Control of grip force during restraint of an object held between finger and thumb: responses of cutaneous afferents from the digits

Unexpected pulling and pushing loads exerted by an object held with a precision grip evoke automatic and graded increases in the grip force (normal to the grip surfaces) that prevent escape of the object; unloading elicits a decrease in grip force. Anesthesia of the digital nerves has shown that these grip reactions depend on sensory signals from the digits. In the present study we assessed the capacity of tactile afferents from the digits to trigger and scale the evoked grip responses. Using tungsten microelectrodes inserted percutaneously into the median nerve of awake human subjects, unitary recordings were made from ten FA I and 13 FA II rapidly adapting afferents, and 12 SA I and 18 SA II slowly adapting afferents. While the subject held a manipulandum between a finger and the thumb, tangential load forces were applied to the receptor-bearing digit (index, middle, or ring finger or thumb) as trapezoidal load-force profiles with a plateau amplitude of 0.5 – 2.0 N and rates of loading and unloading at 2 – 8 N/s, or as step-loads of 0.5 N delivered at 32 N/s. Such load trials were delivered in both the distal (pulling) and proximal (pushing) direction. FA I afferents responded consistently to the load forces, being recruited during the loading and unloading phases. During the loading ramp the ensemble discharge of the FA I afferents reflected the first time-derivative of the load force (i.e., the load-force rate). These afferents were relatively insensitive to the subject's grip force responses. However, high static finger forces appeared to suppress excitation of these afferents during the unloading phase. The FA II afferents were largely insensitive to the load trials: only with the step-loads did some afferents respond. Both classes of SA afferents were sensitive to load force and grip force, and discharge rates were graded by the rate of loading. The firing of the SA I afferents appeared to be relatively more influenced by the subject's grip-force response than the discharge of the SA II afferents, which were more influenced by the load-force stimulus. The direction in which the tangential load force was applied to the skin influenced the firing of most afferents and in particular the SA II afferents. Individual afferents within each class (except for the FA IIs) responded to the loading ramp before the onset of the subject's grip response and may thus be responsible for initiating the automatic increase in grip force. However, nearly half of the FA I afferents recruited by the load trials responded to the loading phase early enough to trigger the subject's gripforce response, whereas only ca. one-fifth of the SA Is and SA IIs did so. These observations, together with the high density of FA I receptors in the digits, might place the FA I afferents in a unique position to convey the information required to initiate and scale the reactive gripforce responses to the imposed load forces.     

Somatosensory control of precision grip during unpredictable pulling loads

During manipulation involving restraint of 'active' (mechanically unpredictable) objects, it is presumed that the control of the grip and other reaction forces more regularly relies on somatosensory input than during manipulation of 'passive' (mechanically predictable) objects. In companion studies we have shown that grip forces are automatically adjusted to the amplitude and the rate of distal pulling loads imposed through an 'active' object held in a precision grip. In this study anesthesia of either one or both digits holding the manipulandum was used to examine whether the grip force regulation was dependent on afferent signals from the digits. Five types of trapezoidal load force profiles of various rate and amplitude combinations were given in an unpredictable sequence while the subject was prevented from seeing the hand. Grip forces, load forces and position of the manipulandum in the pulling direction were recorded. With both digits anesthetized the load amplitude changes yielded considerably less grip force modulation and in many trials obvious grip force responses were absent. Moreover, the latencies between the onset of the load changes and the observed grip force responses were much prolonged. However, there was pronounced inter-individual variation. Subjects exhibiting a lower stiffness in the pulling direction, probably due to more flexed fingers when holding the manipulandum, showed a higher force modulation, higher responsiveness to the load ramps and shorter latencies. Hence, under certain conditions afferent input from receptors proximal to the digits could be utilized to provide some grip regulation. The evoked grip force responses showed an initial response similar to the normally occurring 'catch-up' response, but it was not graded by the load force rate. Also, there was no 'tracking' response, suggesting that the latter was contingent upon a moment-to-moment control using afferent input from the digits. With only one digit anesthetized (thumb) the handicap was less severe. Thus, the grip force regulation was impaired under any condition of digital anesthesia, i.e., afferent input from both index finger and thumb was required for the adequate operation of the grip force regulation.     

