Contributions and co-ordination of individual fingers in multiple finger prehension

Similarities and Differences in Finger Interaction across Typical and Atypical Subpopulations

The method of multidimensional scaling was applied to matrices of finger interaction (IFM) computed for individual participants for finger force production tasks. When IFMs for young controls, elderly, and persons with Down syndrome were pooled, only two dimensions described interpersonal differences; these were related to total force and to the total amount of enslaving. When IFMs for each group were analyzed separately, subpopulation-specific dimensions were found. Potentially, this analysis can be applied to discover meaningful dimensions that reflect differences in indices of finger interaction across and within subpopulations which differ in their apparent ability to use the hand. It may also be useful for tracking changes in finger interaction that occur in the process of specialized training or motor rehabilitation.

Finger interaction during multi-finger tasks involving finger addition and removal

The purpose of this study was to investigate changes in finger forces and their interactions in one-hand multi-finger force production tasks involving finger addition and removal. Eight male subjects were instructed to produce maximal (MVC) forces with explicitly instructed ('master') fingers. After reaching maximal force with a set of master fingers, the subjects added/removed one master finger while continuing to produce the MVC with the new group of master fingers. The non-instructed ('slave') fingers also produced forces (enslaving). Finger addition/removal led to changes in the forces of individual master fingers expected from earlier studies of maximal force production by different finger groups acting synchronously. A significant increase in the forces of remaining master fingers was observed after finger removal and a close-to-significant drop in the forces of previously recruited master fingers was observed after finger addition. These effects were larger when subjects started the task with a smaller number of explicitly involved fingers. The enslaving effects increased after finger addition while they did not change after finger removal. Forces produced by the same group of master fingers acting in different tasks showed no history-dependent effects. However, significant effects of history were seen in enslaving. These observations speak against hypotheses of more independent behaviour of effectors during their asynchronous involvement. They show that finger interaction may show effects of the history of finger involvement in a task.

Coordination of fingertip forces in object transport during locomotion

Walking while carrying a hand-held object requires the generation of appropriate grip forces to offset the inertial forces produced during locomotion. The present study examined the interaction between grip forces and locomotion-induced inertial forces across the gait cycle. Eight subjects transported a container under three conditions: self-paced transport with and without accuracy constraints and a velocity-constrained condition. The results showed that the trunk and transported container moved in a synchronized, sinusoidal pattern during all conditions. Grip and inertial forces of the transporting hand were highly coupled in an anticipatory fashion, regardless of task demands. The inertial forces were higher and the coupling was greater in the faster, unconstrained condition. However, grip force modulation was observed even when the inertial forces acting on the container were small and applied indirectly to the container through the locomotor effects originating in the legs and trunk. We suggest that continuous grip force adjustment is used as a generalized strategy to maximize efficiency during object transport regardless of the size or origin of the inertial forces.

The effects of precision demands during a low intensity pinching task on muscle activation and load sharing of the fingers

High precision demands in manual tasks can be expected to cause more selective use of a part of the muscular synergy involved. To test this expectation, load sharing of the index finger and middle finger was investigated during a pinching task. Myoelectric activation of lower arm and neck-shoulder muscles was measured to see if overall level of effort was affected by precision demands. Ten healthy female subjects performed pinching tasks with three levels of force and three levels of precision demands. The force level did not significantly affect the relative contribution of the index and middle finger to the force. Higher precision demands, however, led to higher contribution of the index finger to the pinch force. Consequently, a more selective load of the forearm and hand occurs during tasks with high precision demands. The variability of the force contribution of the fingers increased during the task. No effects of precision demand on the activation of forearm and neck-shoulder muscles were found. Force level did affect the EMG parameters of several muscles. The effects were most apparent in the muscles responsible for the pinch force, the forearm muscles. Activation of these muscles was higher at higher force levels. In the trapezius muscle at the dominant side EMG amplitudes were lower at the high pinch force compared to the low force and median force conditions.

