Control of grasp stability when humans lift objects with different surface curvatures

In previous investigations of the control of grasp stability, humans manipulated test objects with flat grasp surfaces. The surfaces of most objects that we handle in everyday activities, however, are curved. In the present study, we examined the influence of surface curvature on the fingertip forces used when humans lifted and held objects of various weights. Subjects grasped the test object between the thumb and the index finger. The matching pair of grasped surfaces were spherically curved with one of six different curvatures (concave with radius 20 or 40 mm; flat; convex with radius 20, 10, or 5 mm) and the object had one of five different weights ranging from 168 to 705 g. The grip force used by subjects (force along the axis between the 2 grasped surfaces) increased with increasing weight of the object but was modified inconsistently and incompletely by surface curvature. Similarly, the duration and rate of force generation, when the grip and load forces increased isometrically in the load phase before object lift-off, were not influenced by surface curvature. In contrast, surface curvature did affect the minimum grip forces required to prevent frictional slips (the slip force). The slip force was smaller for larger curvatures (both concave and convex) than for flatter surfaces. Therefore the force safety margin against slips (difference between the employed grip force and the slip force) was higher for the higher curvatures. We conclude that surface curvature has little influence on grip force regulation during this type of manipulation; the moderate changes in slip force resulting from changes in curvature are not fully compensated for by changes in grip force.     

Comparison of plasma lidocaine concentrations after injection of a fixed small volume in the stellate ganglion, the lumbar epidural space, or a single intercostal nerve

Carpal tunnel syndrome may be caused by repeated or sustained elevated carpal tunnel pressure. This study examined the relationship between carpal tunnel pressure, posture, and fingertip load. In 20 healthy individuals, carpal tunnel pressure was measured with a catheter inserted into the carpal tunnel of the dominant hand and connected to a pressure transducer. With the wrist in a pressure-neutral position, the subjects pressed on a force transducer with the index finger to levels of 0, 5, 10, and 15 N. They then pinched the transducer at the same levels of force. For both fingertip-loading postures, the carpal tunnel pressure increased with increasing fingertip load. Carpal tunnel pressures were significantly greater (p < 0.015) for the pinching task (14.2, 29.9, 41.9, and 49.7 mm Hg [1.89, 3.99, 5.59, and 6.63 kPa] for 0, 5, 10, and 15 N force levels, respectively) than for simple finger pressing (7.8, 14.1, 20.0, and 33.8 mm Hg [1.04, 1.88, 2.67, and 4.51 kPa]). This study indicates that although the external load on the finger remained constant between the two tasks, the internal loading, as measured by carpal tunnel pressure, experienced a near 2-fold increase by using a pinch grip. These findings should be given consideration in designing work tasks and tools because relatively low fingertip forces, especially in a pinch grip, elevate carpal tunnel pressures to levels that, if prolonged, may lead to the development or exacerbation of carpal tunnel syndrome.     

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.     

Antibodies to beta2 glycoprotein I are associated with in vitro fertilization implantation failure as well as recurrent miscarriage: results of a prevalence study

The position of the hand during power grip is well-described, but the normal phasic activity of the extrinsic forearm muscles during power grip and release is unknown. People with neurologic impairment may have inadequate power grip or release because of abnormal muscle timing. This study describes the timing of the forearm muscles in 10 normal subjects during power grip and release, which was evaluated using electromyography. During power grip, subjects had consistent timing patterns for extrinsic finger motors and different but individually consistent patterns for wrist motors. This finding supports our hypothesis that different individuals habitually use a specific motor strategy and an intact central nervous system allows them to change their motor strategy to adapt to new environmental parameters.     

