Inter-digit co-ordination and object-digit interaction when holding an object with five digits

Effects of Vibration-Based Generation of Timing of Tactile Perception on Upper Limb Function After Stroke: A Case Study

Sensorimotor dysfunction of the fingers and hands hinders the recovery of motor function post-stroke. Generally, hemiplegic patients are unable to properly control the dynamic friction generated between their fingers and objects during hand/finger muscle activity. In addition to sensory information, a sense of agency generated by the temporal synchronization of sensory prediction and sensory feedback is required to control this dynamic friction. In the present study, we utilized a novel rehabilitation device that transmits real-time fingertip contact information to a transducer in a case of stroke hemiplegia with sensorimotor deficits and stagnated hand/finger motor performance. Post-intervention, the patient's upper extremity motor function score (FMA-UE), which had previously been in a state of arrested recovery, improved from 51/66 to 61/66, especially in the wrist joints. Excessive grip force during object grasping and frequency of falling objects was notably decreased post-intervention. We believe that rehabilitation tasks using perceptual generation via transducer will be a new tool for the rehabilitation of post-stroke hand/finger sensorimotor deficits.

Distinct behavior of the little finger during the vertical translation of an unsteady thumb platform while grasping

Object stabilization while grasping is a common topic of research in motor control and robotics. Forces produced by the peripheral fingers (index and little) play a crucial role in sustaining the rotational equilibrium of a handheld object. In this study, we examined the contribution of the peripheral fingers towards object stabilization when the rotational equilibrium is disturbed. For this purpose, the thumb was placed over an unsteady platform and vertically translated. The task was to trace a trapezoid or an inverted trapezoid pattern by moving the thumb platform in the vertical direction. The thumb displacement data served as visual feedback to trace the pattern displayed. Participants were instructed to maintain the handle in static equilibrium at all times. We observed that the change in the normal force of the little finger due to the downward translation of the thumb was significantly greater than the change in the normal force of the index finger due to the upward translation. We speculate that morphological correlations (between thumb and little finger) during the displacement of the thumb might be a reason for such large increases in the little finger forces.

Evaluation of Intervention Effectiveness of Sensory Compensatory Training with Tactile Discrimination Feedback on Sensorimotor Dysfunction of the Hand after Stroke

We investigated the intervention effect of training using a feedback-type tactile discrimination system on sensorimotor dysfunction of the hand after a stroke. A human male subject with sensorimotor dysfunction in his left hand after a stroke was asked to perform peg manipulation practice, a building block stacking task, and a material identification task for 10 min each for six weeks. During the activities, a tactile discrimination feedback system was used. The system is a device that detects the vibration information generated when touching an object with a hand and that feeds back the captured information in real time as vibration information. After the intervention, in addition to the reorganization of the sensorimotor areas, the deep sensation, sense of agency, numbness, amount of use, and quality of the left-hand movement improved. Our results suggest that training with the use of a feedback system could be a new form of rehabilitation for sensorimotor dysfunction of the hand.

A preliminary study on characterisation of finger interface kinetics using a pressure and shear sensor system

BACKGROUND

Our hands constantly handle objects throughout our lives, where a crucial component of this interaction is the detection of grasping (pressure) and slipping (shear) of the object. While there have been a large amount of studies using pressure sensors for grasping detection, synchronised pressure and shear detection at the finger/object interface is still needed.

OBJECTIVES

This study aims to assess the feasibility of a sensor system designed to detect both pressure and shear at the fingertip/object interface via a single subject test.

STUDY DESIGN

Descriptive study, proof of concept.

METHODS

One healthy subject participated in the study and was asked to perform a single finger test protocol and a simple hand test protocol. The corresponding multidirectional loads at the fingertip/object interface were measured in real time using a pressure and shear sensor system.