Task-dependent modulation of multi-digit force coordination patterns

When grasping and holding an object with five digits under a variety of task constraints, subjects use well-defined force coordination patterns, i.e., consistent force covariations and in-phase synchronization among all digit pairs. The question arises as to whether these force coordination patterns are default mechanisms for controlling multi-digit force production or whether they are specific to lifting and holding an object. To address this question, we asked subjects to grasp a manipulandum and exert forces with five digits simultaneously so as to match a force template measured from an actual object grasp, lift, and hold task (GLH). Unlike GLH, the force production task (FP) lacked the constraint of having to maintain object stability against gravity. The amplitude of individual finger forces and force covariations were similar for both tasks (with the exception of the little finger, which tended to produce less force in FP). Nonetheless, when multiple grip forces were not required to hold the manipulandum against gravity (FP), there was a significantly lower tendency for forces to be synchronized with higher intertrial variability of phase differences between forces exerted by all digit-pairs. Furthermore, the tendency for force phase differences to cluster at 0 degrees was lower for FP than GLH. These results suggest that some aspects of the control of multi-digit grasping, i.e., force synchronization, are specific to object lift and hold rather than to the production of multi-digit forces. Modeling work suggests that motor unit synchronization might play an important role in the modulation of force synchronization patterns.

Prehension synergies: effects of object geometry and prescribed torques

We studied the coordination of forces and moments exerted by individual digits in static tasks that required balancing an external load and torque. Subjects ( n=10) stabilized a handle with an attachment that allowed for change of external torque. Thumb position and handle width systematically varied among the trials. Each subject performed 63 tasks (7 torque values x 3 thumb locations x 3 widths). Forces and moments exerted by the digit tips on the object were recorded. Although direction and magnitude of finger forces varied among subjects, each subject used a similar multidigit synergy: a single eigenvalue accounted for 95.2-98.5% of the total variance. When task parameters were varied, regular conjoint digital force changes (prehension synergies) were observed. Synergies represent preferential solutions used by the subjects to satisfy mechanical requirements of the tasks. In particular, chain effects in force adjustments to changes in the handle geometry were documented. An increased handle width induced the following effects: (a). tangential forces remained unchanged, (b). the same tangential forces produced a larger moment T (t), (c). the increased T (t) was compensated by a smaller moment of the normal forces T(n), and (d). normal finger forces were rearranged to generate a smaller moment. Torque control is a core component of prehension synergies. Observed prehension synergies are only mechanically necessitated in part. The data support a theory of hierarchical organization of prehension synergies.

Dependence of finger flexion force on the posture of the nonperforming fingers during key pressing tasks

The influence of different positions of the nonperforming (idle) fingers on the maximal force contraction of flexion (master) fingers during key pressing tasks was investigated. Ten participants performed maximal voluntary flexion contractions with various combinations of the index, middle, ring, and little fingers while the idle fingers rested on or were lifted away from the supporting surface. The effect of idle finger posture on total finger force production of master fingers was dependent on finger combination. In general, force production by master fingers was higher when the idle fingers were lifted away from the supporting surface than when they rested on it. The average increase in total force production by master fingers caused by the lifting of idle fingers was +12.4% (from -8.3% to +30.2%). Force-production capability of individual master fingers can be facilitated (as high as 34.1%), unchanged, or depressed (as high as -29.0%) by lifting the idle fingers. The effect of idle finger posture on finger force production of master fingers led to changes in force deficit. Neural, anatomical, and mechanical factors might account for the dependence of finger flexion force of master fingers on the posture of the idle fingers.

Modulated cortical control of individual fingers in experienced musicians: an EEG study. Electroencephalographic study

OBJECTIVE

The present research was designed to address the nature of interdependency between fingers during force production tasks in subjects with varying experience in performing independent finger manipulation. Specifically, behavioral and electroencephalographic (EEG) measures associated with controllability of the most enslaved (ring) and the least enslaved (index) fingers was examined in musicians and non-musicians.