Dominant pattern extraction from 3-D kinematic data

The experiment examined the anticipatory modulation of grip force with respect to load force during a drawer opening task. An impact force was introduced by a mechanical stop that arrested movement of the pulling hand. The results showed a typical grip force profile which consisted of two evolving phases, one to control drawer movement onset, and the other to secure grip force at the expected impact. Initially, grip force increased with the load force that was developed to overcome the inertia of the drawer. After the first peak, a small decline was observed, followed by a proactive grip force increase prior to the time of impact. During this ramp-like increase of grip force, load force remained unchanged. In addition, a reactive response was triggered by the impact. That anticipatory control with respect to an impact force is not innate but, rather, is learned by experience was evidenced by a comparison of adults and children. Whereas adults made the characteristic grip force adjustments to anticipate the impact, children used a probing strategy with irregular build-up of force until impact. Furthermore, adults calibrated the second phase of the grip force profile in the initial trials of the task, indicating that grip force was rapidly updated with information related to the impact force. The present results demonstrate that grip-load force coordination during manipulation is a necessity for dealing with destabilizing load perturbations produced by self-induced movement and impact forces. It is concluded that grip force is adjusted automatically, but in a flexible manner, to secure grip in accordance with the characteristics of the pulling synergy.     

New contributions to clinical hypertension from molecular biology

The authors constructed a new dynamic guiding splint assisting the active mobilisation after flexor tendon repair distal to the wrist. In these cases, the "inverse" wrist position seems to be the best position for mobilisation. This means that finger flexion should be carried out during wrist extension, and finger extension during wrist flexion. The splint guides and co-ordinates the movements of the wrist and the fingers, and it limits the free usage of the hand.     

Control of grip force when tilting objects: effect of curvature of grasped surfaces and applied tangential torque

When we manipulate objects in everyday tasks, there are variations in the shape of the grasped surfaces, and the loads that potentially destabilize the grasp include time-varying linear forces and torques tangential to the grasped surfaces. Previous studies of the control of fingertip forces for grasp stability have dealt principally with flat grip surfaces and linear force loads. Here, we studied the regulation of grip force with changes in curvature of grasped surfaces and changes in tangential torque applied by the index finger and thumb when humans lifted an object and rotated it about the horizontal grip axis through an angle of 65 degrees. The curvatures of the matched pair of spherical surfaces varied from -50 m-1 (concave with radius 20 mm) to 200 m-1 (convex with radius 5 mm). The applied tangential torque at the orientation of 65 degrees was varied sixfold. Regardless of the values of curvature and end torque, grip force and tangential torque were coordinated, increasing in parallel throughout the tilt with an approximately linear relationship; the slope of the line increased progressively with increasing surface curvature. This parametric scaling of grip force was directly related to the minimum grip force required to prevent rotational slip, resulting in an adequate safety margin against slip in all cases. We conclude that surface curvature parametrically influences grip force regulation when the digits are exposed to torsional loads. Furthermore, the sensorimotor programs that control the grip force apparently predict the effect of the total load comprising linear forces and tangential torques.     

A dynamic model for simulating movements of the elbow, forearm, an wrist

We developed a dynamic model of the upper extremity to simulate forearm and wrist movements. The model is based on the skeletal structure of the arm and is capable of elbow flexion/extension, forearm pronosupination, and wrist flexion/extension and radial/ulnar deviation movements. Movements are produced by activation of a Hill-type model of muscle, and limits on joint motion are imposed by passive moments modeled after experimental results. We investigated the muscle output force sensitivity, as well as wrist flexion/extension motion sensitivity to parameter variations. The tendon slack length and muscle fiber length were found to have the greatest influence on muscle output and flexion/extension wrist motion. The model captured the direction of the moment vectors at the wrist well, but predicted much higher moments than were measured by stimulating the paralyzed muscles of one tetraplegic subject.     

Anticipatory load torques produced by voluntary movements.