RESULTS

Results from the finger test protocol show peak values of up to approximately 50 kPa (5 N) and 30 kPa (3 N) of pressure for each test, respectively. Results from the hand test protocol show a pressure and shear profile that shows a large increase in grip force during the initial grasping of the object, with a peak pressure of approximately 50 kPa (5 N). The pressure and shear profile demonstrates that the load is not evenly distributed across all digits.

CONCLUSION

This study provides evidence that the reported sensor system has sufficient resolution, dynamic response and load capability to capture biomechanical information during basic protocols and hand-grasping tasks. Clinical relevance The presented sensor system could be potentially used as a tool for measuring and evaluating hand function and could be incorporated into a prosthetic hand as a tactile feedback system.

Is the Control of Applied Digital Forces During Natural Five-digit Grasping Affected by Carpal Tunnel Syndrome?

BACKGROUND

The impaired sensory function of the hand induced by carpal tunnel syndrome (CTS) is known to disturb dexterous manipulations. However, force control during daily grasping configuration among the five digits has not been a prominent focus of study. Because grasping is so important to normal function and use of a hand, it is important to understand how sensory changes in CTS affect the digit force of natural grasp.

QUESTIONS/PURPOSES

We therefore examined the altered patterns of digit forces applied during natural five-digit grasping in patients with CTS and compared them with those seen in control subjects without CTS. We hypothesized that the patients with CTS will grasp by applying larger forces with lowered pair correlations and more force variability of the involved digits than the control subjects. Specifically, we asked: (1) Is there a difference between patients with CTS and control subjects in applied force by digits during lift-hold-lower task? (2) Is there a difference in force correlation coefficient of the digit pairs? (3) Are there force variability differences during the holding phase?

METHODS

We evaluated 15 female patients with CTS and 15 control subjects matched for age, gender, and hand dominance. The applied radial forces (Fr) of the five digits were recorded by respective force transducers on a cylinder simulator during the lift-hold-lower task with natural grasping. The movement phases of the task were determined by a video-based motion capture system.

RESULTS

The applied forces of the thumb in patients with CTS (7 ± 0.8 N; 95% CI, 7.2-7.4 N) versus control subjects (5 ± 0.8 N; 95% CI, 5.1-5.3 N) and the index finger in patients with CTS (3 ± 0.3 N; 95% CI, 3.2-3.3 N) versus control subjects (2 ± 0.3 N; 95% CI, 2.2-2.3 N) observed throughout most of the task were larger in the CTS group (p ranges 0.035-0.050 for thumb and 0.016-0.050 for index finger). In addition, the applied force of the middle finger in patients with CTS (1 ± 0.1 N; 95% CI, 1.3-1.4 N) versus the control subjects (2 ± 0.2 N; 95% CI, 1.9-2.0 N) during the lowering phase was larger in CTS group (p ranges 0.039-0.050). The force correlations of the thumb-middle finger observed during the lowering phase in the patients with CTS (0.8 ± 0.2; 95% CI, 0.6-0.9) versus the control subjects (0.9 ± 0.1; 95% CI, 0.8-1.0; p = 0.04) were weaker in the CTS group. The thumb-little finger during holding in the patients with CTS (0.5 ± 0.2; 95% CI, 0.3-0.7) versus the control subjects (0.8 ± 0.2; 95% CI, 0.6-0.9; p = 0.02), and the lowering phase in the patients with CTS (0.6 ± 0.2; 95% CI, 0.3-0.8) versus the control subjects (0.9 ± 0.1; 95% CI, 0.8-1.0; p = 0.01) also were weaker. The force variabilities of patients with CTS were greater in the CTS group than in the control subjects: in the thumb ([0.26 ± 0.11 N, 95% CI, 0.20-0.32 N] versus [0.19 ± 0.06 N; 95% CI, 0.16-0.22 N], p = 0.03); index finger ([0.09 ± 0.07 N; 95% CI, 0.05-0.13 N] versus [0.05 ± 0.03 N; 95% CI, 0.04-0.07 N], p = 0.03); middle finger ([0.06 ± 0.04 N; 95% CI, 0.04-0.08 N] versus [0.03 ± 0.01 N; 95% CI, 0.02-0.04 N], p = 0.02), and ring finger ([0.04 ± 0.03 N; 95% CI, 0.20-0.06 N] versus [0.02 ± 0.01 N; 95% CI, 0.02-0.02 N], p = 0.01).