METHODS

Six piano players and 6 age-matched control subjects performed a series of isometric force production tasks with the index and ring fingers. Subjects produced 3 different force levels with either their index or ring fingers. We measured the isometric force output produced by all 4 fingers (index, ring, middle and little), including both ramp and static phases of force production. We applied time-domain averaging of EEG single trials in order to extract 4 components of the movement-related cortical potentials (MRCP) preceding and accompanying force responses.

RESULTS

Three behavioral findings were observed. First, musicians were more accurate than non-musicians at reaching the desired force level. Second, musicians showed less enslaving as compared to non-musicians. And third, the amount of enslaving increased with the increment of nominal force levels regardless of whether the index or ring finger was used as the master finger. In terms of EEG measures, we found differences between tasks performed with the index and ring fingers in non-musicians. For musicians, we found larger MRCP amplitudes at most electrode sites for the ring finger.

CONCLUSIONS

Our data extends previous enslaving research and suggest an important role for previous experience in terms of the independent use of the fingers. Given that a variety of previous work has shown finger independence to be reflected in cortical representation in the brain and our findings of MRCP amplitude associated with greater independence of fingers in musicians, this suggests that what has been considered to be stable constraints in terms of finger movements can be modulated by experience.

SIGNIFICANCE

This work supports the idea that experience is associated with changes in behavioral and EEG correlates of task performance and may have clinical implications in disorders such as stroke or focal hand dystonia. Practice-related procedures offer useful approaches to rehabilitation strategies.

Two virtual fingers in the control of the tripod grasp

To investigate the organization of multi-fingered grasping, we asked subjects to grasp an object using three digits: the thumb, the index finger, and the middle or ring finger. The object had three coarse flat contact surfaces, whose locations and orientations were varied systematically. Subjects were asked to grasp and lift the object and then to hold it statically. We analyzed the grasp forces in the horizontal plane that were recorded during the static hold period. Static equilibrium requires that the forces exerted by the three digits intersect at a common point, the force focus. The directions of the forces exerted by the two fingers opposing the thumb depended on the orientation of the contact surfaces of both fingers but not on the orientation of the contact surface of the thumb. The direction of the thumb's force did not depend on the orientation of the contact surfaces of the two fingers and depended only weakly on the orientation of the thumb's contact surface. In general, the thumb's force was directed to a point midway between the two fingers. The results are consistent with a hierarchical model of the control of a tripod grasp. At the first level, an opposition space is created between the thumb and a virtual finger located approximately midway between the two actual fingers. The directions of the forces exerted by the two fingers are constrained to be mirror symmetric about the opposition axis. The actual directions of finger force are elaborated at the next level on the basis of stability considerations.

Coordination of grip configurations as a function of force output

In this investigation, the authors examined the coordination and control of force production by the digits of the hand as a function of criterion force level and grip configuration. Each adult participant (N = 6: 3 men and 3 women) was required to place the thumb and a finger (or fingers) upon load cells that were fixed to a grasping apparatus that was clamped to a table. In the task, participants had to match a criterion continuous constant total force level displayed on a computer screen. There were 10 trials at each grip configuration and criterion force level combination on each of 3 consecutive days. The results showed that (a) different grip configurations minimized error at each force level; (b) there was a specific digit pairing within a given grip configuration that produced the highest correlation of force output; (c) the correlation between the force output of digits generally increased at higher force levels; (d) error was reduced at each force level and grip configuration over the practice period; and (e) the organization of the force output of each digit varied as a function of digit, force level, grip configuration, and practice. The findings are consistent with the hypothesis that coordination of the digits in prehension is reflective of an adaptive, task-specific solution that is modified with practice.

