well by a logarithmic function. The average F(T) relaxation duration (tau) was 11.8 s. Apparent tangential stiffness (kT), determined by F(T) after relaxation, varied linearly with normal force. With the DIP joints constrained the fingers showed significantly larger stiffness than with the PIP joints constrained (p<0.001). Implications for finger force coordination studies are discussed.

Connective Tissue↗