CONCLUSIONS

Patients with CTS grasped with greater digit force associated with weaker correlation and higher variability on specific digits in different task demands. These altered patterns in daily grasping may lead to secondary problems, which will need to be assessed in future studies with this model to see if they are reversible in patients undergoing carpal tunnel release.

CLINICAL RELEVANCE

The current results helped to identify altered patterns of grasping force during simulated daily function in patients with CTS and to provide the clinician with potential information that might help guide the rehabilitation of grasp in these patients.

Directional coordination of thumb and finger forces during precision pinch

The human opposable thumb enables the hand to perform dexterous manipulation of objects, which requires well-coordinated digit force vectors. This study investigated the directional coordination of force vectors generated by the thumb and index finger during precision pinch. Fourteen right-handed, healthy subjects were instructed to exert pinch force on an externally stabilized apparatus with the pulps of the thumb and index finger. Subjects applied forces to follow a force-ramp profile that linearly increased from 0 to 12 N and then decreased to 0 N, at a rate of ±3 N/s. Directional relationships between the thumb and index finger force vectors were quantified using the coordination angle (CA) between the force vectors. Individual force vectors were further analyzed according to their projection angles (PAs) with respect to the pinch surface planes and the shear angles (SAs) within those planes. Results demonstrated that fingertip force directions were dependent on pinch force magnitude, especially at forces below 2 N. Hysteresis was observed in the force-CA relationship for increasing and decreasing forces and fitted with exponential models. The fitted asymptotic values were 156.0±6.6° and 150.8±9.3° for increasing and decreasing force ramps, respectively. The PA of the thumb force vector deviated further from the direction perpendicular to the pinching surface planes than that of the index finger. The SA showed that the index finger force vector deviated in the ulnar-proximal direction, whereas the thumb switched its force between the ulnar-proximal and radial-proximal directions. The findings shed light on the effects of anatomical composition, biomechanical function, and neuromuscular control in coordinating digit forces during precision pinch, and provided insight into the magnitude-dependent force directional control which potentially affects a range of dexterous manipulations.

Removal of visual feedback lowers structural variability of inter-digit force coordination during sustained precision pinch

This study examined the effects of visual feedback on inter-digit force coordination during a precision pinch. Sixteen healthy, right-handed subjects were instructed to pinch an instrumented apparatus for 1 min with a stable force output. Visual feedback was provided for the first 30s and withdrawn for the second 30s. Detrended fluctuation analysis (DFA) and detrended cross-correlation analysis (DCCA) methods were used to quantify the time-dependent structures of each digit's force and of the force correlation between the digits. After removing visual feedback, the DFA scaling exponent, αDFA, increased from 1.10±0.12 to 1.29±0.13 for the thumb and from 0.95±0.08 to 1.33±0.13 for the index finger (F1,95=372.47, p<0.001); the DCCA scaling exponent, αDCCA, increased from 1.00±0.08 to 1.33±0.13 (t95=20.33, p<0.001). Structural changes were observed beginning with the first 5s epoch after the removal of visual feedback. The results provide evidence that removing visual feedback lowers the structural variability of inter-digit force coordination. This change is reflected in the high-level control strategy, resulting in the two digits being more tightly coupled under somatosensory feedback without visual inputs.