Control of grasp stability in humans under different frictional conditions during multidigit manipulation

Control of grasp stability under different frictional conditions has primarily been studied in manipulatory tasks involving two digits only. Recently we found that many of the principles for control of forces originally demonstrated for two-digit grasping also apply to various three-digit grasps. Here we examine the control of grasp stability in a multidigit task in which subjects used the tips of the thumb, index, and middle finger to lift an object. The grasp resembled those used when lifting a cylindrical object from above. The digits either all contacted the same surface material or one of the digits contacted a surface material that was more, or less, slippery than that contacted by the other two digits. The three-dimensional forces and torques applied by each digit and the contact positions were measured along with the position and orientation of the object. The distribution of forces among the digits strongly reflected constraints imposed by the geometric relationship between the object's center of mass and the contact surfaces. On top of this distribution, we observed changes in force coordination related to changes in the combination of surface materials. When all digits contacted the same surface material, the ratio between the normal force and tangential load (F(n):L ratio) was similar across digits and scaled to provide an adequate safety margin against slip. With different contact surfaces subjects adapted the F(n):L ratios at the individual digits to the local friction with only small influences by the friction at the other two digits. They accomplished this by scaling the normal forces similarly at all digits and changing the distribution of load among the digits. The surface combination did not, however, influence digit position, tangential torque, or object tilting systematically. The change in load distribution, rather, resulted from interplay between these factors, and the nature of this interplay varied between trials. That is, subjects achieved grasp stability with various combinations of fingertip actions and appeared to exploit the many degrees of freedom offered by the multidigit grasp. The results extend previous findings based on two-digit tasks to multidigit tasks by showing that subjects adjust fingertip forces at each digit to the local friction. Moreover, our findings suggest that subjects adapted the load distribution to the current frictional condition by regulating the normal forces to allow slips to occur early in the lift task, prior to object lift-off.

Effect of gloves on prehensile forces during lifting and holding tasks

The effect of gloves on the spatio-temporal characteristics of prehensile forces during lifting and holding tasks was investigated. Participants (n = 10) lifted a force transducer equipped object (weight = 0.29 N) with various types of gloves and barehanded using a two-fingered precision grip. Rubber surgical gloves of varied thicknesses (0.24, 0.61 and 1.02 mm) were worn to examine the effect of glove thickness on a rayon surface. It was found that grip force increased with thickness because the participants employed a higher safety margin above the minimum force required to hold the object. The safety margin for the barehanded condition was the smallest. The performance time for lifting the object was not influenced by the variation of glove thickness. The findings suggest that glove thickness, which presumably modifies the cutaneous sensation, influences grip force regulation. The effect of glove material (rubber and cotton) was also examined in relation to slippery (rayon) and non-slippery (sandpaper) surfaces. It was found that the participants used a larger grip force with the cotton glove than the rubber glove for the slippery surface, but not with the non-slippery surface. With use of the rubber glove, a relatively low grip force level was maintained for both slippery and non-slippery surfaces. The performance time for the cotton glove was longer than that for the rubber glove. The findings suggest that the rubber glove provides better efficiency of force and temporal control than the cotton glove in precision handling of small objects.

Control of fingertip forces in multidigit manipulation

Previous studies of control of fingertip forces in skilled manipulation have focused on tasks involving two digits, typically the thumb and index finger. Here we examine control of fingertip actions in a multidigit task in which subjects lifted an object using unimanual and bimanual grasps engaging the tips of the thumb and two fingers. The grasps resembled those used when lifting a cylindrical object from above; the two fingers were some 4.25 cm apart and the thumb was approximately 5.54 cm from either finger. The three-dimensional forces and torques applied by each digit and the digit contact positions were measured along with the position and orientation of the object. The vertical forces applied tangential to the grasp surfaces to lift the object were synchronized across the digits, and the contribution by each digit to the total vertical force reflected intrinsic object properties (geometric relationship between the object's center of mass and the grasped surfaces). Subjects often applied small torques tangential to the grasped surfaces even though the object could have been lifted without such torques. The normal forces generated by each digit increased in parallel with the local tangential load (force and torque), providing an adequate safety margin against slips at each digit. In the present task, the orientations of the force vectors applied by the separate digits were not fully constrained and therefore the motor controller had to choose from a number of possible solutions. Our findings suggest that subjects attempt to minimize (or at least reduce) fingertip forces while at the same time ensure that grasp stability is preserved. Subjects also avoid horizontal tangential forces, even at a small cost in total force. Moreover, there were subtle actions exerted by the digits that included changes in the distribution of vertical forces across digits and slight object tilt. It is not clear to what extent the brain explicitly controlled these actions, but they could serve, for instance, to keep tangential torques small and to compensate for variations in digit contact positions. In conclusion, we have shown that when lifting an object with a three-digit grip, the coordination of fingertip forces, in many respects, matches what has been documented previously for two-digit grasping. At the same time, our study reveals novel aspects of force control that emerge only in multidigit manipulative tasks.