Thumb and finger forces produced by motor units in the long flexor of the human thumb

The uncommonly good proprioceptive performance of the long flexor of the thumb, flexor pollicis longus (FPL), may add significantly to human manual dexterity. We investigated the forces produced by FPL single motor units during a weak static grip involving all digits by spike-triggered averaging from single motor units, and by averaging from twitches produced by intramuscular stimulation. Nine adult subjects were studied. The forces produced at each digit were used to assess how forces produced in FPL are distributed to the fingers. Most FPL motor units produced very low forces on the thumb and were positively correlated with the muscle force at recruitment. Activity in FPL motor units commonly loaded the index finger (42/55 units), but less commonly the other fingers ( P < 0.001). On average, these motor units produced small but significant loading forces on the index finger (∼5.3% of their force on the thumb) with the same time-to-peak force as the thumb (∼50 ms), but had no significant effect on other fingers. However, intramuscular stimulation within FPL did not produce significant forces in any finger. Coherence at 2–10 Hz between the thumb and index finger force was twice that for the other finger forces and the coherence to the non-index fingers was not altered when the index finger did not participate in the grasp. These results indicate that, within the long-term coordinated forces of all digits during grasping, FPL motor units generate forces highly focused on the thumb with minimal peripheral transfer to the fingers and that there is a small but inflexible neural coupling to the flexors of the index finger.     

Anticipatory postural adjustments in stance and grip

 The reactive forces and torques associated with moving a hand-held object between two points are potentially destabilising, both for the object’s position in the hand and for body posture. Previous work has demonstrated that there are increases in grip force ahead of arm motion that contribute to object stability in the hand. Other studies have shown that early postural adjustments in the legs and trunk minimise the potential perturbing effects on body posture of rapid voluntary arm movement. This paper documents the concurrent evolution of grip force and postural adjustments in anticipation of dynamic and static loads. Subjects held a manipulandum in precision grasp between thumb and index finger and pulled or pushed either a dynamic or a fixed load horizontally towards or away from the body (the grasp axis was orthogonal to the line of the load force). A force plate measured ground reaction torques, and force transducers in the manipulandum measured the load (tangential) and grip (normal) forces acting on the thumb and finger. In all conditions, increases in grip force and ground reaction torque preceded any detectable rise in load force. Rates of change of grip force and ground reaction torque were correlated, even after partialling out a common dependence on load force rate. Moreover, grip force and ground reaction torque rates at the onset of load force were correlated. These results imply the operation of motor planning processes that include anticipation of the dynamic consequences of voluntary action. Received: 28 June 1996 / Accepted: 4 February 1997     

Force synergies for multifingered grasping

Compared with the control of precision grips involving the thumb and one or two fingers, the control of grasping using the entire hand involves a larger number of degrees of freedom that has to be controlled simultaneously, and it introduces indeterminacies in the distribution of grip forces suitable for holding an object. We studied the control of five-digit grasping by measuring contact forces when subjects lifted, held, and replaced a manipulandum. This study focused primarily on the patterns of coordination among the normal forces exerted by each of the digits, assessed by varying the center of mass of the manipulandum. The force patterns during the lift and hold phases were modulated as a function of the location of the center of mass. A frequency domain analysis revealed a consistent temporal synergy by which digits tended to exert normal forces in phase with each other across all experimental conditions. This tendency for in-phase covariations by the normal forces exerted by the digits extended over the entire functional frequency range (up to 10 Hz). When the effect of thumb force was removed, a second synergy was revealed in which forces in two fingers could be modulated 180 degrees out of phase (also prevailing throughout the range of frequencies studied). The first synergy suggests the presence of a "common drive" to all of the extrinsic finger muscles, whereas the second one suggests another input, ultimately resulting in a reciprocally organized pattern of activity of some of these muscles.     