Quantifying Digit Force Vector Coordination during Precision Pinch

A methodology was established to investigate the contact mechanics of the thumb and the index finger at the digit-object interface during precision pinch. Two force/torque transducers were incorporated into an apparatus designed to overcome the thickness of each transducer and provide a flexible pinch span for digit placement and force application. To demonstrate the utility of the device, five subjects completed a pinch task with the pulps of their thumb and index finger. Inter-digit force vector coordination was quantified by examining the 1) force vector component magnitudes, 2) resultant force vector magnitudes, 3) coordination angle - the angle formed by the resultant vectors of each digit, 4) direction angles - the angle formed by each vector and the coordinate axes, and 5) center of pressure locations. It was shown that the resultant force magnitude of the index finger exceeded that of the thumb by 0.8 ± 0.3 N and that the coordination angle between the digit resultant force vectors was 160.2 ± 4.6°. The experimental apparatus and analysis methods provide a valuable tool for the quantitative examination of biomechanics and motor control during dexterous manipulation.

The force synergy of human digits in static and dynamic cylindrical grasps

This study explores the force synergy of human digits in both static and dynamic cylindrical grasping conditions. The patterns of digit force distribution, error compensation, and the relationships among digit forces are examined to quantify the synergetic patterns and coordination of multi-finger movements. This study recruited 24 healthy participants to perform cylindrical grasps using a glass simulator under normal grasping and one-finger restricted conditions. Parameters such as the grasping force, patterns of digit force distribution, and the force coefficient of variation are determined. Correlation coefficients and principal component analysis (PCA) are used to estimate the synergy strength under the dynamic grasping condition. Specific distribution patterns of digit forces are identified for various conditions. The compensation of adjacent fingers for the force in the normal direction of an absent finger agrees with the principle of error compensation. For digit forces in anti-gravity directions, the distribution patterns vary significantly by participant. The forces exerted by the thumb are closely related to those exerted by other fingers under all conditions. The index-middle and middle-ring finger pairs demonstrate a significant relationship. The PCA results show that the normal forces of digits are highly coordinated. This study reveals that normal force synergy exists under both static and dynamic cylindrical grasping conditions.

Coordination of Contact Forces During Multifinger Static Prehension

This study investigated the effects of modifying contact finger forces in one direction—normal or tangential—on the entire set of the contact forces, while statically holding an object. Subjects grasped a handle instrumented with finger force-moment sensors, maintained it at rest in the air, and then slowly: (1) increased the grasping force, (2) tried to spread fingers apart, and (3) tried to squeeze fingers together. Analysis was mostly performed at the virtual finger (VF) level (the VF is an imaginable finger that generates the same force and moment as the four fingers combined). For all three tasks there were statistically significant changes in the VF normal and tangential forces. For finger spreading/squeezing the tangential force neutral point was located between the index and middle fingers. We conclude that the internal forces are regulated as a whole, including adjustments in both normal and tangential force, instead of only a subset of forces (normal or tangential). The effects of such factors as EFFORT and TORQUE were additive; their interaction was not statistically significant, thus supporting the principle of superposition in human prehension.

Limits to the Control of the Human Thumb and Fingers in Flexion and Extension

In humans, hand performance has evolved from a crude multidigit grasp to skilled individuated finger movements. However, control of the fingers is not completely independent. Although musculotendinous factors can limit independent movements, constraints in supraspinal control are more important. Most previous studies examined either flexion or extension of the digits. We studied differences in voluntary force production by the five digits, in both flexion and extension tasks. Eleven healthy subjects were instructed either to maximally flex or extend their digits, in all single- and multidigit combinations. They received visual feedback of total force produced by “instructed” digits and had to ignore “noninstructed” digits. Despite attempts to maximally flex or extend instructed digits, subjects rarely generated their “maximal” force, resulting in a “force deficit,” and produced forces with noninstructed digits (“enslavement”). Subjects performed differently in flexion and extension tasks. Enslavement was greater in extension than in flexion tasks ( P = 0.019), whereas the force deficit in multidigit tasks was smaller in extension ( P = 0.035). The difference between flexion and extension in the relationships between the enslavement and force deficit suggests a difference in balance of spillover of neural drive to agonists acting on neighboring digits and focal neural drive to antagonist muscles. An increase in drive to antagonists would lead to more individualized movements. The pattern of force production matches the daily use of the digits. These results reveal a neural control system that preferentially lifts fingers together by extension but allows an individual digit to flex so that the finger pads can explore and grasp.