Sensory input and control of grip

When we use our digits to manipulate objects the applied fingertip forces and torques tangential to the grip surfaces are a result of complex muscle activity. These patterns are acquired during our ontogenetic development and we select them according to the manipulative intent. But the basic force coordination expressed in these patterns has to be tuned to the physical properties of the current object, e.g. shape, surface friction and weight. This takes place primarily by parametric adjustments of the force output based on internal models of the target object, i.e. implicit memory systems that represent critical object properties. From visual or haptic information we identify objects and automatically retrieve the relevant models. These models are then used to adapt the motor commands prior to their execution. The formation of models and their swift updating with changes in object properties depend, however, on signals from tactile sensors in the fingertips.

Mechanisms for force adjustments to unpredictable frictional changes at individual digits during two-fingered manipulation

Previous studies on adaptation of fingertip forces to local friction at individual digit-object interfaces largely focused on static phases of manipulative tasks in which humans could rely on anticipatory control based on the friction in previous trials. Here we instead analyze mechanisms underlying this adaptation after unpredictable changes in local friction between consecutive trials. With the tips of the right index and middle fingers or the right and left index fingers, subjects restrained a manipulandum whose horizontal contact surfaces were located side by side. At unpredictable moments a tangential force was applied to the contact surfaces in the distal direction at 16 N/s to a plateau at 4 N. The subjects were free to use any combination of normal and tangential forces at the two fingers, but the sum of the tangential forces had to counterbalance the imposed load. The contact surface of the right index finger was fine-grained sandpaper, whereas that of the cooperating finger was changed between sandpaper and the more slippery rayon. The load increase automatically triggered normal force responses at both fingers. When a finger contacted rayon, subjects allowed slips to occur at this finger during the load force increase instead of elevating the normal force. These slips accounted for a partitioning of the load force between the digits that resulted in an adequate adjustment of the normal:tangential force ratios to the local friction at each digit. This mechanism required a fine control of the normal forces. Although the normal force at the more slippery surface had to be comparatively low to allow slippage, the normal forces applied by the nonslipping digit at the same time had to be high enough to prevent loss of the manipulandum. The frictional changes influenced the normal forces applied before the load ramp as well as the size of the triggered normal force responses similarly at both fingers, that is, with rayon at one contact surface the normal forces increased at both fingers. Thus to independently adapt fingertip forces to the local friction the normal forces were controlled at an interdigital level by using sensory information from both engaged digits. Furthermore, subjects used both short- and long-term anticipatory mechanisms in a manner consistent with the notion that the central nervous system (CNS) entertains internal models of relevant object and task properties during manipulation.

Control of forces applied by individual fingers engaged in restraint of an active object