The stability of an object held between the finger and thumb depends on friction developed by grip force, normal to the contact surfaces, to overcome tangential load force. Previous research has shown that in lifting an object, grip force rises with the increase in gravitational load force as the hand takes the weight and that in moving an object, grip force is adjusted to meet movement-induced inertial load force. Those results demonstrated the anticipatory nature of coordination of grip force with load force. Whether grip force anticipates load torque was studied in this research. When participants were constrained to use grasp points where the grasp axis was manifestly distant from object center of mass, it was found that they made grip force adjustments in anticipation of load torques that tended to destabilize an object as a result of lifting or moving it. These adjustments imply use of information about object center of mass in movement planning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Importance of the intersegmental trunk muscles for the stability of the lumbar spine. A biomechanical study in vitro

STUDY DESIGN: A biomechanical study was performed to determine the consequences of a simulation of muscle forces on the loads imposed on the functional spinal units. OBJECTIVES: No biomechanical study has investigated the effect of incorporation of agonist and antagonist muscle forces on the loading of functional spinal units. SUMMARY OF BACKGROUND DATA: Spinal disorders and low back pain are increasingly becoming a worldwide problem. Traditional conservative therapies are intended to strengthen the muscles of the trunk using a judicious regimen of physical exercises. METHODS: Eighteen whole, fresh-frozen human cadaveric lumbar spine specimens (L2-S2; average age, 53.4 years) were tested in a spine tester using pure flexion-extension, lateral bending, and axial moments. The effects of coactivation of psoas and multifidus muscles on L4-L5 mobility were simulated in vitro by applying two pairs of corresponding force vectors to L4. The segmental stability was defined by the correlation of an applied moment to the resultant deformation as shown in load-displacement curves, and the range of motion was defined as the angular deformation at maximum load. RESULTS: The coactivation of muscles was accompanied by a 20% decrease in the range of motion (i.e., a significant increase in stability) during lateral bending and axial moments. Application of flexion-extension moments and muscle coactivation resulted in a 13% increase in the sagittal range of motion. CONCLUSIONS: The action of the intersegmental agonist and antagonist muscles biomechanically increases the overall stiffness (stability) of the intervertebral joints in axial torque and lateral bending, whereas it may destabilize the segment in flexion.     

Coupling of grip force and load force during arm movements with grasped objects

Numerous studies have investigated the kinematics of arm movements; others have examined grip forces during static holding of objects. However, the coordination of grip force and arm movement when moving grasped objects has not been documented. We show that grip force is finely modulated in phase with load force during movements with grasped objects in which load force varies with acceleration. A tight coupling between grip and load force is seen in point-to-point and cyclic movements of varying rate and direction. We conclude that in transporting an object, the programming of grip force is part and parcel of the process of planning the arm movement.     

The effect of peritoneal air exposure on postoperative tumor growth

Botulinum toxin A has been used to treat wrist and finger spasticity mainly through injection of the forearm flexor muscles. This case study describes its first reported use in managing spastic lumbricals of the hand. A 19-year-old male had significant flexion deformity and hypertonicity of the left wrist and hand, particularly the second through fifth metacarpophalangeal joints, after traumatic brain injury. By using the 0-4 Ashworth scale, spasticity of the lumbricals across the second to fourth metacarpophalangeal joints was rated 2, with persistent clonus of the finger flexors as confirmed by electromyography to the middle and ring fingers, even after botulinum toxin A injection of the flexor digitorum sublimis and profundus muscles. By using the electromyography-guided technique, botulinum toxin A was injected into the first lumbrical of the index finger (12 units), second and third lumbricals of the middle and ring fingers, respectively (15 units each), and fourth lumbrical of the little finger (10 units). At follow-up, clinical and electromyographic examination revealed a significant reduction in tone and clonus of the injected lumbricals. Ashworth scores of the lumbricals from the index to little finger improved to 1. Botulinum toxin A injection of the lumbricals can be beneficial in managing spasticity of these muscles. It is well tolerated and effective at doses of 10 to 15 units. Lumbrical injection of botulinum toxin A is a useful adjunct in our percutaneous armamentarium for managing the spastic hand.     