Muscle-pair specific distribution and grip-type modulation of neural common input to extrinsic digit flexors

To gain insight into the synergistic control of hand muscles, we have recently quantified the strength of correlated neural activity across motor units from extrinsic digit flexors during a five-digit object-hold task. We found stronger synchrony and coherence across motor units from thumb and index finger flexor muscle compartment than between the thumb flexor and other finger flexor muscle compartments. The present study of two-digit object hold was designed to determine the extent to which such distribution of common input among thumb-finger flexor muscle compartments, revealed by holding an object with five digits, is preserved when varying the functional role of a given digit pair. We recorded normal force exerted by the digits and electrical activity of single motor units from muscle flexor pollicis longus (FPL) and two compartments of the m. flexor digitorum profundus (FDP2 and FDP3; index and middle finger, respectively). Consistent with our previous results from five-digit grasping, synchrony and coherence across motor units from FPL-FDP2 was significantly stronger than in FPL-FDP3 during object hold with two digits [common input strength: 0.49 +/- 0.02 and 0.35 +/- 0.02 (means +/- SE), respectively; peak coherence: 0.0054 and 0.0038, respectively]. This suggests that the distribution of common neural input is muscle-pair specific regardless of grip type. However, the strength of coherence, but not synchrony, was significantly stronger in two- versus five-digit object hold for both muscle combinations, suggesting the periodicity of common input is sensitive to grip type.     

Nondigital afferent input in reactive control of fingertip forces during precision grip

Sensory inputs from the digits are important in initiating and scaling automatic reactive grip responses that help prevent frictional slips when grasped objects are subjected to destabilizing load forces. In the present study we analyzed the contribution to grip-force control from mechanoreceptors located proximal to the digits when subjects held a small manipulandum between the tips of the thumb and index finger. Loads of various controlled amplitudes and rates were delivered tangential to the grip surfaces at unpredictable times. Grip forces (normal to the grip surfaces) and the position of the manipulandum were recorded. In addition, movements of hand and arm segments were assessed by recording the position of markers placed at critical points. Subjects performed test series during normal digital sensibility and during local anesthesia of the index finger and thumb. To grade the size of movements of tissues proximal to the digits caused by the loadings, three different conditions of arm and hand support were used; (1) in the hand-support condition the subjects used the three ulnar fingers to grasp a vertical dowel support and the forearm was supported in a vacuum cast; (2) in the forearm-support condition only the forearm was supported; finally, (3) in the no-support condition the arm was free. With normal digital sensibility the size of the movements proximal to the digits had small effects on the grip-force control. In contrast, the grip control was markedly influenced by the extent of such movements during digital anesthesia. The poorest control was observed in the hand-support condition, allowing essentially only digital movements. The grip responses were either absent or attenuated, with greatly prolonged onset latencies. In the forearm and no-support conditions, when marked wrist movements took place, both the frequency and the strength of grip-force responses were higher, and the grip response latencies were shorter. However, the performance never approached normal. It is concluded that sensory inputs from the digits are dominant in reactive grip control. However, nondigital sensory input may be used for some grip control during impaired digital sensibility. Furthermore, the quality of the control during impaired sensibility depends on the extent of movements evoked by the load in the distal, unanesthetized parts of the arm. The origin of these useful sensory signals is discussed.     

Loads applied tangential to a fingertip during an object restraint task can trigger short-latency as well as long-latency EMG responses in hand muscles

Electrical stimulation of the digital nerves can cause short- and long-latency increases in electromyographic activity (EMG) of the hand muscles, but mechanical stimulation of primarily tactile afferents in the digits generally evokes only a long-latency increase in EMG. To examine whether such stimuli can elicit short-latency reflex responses, we recorded EMG over the first dorsal interosseous muscle when subjects (n=13) used the tip of the right index finger to restrain a horizontally oriented plate from moving when very brisk tangential forces were applied in the distal direction. The plate was subjected to ramp-and-hold pulling loads at two intensities (a 1-N load applied at 32 N/s or a 2-N load applied at 64 N/s) at times unpredictable to the subjects (mean interval 2 s; trial duration 500 ms). The contact surface of the manipulandum was covered with rayon—a slippery material. For each load, EMG was averaged for 128 consecutive trials with reference to the ramp onset. In all subjects, an automatic increase in grip force was triggered by the loads applied at 32 N/s; the mean onset latency of the EMG response was 59.8±0.9 (mean ± SE) ms. In seven subjects (54%) this long-latency response was preceded by a weak short-latency excitation at 34.6±2.9 ms. With the loads applied at 64 N/s, the long-latency response occurred slightly earlier (58.9±1.7 ms) and, with one exception, all subjects generated a short-latency EMG response (34.9±1.3 ms). Despite the higher background grip force that subjects adopted during the stronger loads (4.9±0.3 N vs 2.5±0.2 N), the incidence of slips was higher—the manipulandum escaped from the grasp in 37±5% of trials with the 64 N/s ramps, but in only 18±4% with the 32-N/s ramps. The deformation of the fingertip caused by the tangential load, rather than incipient or overt slips, triggered the short-latency responses because such responses occurred even when the finger pad was fixed to the manipulandum with double-sided adhesive tape so that no slips occurred.     