Kinetic analysis of the thumb in jar-opening activity among female adults

Jar opening is commonly viewed as a challenging task for female adults in daily living. The thumb plays a particular role in grasping the jar lid and leading the turning activity through opposition to the other digits. This study measured and compared the force, torque and torque contribution of the thumb in the activity under ordinary grasp patterns and jar-holding positions. A steel jar-like cylinder was custom made to measure the force and torque generated simultaneously by thumb and wrist. Sixteen young females without a history of hand injury were recruited as subjects. The force and torque of the thumb were found to be influenced significantly only by the grasp pattern but its torque contribution was significantly affected only by the jar-holding position. However, overall torque was influenced by both the grasp pattern and the jar-holding position. The torque contribution of the thumb under the four different grasp patterns and jar-holding positions was found to range from 17.4% to 23.9%. The contribution ratios suggest that the thumb may offer a force equivalent to the other digits rather than just a counter force in this activity.

Lower median nerve block impairs precision grip

The purpose of this study was to investigate precision grip impairment caused by a lower median nerve block at the wrist. The median nerve block was achieved by injecting bupivacaine hydrochloride into the carpal tunnel, which acutely simulated a median neuropathy. Seven healthy male subjects were instructed to grip, lift, and hold an instrumented handle within 60s using precision grip. The same tasks were performed before and after the nerve block. Force and torque data were recorded using two miniature 6-component force/torque transducers. The precision grip was quantified by the safety margin (i.e. the difference between the actual grip force and the minimal grip force to keep the object from dropping), the variation of grip force, and the migration area of center of pressure (i.e. the area defined by the center of pressure at a digit-transducer surface while holding the handle). Two subjects were unable to complete the precision grip tasks after the nerve block, and their data were excluded from the analyses. The median nerve block caused significant increases (P<0.05) in the safety margin of the grip force (>50%), the grip force variation (>80%), and the area of center of pressure migration (>250%). Median nerve block at the wrist impairs the fine motor control during precision grip. Our results corroborate the important role played by sensory function in hand fine motor control. Clinically, the measures related to precision grip have the potential to quantify impairment of hand function caused by neuromuscular disorders, to monitor the progress of a hand disorder, and to evaluate the efficacy of a treatment or rehabilitation procedure.

Soft tissue strain energy minimization: a candidate control scheme for intra-finger normal–tangential force coordination

The safety margin (SM) measure has been used to quantify the phenomenon that humans grasp objects more firmly than is necessary to prevent slip. The biomechanical basis for the SM phenomenon is addressed herein. A hypothesis is proposed regarding intra-finger normal-tangential force coordination. The idea is that the central nervous system (CNS) minimizes the strain energy of the soft finger pad tissue by varying normal force when presented with a certain tangential force. This control scheme requires no knowledge of the frictional conditions at the finger-object interface; the CNS needs only to detect the strain energy in the contact region, an area abundant with strain-sensitive mechanoreceptors. The scheme is not independent, but is rather a possible component of a more complicated system. The strain energy minimization problem was solved using the finite element model (FEM) of Wu et al. (Med. Eng. Phys. 24(4)(2002) 253). Optimization results revealed that the suggested control scheme produced SM values of 30-50%, corresponding closely to those reported experimentally. Slip prevention naturally emerges from the control scheme provided that the friction coefficient exceeds 0.7, a value lower than typically encountered.