Racial differences in muscle strength in disabled older women

This study examines racial differences in muscle strength, and associations of muscle strength to level of physical activity and severity of disability, among a community sample of 254 black and 665 white, moderately to severely disabled women aged 65 and older. Potential confounders that were adjusted for in the models included age, body weight and height, joint pain, number of chronic conditions, and socioeconomic status. Hand grip, hip flexion, and knee extension forces were measured using portable hand-held dynamometers in the participants' homes. Hand grip strength was measured as the maximal isometric force. Hip flexion and knee extension forces were measured as the greatest force the tester had to apply to break the isometric contraction. A declining strength gradient was observed with increasing severity of disability and for decreasing level of physical activity in both races. At equal levels of disability or physical activity, blacks had better hand grip and hip flexion strength, but knee extension strength did not differ by race. The greater hand grip and hip flexion strength found in black women may be related to their greater muscle mass and known racial differences in body dimensions. No consistent racial differences were observed in the relationship between physical activity and muscle strength, or muscle strength and disability, suggesting that the role of muscle strength in the disablement process does not differ between races. Physical activity and exercise programs may be feasible ways to prevent worsening of disability in blacks and whites.     

Co-contraction of lumbar muscles during the development of time-varying triaxial moments

The recruitment and co-contraction of lumbar muscles were investigated during the voluntary development of slowly and rapidly varying trunk flexion and extension, lateral bending, and axial twisting moments. Myoelectric signals were recorded from 14 lumbar muscles in nine young men during maximum voluntary exertions and cyclic isometric exertions. System identification techniques were used to calibrate dynamic models of the relationship between myoelectric signals and force. To assess co-contraction, the predicted muscle forces were subdivided into a task-moment set of muscle forces that minimally satisfied moment equilibrium and a co-contraction set of muscle forces that produced zero net moment. The sum of co-contraction muscle forces was used to quantify the degree of co-contraction present. Co-contraction was largely dependent on the direction of exertion and relatively less dependent on the subject or the rate of exertion. Co-contractions were estimated to contribute approximately 16-19% to the sum of muscle forces at a lumbar cross section during attempted extension of the trunk. Estimated co-contractions during attempted lateral bending and axial twisting were two to three times greater, which demonstrates that co-contraction is a major determinant of spinal loading in these tasks. This analysis suggests that substantial contractions of lumbar muscles, especially during asymmetric exertions, are used for reasons other than equilibrating moments at the L3-L4 level.     

Large index-fingertip forces are produced by subject-independent patterns of muscle excitation

Are fingertip forces produced by subject-independent patterns of muscle excitation? If so, understanding the mechanical basis underlying these muscle coordination strategies would greatly assist surgeons in evaluating options for restoring grasping. With the finger in neutral ad- abduction and flexed 45 degrees at the MCP and PIP, and 10 degrees at DIP joints, eight subjects attempted to produce maximal voluntary forces in four orthogonal directions perpendicular to the distal phalanx (palmar, dorsal, lateral and medial) and in one direction collinear with it (distal). Forces were directed within 4.7 +/- 2.2 degrees (mean +/- S.D.) of target and their magnitudes clustered into three distinct levels (p < 0.05; post hoc pairwise RMANOVA). Palmar (27.9 +/- 4.1 N), distal (24.3 +/- 8.3 N) and medial (22.9 +/- 7.8 N) forces were highest, lateral (14.7 +/- 4.8 N) was intermediate, and dorsal (7.5 +/- 1.5 N) was lowest. Normalized fine-wire EMGs from all seven muscles revealed distinct muscle excitation groups for palmar, dorsal and distal forces (p < 0.05; post hoc pairwise RMANOVA). Palmar force used flexors, extensors and dorsal interosseous; dorsal force used all muscles; distal force used all muscles except for extensors; medial and lateral forces used all muscles including significant co-excitation of interossei. The excitation strategies predicted to achieve maximal force by a 3-D computer model (four pinjoints, inextensible tendons, extensor mechanism and isometric force models for all seven muscles) reproduced the observed use of extensors and absence of palmar interosseous to produce palmar force (to regulate net joint flexion torques), the absence of extensors for distal force, and the use of intrinsics (strong MCP flexors) for dorsal force. The model could not predict the interossei co-excitation seen for medial and lateral forces, which may be a strategy to prevent MCP joint damage. The model predicts distal force to be most sensitive to dorsal interosseous strength, and palmar and distal forces to be very sensitive to MCP and PIP flexor moment arms, and dorsal force to be sensitive to the moment arm of and the tension allocation to the PIP extensor tendon of the extensor mechanism.     