Coordinated isometric muscle commands adequately and erroneously programmed for the weight during lifting task with precision grip

Summary Small objects were lifted from a table, held in the air, and replaced using the precision grip between the index finger and thumb. The adaptation of motor commands to variations in the object's weight and sensori-motor mechanisms responsible for optimum performance of the transition between the various phases of the task were examined. The lifting movement involved mainly a flexion of the elbow joint. The grip force, the load force (vertical lifting force) and the vertical position were measured. Electromyographic activity (e.m.g.) was recorded from four antagonist pairs of hand/arm muscles primarily influencing the grip force or the load force. In the lifting series with constant weight, the force development was adequately programmed for the current weight during the loading phase (i.e. the phase of parallel increase in the load and grip forces during isometric conditions before the lift-off). The grip and load force rate trajectories were mainly single-peaked, bell-shaped and roughly proportional to the final force. In the lifting series with unexpected weight changes between lifts, it was established that these force rate profiles were programmed on the basis of the previous weight. Consequently, with lifts programmed for a lighter weight the object did not move at the end of the continuous force increase. Then the forces increased in a discontinous fashion until the force of gravity was overcome. With lifts programmed for a heavier weight, the high load and grip force rates at the moment the load force overcame the force of gravity caused a pronounced positional overshoot and a high grip force peak, respectively. In these conditions the erroneous programmed commands were automatically terminated by somatosensory signals elicited by the start of the movement. A similar triggering by somatosensory information applied to the release of programmed motor commands accounting for the unloading phase (i.e. the parallel decrease in the grip and load forces after the object contacted the table following its replacement). These commands were always adequately programmed for the weight.     

Altered digit force direction during pinch grip following stroke

This study examined grip force development in individuals with hemiparesis following unilateral stroke. Eleven patients with chronic stroke with severe hand impairment and five age-matched neurologically intact subjects grasped an instrumented object between the index finger and thumb while fingertip forces, digit posture, and muscle electromyographic activity were recorded. We tested a range of different grip conditions with varying grip sizes, object stability, and grip force level. We found that fingertip force direction in the paretic digits deviated from the direction normal to the grip surface by more than twice as much as for asymptomatic digits. Additionally, the paretic thumb had, on average, 18% greater deviation of grip force direction than the paretic index finger. This large deviation of finger force direction for the paretic digits was consistently observed regardless of grip size, grip force level, and object stability. Due to the large deviation of the force direction from the normal direction, the paretic digits slipped and moved more than 1 cm during 55% of all grasping trials. A regression analysis suggests that this altered grip force direction was associated with altered hand muscle activation patterns, but not with the posture at which the digit made contact with the object. Therapies to redirect the force direction at the digits may improve stroke survivors’ ability to stably grip an object.     