We investigated the coordination of fingertip forces in subjects who used the tips of two fingers to restrain an instrumented manipulandum with horizontally oriented grip surfaces. The grip surfaces were subjected to tangential pulling forces in the distal direction in relation to the fingers. The subjects used either the right index and middle fingers (unimanual grasp) or both index fingers (bimanual grasp) to restrain the manipulandum. To change the frictional condition at the digit-object interfaces, either both grip surfaces were covered with sandpaper or one was covered with sandpaper and the other with rayon. The forces applied normally and tangentially to the grip surfaces were measured separately at each plate along with the position of the plates. Subjects could have performed the present task successfully with many different force distributions between the digits. However, they partitioned the load in a manner that reflected the frictional condition at the local digit-object interfaces. When both digits contacted sandpaper, they typically partitioned the load symmetrically, but when one digit made contact with rayon and the other with sandpaper, the digit contacting the less slippery material (sandpaper) took up a larger part of the load. The normal forces were also influenced by the frictional condition, but they reflected the average friction at the two contact sites rather than the local friction. That is, when friction was low at one of the digit-object interfaces, only the applied normal forces increased at both digits. Thus sensory information related to the local frictional condition at the respective digit-object interfaces controlled the normal force at both digits. The normal:tangential force ratio at each digit appeared to be a controlled variable. It was adjusted independently at each digit to the minimum ratio required to prevent frictional slippage, keeping an adequate safety margin against slippage. This was accomplished by the scaling of the normal forces to the average friction and by partitioning of the load according to frictional differences between the digit-object interfaces. In conclusion, by adjusting the normal:tangential force ratios to the local frictional condition, subjects avoided excessive normal forces at the individual digit-object interfaces, and by partitioning the load according the frictional difference, subjects avoided high normal forces. Thus the local frictional condition at the separate digit-object interfaces is one factor that can strongly influence the distribution of forces across digits engaged in a manipulative act.

Grip posture and forces during holding cylindrical objects with circular grips

Individual finger position and external grip forces were investigated while subjects held cylindrical objects from above using circular precision grips. Healthy females (n = 11) and males (n = 15) lifted cylindrical objects of various weights (0.5, 1.0 and 2.0 kg), and varied diameters (5.0, 7.5 and 10.0 cm) using the 5-finger grip mode. The effects of 4-, 3- and 2-finger grip modes in the circular grip were also investigated. Individual finger position was nearly constant for all weights and for diameters of 5.0 and 7.5 cm. The mean angular positions for the index, middle, ring and little fingers relative to the thumb were 98 degrees, 145 degrees, 181 degrees, and 236 degrees, respectively. At the 10-cm diameter, the index and middle finger positions increased, while the ring and little finger positions decreased. There were no differences in individual finger position with regard to gender, hand dimension, or hand strength. Total grip force increased with weight, and at diameters greater or lesser than 7.5 cm. Total grip force also increased as the number of fingers used for grasping decreased. Although the contribution of the individual fingers to the total grip force changed with weight and diameter, the thumb contribution always exceeded 38% followed by the ring and little fingers, which contributed approximately 18-23% for all weights and diameters. The contribution of the index finger was always smallest (> or = 11%). There was no gender difference for any of the grip force variables. The effects of hand dimension and hand strength on the individual finger grip forces were subtle.

Individual finger forces acting on a grasped object during shaking actions

Individual finger grip forces acting on a hand-held object were examined during shaking tasks with a five-finger precision grip. The subjects (n = 13) shook a force transducer-equipped grip object (mass = 400 g) in vertical, horizontal, and mediolateral directions at an average movement speed of 33 cm/s (moderate) and 66 cm/s (fast). In addition, grip forces were examined while the subjects (n = 10) held the object in front of the body and walked or ran in place. It was found that the grip forces for all the fingers changed temporally and spatially coupling with the acceleration of the object resulting from shaking. The results suggest that grip force control is accomplished in an active and anticipatory fashion. Regardless of the shaking direction and speed, among the four fingers the absolute grip force in the index finger was largest, followed by the middle, ring, and little finger forces. The index finger therefore plays a primary role in grip force control during shaking. The percent force contribution by each finger varied depending on the direction of shaking. Contributions of the ring and little fingers were larger when shaken in the horizontal and mediolateral directions than they were in the vertical direction. The results suggest that different finger co-ordination is required in relation to shaking direction. Changes in shaking speed from moderate to fast changed the grip forces for all the fingers. During walking and running, grip force control similar to that during active vertical shaking was required to hold the object safely in the hand.