Selective recruitment of single motor units in human flexor digitorum superficialis muscle during flexion of individual fingers

Flexor digitorum superficialis (FDS) is an extrinsic multi-tendoned muscle which flexes the proximal interphalangeal joints of the four fingers. It comprises four digital components, each with a tendon that inserts onto its corresponding finger. To determine the degree to which these digital components can be selectively recruited by volition, we recorded the activity of a single motor unit in one component via an intramuscular electrode while the subject isometrically flexed each of the remaining fingers, one at a time. The finger on which the unit principally acted was defined as the 'test finger' and that which flexed isometrically was the 'active' finger. Activity in 79 units was recorded. Isometric finger flexion forces of 50% maximum voluntary contraction (MVC) activated less than 50% of single units in components of FDS acting on fingers that were not voluntarily flexed. With two exceptions, the median recruitment threshold for all active-test finger combinations involving the index, middle, ring and little finger test units was between 49 and 60% MVC (60% MVC being the value assigned to those not recruited). The exceptions were flexion of the little finger while recording from ring finger units (median: 40% MVC), and vice versa (median: 2% MVC). For all active-test finger combinations, only 35/181 units were activated when the active finger flexed at less than 20% MVC, and the fingers were adjacent for 28 of these. Functionally, to recruit FDS units during grasping and lifting, relatively heavy objects were required, although systematic variation occurred with the width of the object. In conclusion, FDS components can be selectively activated by volition and this may be especially important for grasping at high forces with one or more fingers.

Motor control of rapid sequential finger tapping in humans

We investigated in 29 healthy subjects a simple model of rapid independent finger movement: the rapid sequential tapping of adjacent fingers. Inter-tap interval (ITI) was measured for adjacent pairs of fingers in each direction. ITI was shorter in the ulnar-->radial direction than in the reverse direction [P < 0.001 for middle to index (M-->I) compared with index to middle (I-->M)]. There was a gradient across the hand such that in the ulnar-->radial direction, little to ring (L-->R) tapping was fastest and M-->I slowest; in the radial-->ulnar direction, the reverse was the case. Rectified surface electromyography (EMG) from finger extensors and flexors was averaged with respect to either the first or second tap. The interval between the flexor EMG burst and the tap was similar irrespective of the order of finger tapping, excluding a mechanical explanation of the timing difference. Transcranial magnetic stimulation (TMS) was applied at 0- to 50-ms intervals after the first tap. Interposed TMS delayed the second tap significantly more (P < 0.001) in the M-->I direction than in the I-->M direction. Motor-evoked potentials (MEPs) evoked by TMS interposed between taps showed a greater facilitation in the M-->I than in the I-->M direction (P < 0.001). Increasing intensity of TMS rendered subjects unable to produce the second tap, more frequently in the I-->M direction than in the M-->I direction. We have demonstrated a consistent pattern across the hand and postulate that finger-order-dependent differences in ITI and the gradient of these across the hand may reflect the mechanism of grasping and further that the cortical programming of finger tapping differs depending on finger order.

Prehension synergies during nonvertical grasping, II: Modeling and optimization

This study examines various optimization criteria as potential sources of constraints that eliminate (or at least reduce the degree of) mechanical redundancy in prehension. A model of nonvertical grasping mimicking the experimental conditions of Pataky et al. (current issue) was developed and numerically optimized. Several cost functions compared well with experimental data including energylike functions, entropylike functions, and a ''motor command'' function. A tissue deformation function failed to predict finger forces. In the prehension literature, the ''safety margin'' (SM) measure has been used to describe grasp quality. We demonstrate here that the SM is an inappropriate measure for nonvertical grasps. We introduce a new measure, the ''generalized safety margin'' (GSM), which reduces to the SM for vertical and two-digit grasps. It was found that a close-to-constant GSM accounts for many of the finger force patterns that are observed when grasping an object oriented arbitrarily with respect to the gravity field. It was hypothesized that, when determining finger forces, the CNS assumes that a grasped object is more slippery than it actually is. An ''operative friction coefficient'' of approximately 30% of the actual coefficient accounted for the offset between experimental and optimized data. The data suggest that the CNS utilizes an optimization strategy when coordinating finger forces during grasping.