Movement detection at the distal joint of the human thumb and fingers

To determine whether proprioceptive acuity is the same at all digits, particularly when postured as in a ’grasp’, we imposed 10° movements at the distal joint of the thumb, index and ring finger, at three velocities; 1.25°/s, 2.5°/s and 5°/s. The test joint was initially flexed by 25° and the joints proximal to the test joint were maintained in a standard posture for each study. When in a grasp posture that disengaged the extensor muscles at the distal joint of the finger, movement detection at the thumb was superior to that at the fingers for all velocities. However, when the fingers were positioned so that all proprioceptive inputs were able to contribute (i.e. cutaneous, joint and both flexor and extensor muscle afferents), proprioceptive acuity was similar for the three digits. Loss of local cutaneous (and joint) inputs by digital anaesthesia significantly impaired performance at all digits, suggesting a critical role for cutaneous input in normal proprioceptive sensibility at all distal joints of the digits. Anaesthesia of the extensor muscle afferents innervating the thumb did not affect its proprioceptive acuity. Thus, for the thumb, the extensor muscle afferents do not provide critical information. The greater change in muscle fascicle length for the thumb’s long flexor muscle (3% per 10°) compared with that in the finger flexor muscles (e.g. 0.1% per 10°) could contribute to the thumb’s performance. There appears to be less redundancy of muscle and non-muscle signals for the fingers than for the thumb, because a reduction in either cutaneous or muscle input significantly impaired acuity at the fingers. Overall, when the hand is in a grasping posture, irrespective of the contribution of local cutaneous inputs, the long flexor acting on the thumb may contribute more to its proprioceptive acuity than the long finger flexors contribute to acuity at the fingers. Received: 20 January 1998 / Accepted:25 March 1998     

The effects of muscimol inactivation of small regions of motor and somatosensory cortex on independent finger movements and force control in the precision grip

This study investigated the effects of inactivating small regions of the primary somatosensory (SI) and motor (MI) cortex on the control of finger forces in a precision grip. A monkey was trained to grasp and lift a computer-controlled object between the thumb and index finger and to hold it stationary within a narrow position window for 2 s. The grip force applied perpendicular to the object surface, the lifting or load force applied tangentially in the vertical direction, and the vertical displacement were sampled at 100 Hz. Also, the ability of the monkey to extract small pieces of food from narrow wells of a Klüver board was analyzed from video-tape. Preliminary single-unit recordings and microstimulation studies were used to map the extent of the thumb and index-finger representation within SI and MI. Two local injections of 1 μl each (5 μg/μl) of the GABA_A-agonist muscimol were used to inactivate the thumb and index region of either the pre- or post-central gyrus. The precision grip was differently affected by muscimol injection into either SI or MI. MI injections produced a deficit in the monkey’s ability to perform independent finger movements and a general weakness in the finger muscles. Whole-hand grasping movements were inappropriately performed in an attempt to grasp either the instrumented object or morsels of food. Although the effect seemed strongest on intrinsic hand muscles, a clear deficit in digit extension was also noted. As a result, the monkey was unable to lift and maintain the object within the position window for the required 2 s, and, over time, the grip force decreased progressively until the animal stopped working. Following SI injections, the most obvious effect was a loss of finger coordination. In grasping, the placement of the fingers on the object was often abnormal and the monkey seemed unable to control the application of prehensile and lifting forces. However, the detailed analysis of forces revealed that a substantial increase in the grip force occurred well before any deficit in the coordination of finger movements was noted. This observation suggests that cutaneous feedback to SI is essential for the fine control of grip forces. Received: 05 October 1998 / Accepted: 30 March 1999     

Evidence for the involvement of the posterior parietal cortex in coordination of fingertip forces for grasp stability in manipulation

Grasp stability during object manipulation is achieved by the grip forces applied normal to the grasped surfaces increasing and decreasing in phase with increases and decreases of destabilizing load forces applied tangential to the grasped surfaces. This force coordination requires that the CNS anticipates the grip forces that match the requirements imposed by the self-generated load forces. Here, we use functional MRI (fMRI) to study neural correlates of the grip-load force coordination in a grip-load force task in which six healthy humans attempted to lift an immovable test object held between the tips of the right index finger and thumb. The recorded brain activity was compared with the brain activity obtained in two control tasks in which the same pair of digits generated forces with similar time courses and magnitudes; i.e., a grip force task where the subjects only pinched the object and did not apply load forces, and a load force task, in which the subjects applied vertical forces to the object without generating grip forces. Thus neither the load force task nor the grip force task involved coordinated grip-load forces, but together they involved the same grip force and load force output. We found that the grip-load force task was specifically associated with activation of a section of the right intraparietal cortex, which is the first evidence for involvement of the posterior parietal cortex in the sensorimotor control of coordinated grip and load forces in manipulation. We suggest that this area might represents a node in the network of cortical and subcortical regions that implement anticipatory control of fingertip forces for grasp stability.     

Corticospinal Influences on the Distal Muscles of the Hand in Conditions of Inertial Loading

Electromyographic activity and synchronous discharges in the muscles of the wrist induced by transcranial magnetic stimulation of the motor cortex as the thumb and index finger were used to hold a handle bearing a weight were studied during performance of a number of motor tasks. When the subject increased grip force, for example, in response to increases in the weight of the attached load or by voluntarily squeezing the handle, the evoked response increased proportionally to muscle activity. If the subject moved the hand holding the handle up and down with an amplitude of 10 cm and a frequency of 0.5–1 Hz, grip force changed in accordance with the predicted inertial loading. The muscle response in the adductor pollicis muscle increased to a greater extent than the activity in the muscle. The response to sudden inertial loading consisted of a reflex increase in grip force, the muscle response increasing to a lesser extent than activity in the muscle. This suggests that larger increases in evoked muscle responses on up and down movement of the hand with a load are associated with anticipatory changes in grip force. These results are assessed from the point of view of the involvement of the motor cortex in generating anticipatory changes in muscle activity in the distal muscles.     

Grip-force responses to unanticipated object loading: load direction reveals body- and gravity-referenced intrinsic task variables

Humans preserve grasp stability by automatically regulating the grip forces when loads are applied tangentially to the grip surfaces of a manipulandum held in a precision grip. The effects of the direction of the load force in relation to the palm, trunk, and gravity were investigated in blindfolded subjects. Controlled, tangential load-forces were delivered in an unpredictable manner to the grip surface in contact with the index finger either in the distal and proximal directions (away from and toward the palm) or in the ulnar and radial directions (transverse to the palm). The hand was oriented in: (1) a standard position, with the forearm extended horizontally and anteriorly in intermediate pronosupination; (2) an inverted position, reversing the direction of radial and ulnar loads in relation to gravity; and (3) a horizontally rotated position, in which distal loads were directed toward the trunk. The amplitude of the grip-force responses (perpendicular to the grip surface) varied with the direction of load in a manner reflecting frictional anisotropies at the digit-object interface; that is, the subjects automatically scaled the grip responses to provide similar safety margins against frictional slips. For all hand positions, the time from onset of load increase to start of the gripforce increase was shorter for distal loads, which tended to pull the object out of the hand, than for proximal loads. Furthermore, this latency was shorter for loads in the direction of gravity, regardless of hand position. Thus, shorter latencies were observed when frictional forces alone opposed the load, while longer latencies occurred when gravity also opposed the load or when the more proximal parts of the digits and palm were positioned in the path of the load. These latency effects were due to different processing delays in the central nervous system and may reflect the preparation of a default response in certain critical directions. The response to loads in other directions would incur delays required to implement a new frictional scaling and a different muscle activation pattern to counteract the load forces. We conclude that load direction, referenced to gravity and to the hand's geometry, represents intrinsic task variables in the automatic processes that maintain a stable grasp on objects subjected to unpredictable load forces. In contrast, the grip-force safety margin against frictional slips did not vary systematically with respect to these task variables. Instead, the magnitude of the grip-force responses varied across load direction and hand orientation according to frictional differences providing similar safety margins supporting grasp stability.     