Prehension synergies during nonvertical grasping, I: experimental observations

The mechanical complexities of rotating an object through the gravity field present a formidable challenge to the human central nervous system (CNS). The current study documents the finger force patterns selected by the CNS when performing one-, two-, and four-finger grasping while holding an object statically at various orientations with respect to vertical. Numerous mechanically "unnecessary" behaviors were observed. These included: nonzero tangential forces for horizontal handle orientations, large internal forces (i.e., those in excess of equilibrium requirements) for all orientations, and safety margins between 50 and 90%. Additionally, none of the investigated measures were constant across orientations or could be represented as a simple trigonometric function of orientation. Nonetheless, all measures varied in systematic (and sometimes symmetric) ways with orientation. The results suggest that the CNS selects force patterns that are based on mechanical principles but also that are not simply related to object orientation. This study is complemented by a second paper that provides an in-depth analysis of the mechanics of nonvertical grasping and accounts for many of the observed results with numerical optimization (see Part II - current issue). Together, the papers demonstrate that the CNS is likely to utilize optimization processes when controlling prehensile actions.

Tangential load sharing among fingers during prehension

The goal of the study was to examine the force sharing among the fingers during static prehension under systematic loading and postural changes. A custom-built handle was constructed that allowed for bi-directional loading (upward and downward) with different load magnitudes (250, 750 and 1250 g). Five- and three-digit grasps were tested. The fingers were spaced 2, 3 or 6 cm apart. The handle was oriented vertically such that the tangential forces acted parallel to the applied load. There were no differences in tangential sharing patterns between males and females. The factors that did affect the sharing pattern (ranked from the smallest to the largest effect) were: TRIAL (i.e., inter-trial variability), LOAD MAGNITUDE, SUBJECT, HAND POSTURE and LOAD DIRECTION. Normal force sharing accounted for much but not all of the variability in tangential sharing. The results suggest that loading direction should be considered when designing tools that require functional tangential forces for successful task completion (e.g., screwdrivers, jar lids, etc.). A hypothesis to account for the directional changes based on the passive properties of the fingers is proposed.

Measurement of prehensile grasp capabilities by a force and moment wrench: methodological development and assessment of manual workers

Prehensile grasp capability is typically quantified by pinch and grasp forces. This work was undertaken to develop a methodology to assess complex, multi-axis hand exertions through the measurement of forces and moments exerted by the hand along and about three orthogonal axes originating at the grip centre; termed an external wrench. Instrumentation consisting of a modified pinch/grip dynamometer affixed to a 6 df force cube was developed to simultaneously measure three forces, three moments and the pinch/grip force about the centre of the grip. Twenty right hand dominant manual workers (10 male and 10 female), free of hand or wrist disorders, completed a variety of maximal strength tasks. The randomized block design involved three separate grips--power grip, lateral pinch and pulp pinch. Randomized within each block were three non-concurrent repetitions of isolated maximal force and moment generations along and about the three principle orthogonal axes and a maximal grip force exertion. Trials were completed while standing, with the arm abducted and elbow flexed to 90 degrees with a wrist posture near neutral. Where comparable protocols existed in the literature, forces and moments exerted were found to be of similar magnitude to those reported previously. Female and male grip strengths on a Jamar dynamometer were 302.6 N and 450.5 N, respectively. Moment exertions in a power grip (female and male) were 4.7 Nm and 8.1 Nm for pronator, 4.9 Nm and 8.0 Nm for supinator, 6.2 Nm and 10.3 Nm for radial deviator, 7.7 Nm and 13.0 Nm for ulnar deviator, 6.2 Nm and 8.2 Nm for extensor, and 7.1 Nm and 9.3 Nm for flexor moments. Correlations with and between maximal force and moment exertions were only moderate. This paper describes instrumentation that allows comprehensive characterization of prehensile force and moment capability.

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.