Somatosensory control of precision grip during unpredictable pulling loads

In manipulating 'passive' objects, for which the physical properties are stable and therefore predictable, information essential for the adaptation of the motor output to the properties of the current object is principally based on 'anticipatory parameter control' using sensorimotor memories, i.e., an internal representation of the object's properties based on previous manipulative experiences. Somatosensory afferent signals only intervene intermittently according to an 'event driven' control policy. The present study is the first in a series concerning the control of precision grip when manipulating 'active' objects that exert unpredictable forces which cannot be adequately represented in a sensorimotor memory. Consequently, the manipulation may be more reliant on a moment-to-moment sensory control. Subjects who were prevented from seeing the hand used the precision grip to restrain a manipulandum with two parallel grip surfaces attached to a force motor which produced distally directed (pulling) loads tangential to the finger tips. The trapezoidal load profiles consisted of a loading phase (4 N/s), plateau phase and an unloading phase (4 N/s) returning the load force to zero. Three force amplitudes were delivered in an unpredictable sequence; 1 N, 2 N and 4 N. In addition, trials with higher load rate (32 N/s) at a low amplitude (0.7 N), were superimposed on various background loads. The movement of the manipulandum, the load forces and grip forces (normal to the grip surfaces) were recorded at each finger. The grip force automatically changed with the load force during the loading and unloading phases. However, the grip responses were initiated after a brief delay. The response to the loading phase was characterized by an initial fast force increase termed the 'catch-up' response, which apparently compensated for the response delay--the grip force adequately matched the current load demands by the end of the catch-up response. In ramps with longer lasting loading phases (amplitude greater than or equal to 2 N) the catch-up response was followed by a 'tracking' response, during which the grip force increased in parallel with load force and maintained an approximately constant force ratio that prevented frictional slips. The grip force during the hold phase was linearly related to the load force, with an intercept close to the grip force used prior to the loading. Likewise, the grip force responses evoked by the fast loadings superimposed on existing loads followed the same linear relationship.(ABSTRACT TRUNCATED AT 400 WORDS)     

Static dynamometer for the measurement of multidirectional forces exerted by the thumb

The functioning of a static dynamometer designed to measure simultaneous forces exerted by the thumb in the vertical and horizontal axes is described. The analysis of the output signals by a desktop computer program provides information regarding the forces generated in eight directions covering a plane transverse to the thumb by 45° increments. In 12 normal female subjects, the maximum voluntary torques exerted at the trapezo-metacarpal joint of the thumb were examined and the muscle activation patterns of the interosseus, flexor pollicis brevis and adductor pollicis brevis muscles were recorded in one subject. Torques and muscle activation patterns were depicted using polar plots. Dynamometric data indicate that strength varies with direction and that higher torques are obtained in directions that bring the thumb towards the palm, i.e. flexion, adduction, combined flexion-adduction and extension-adduction. Patterns of muscle activity vary according to the direction evaluated suggesting that strength depends on the number of activated muscles as well as the relative force contribution of each muscle.     

Control of grasp stability during pronation and supination movements

 We analyzed the control of grasp stability during a major manipulative function of the human hand: rotation of a grasped object by pronation and supination movement. We investigated the regulation of grip forces used to stabilize an object held by a precision grip between the thumb and index finger when subjects rotated it around a horizontal axis. Because the center of mass was located distal to the grip axis joining the fingertips, destabilizing torque tangential to the grasp surfaces developed when the grip axis rotated relative to the field of gravity. The torque load was maximal when the grip axis was horizontal and minimal when it was vertical. An instrumented test object, with a mass distribution that resulted in substantial changes in torque load during the rotation task, measured forces and torques applied by the digits. The mass distribution of the object was unpredictably changed between trials. The grip force required to stabilize the object increased directly with increasing torque load. Importantly, the grip force used by the subjects also changed in proportion to the torque load such that subjects always employed adequate safety margins against rotational slips, i.e., some 20–40% of the grip force. Rather than driven by sensory feedback pertaining to the torque load, the changes in grip force were generated as an integral part of the motor commands that accounted for the rotation movement. Subjects changed the grip force in parallel with, or even slightly ahead of, the rotation movement, whereas grip force responses elicited by externally imposed torque load changes were markedly delayed. Moreover, blocking sensory information from the digits did not appreciably change the coordination between movement and grip force. We thus conclude that the grip force was controlled by feedforward rather than by feedback mechanisms. These feedforward mechanisms would thus predict the consequences of the rotation movement in terms of changes in fingertip loads when the orientation of the grip axis changed in the field of gravity. Changes in the object’s center of mass between trials resulted in a parametric scaling of the motor commands prior to their execution. This finding suggests that the sensorimotor memories used in manipulation to adapt the motor output for the physical properties of environmental objects also encompass information related to an object’s center of mass. This information was obtained by somatosensory cues when subjects initially grasped the object with the grip axis vertical, i.e., during minimum tangential torque load. Received: 16 October 1998 / Accepted: 1 February 1999     

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.     

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.