Division of the transverse carpal ligament and flexor tendon excursion: open and endoscopic carpal tunnel release

Ten fresh cadaver upper extremities from 10 different subjects were used in this study of the effect of both open and endoscopic carpal tunnel release on flexor tendon excursion. The amount of excursion necessary to bring each finger from the fully extended to the fully flexed position with the fingertip just touching the palm was measured with the extremity mounted in a device that moved the wrist from extension through flexion. Endoscopic carpal tunnel release, open release, and transverse carpal ligament reconstruction were performed with tendon excursion measurements made in each of four wrist positions after each procedure. Fingertip to palm distance was also measured. The measurements of flexor tendon excursion in neutral wrist position with intact transverse carpal ligament served as the norm for each finger and as the denominator in the ratio of postoperative to preoperative excursion distances. The study confirmed the importance of the transverse carpal ligament as a flexor pulley; transection of the ligament increased the amount of flexor tendon excursion necessary to achieve finger flexion and fingertip-to-palm contact. Tendon excursion/digital flexion improved after transposition flap repair. Neither open nor endoscopic carpal tunnel release conferred any particular benefit to flexor tendon excursion postoperatively. The proximal palmar aponeurosis does not seem to have the same pulley effect as the transverse fibers of the distal palm.     

Coordination of grip and load forces during vertical point-to-point movements with a grasped object in Parkinson's disease.

Thirteen patients with Parkinson's disease and 13 age-matched control subjects performed vertical point-to-point arm movements with an instrumented object, starting and ending with the object being held stationary. All Parkinsonian patients were tested on medication. Parkinsonian patients retained all aspects of predictive grip force control. Compared with healthy controls, they generated similar static grip forces during stationary holding and similar force ratios between maximum grip and load force, reflecting effective grip force adjustments in relation to movement-induced inertial loads. Grip and load force maximums coincided very closely, indicating that temporal aspects of predictive grip force regulation were also unaffected. However, Parkinsonian subjects showed additional oscillations in acceleration and grip force due to tremor and produced significantly slower arm accelerations due to bradykinesia. The results suggest that Parkinson's disease does not significantly impair the anticipation of movement-induced load fluctuations during voluntary arm movements with a grasped object performed on medication. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Transfer of loads between lumbar tissues during the flexion-relaxation phenomenon

STUDY DESIGN AND METHODS: This study used an anatomically detailed model of the lumbar tissues, driven from biologic signals of vertebral displacement and myoelectric signals, to estimate individual muscle and passive tissue force-time histories during the performance of the "flexion-relaxation" maneuver. Eight male university students performed three trials each of the "flexion-relaxation" maneuver with six pairs of surface myoelectric electrodes monitoring the right side of the trunk musculature, an electromagnetic device to record lumbar flexion, and videotape to record body segment displacement. OBJECTIVES: To examine the loads on individual tissues during the transfer of moment support responsibility from predominantly active muscle to predominantly passive tissue. SUMMARY OF BACKGROUND DATA: No previous studies, to the authors' knowledge, have examined individual tissue loading during the flexion-relaxation maneuver. RESULTS: Although most subjects were able to "relax" their lumbar extensors in full flexion, activity remained in the thoracic extensors and abdominals. Tissue load predictions suggested that while the lumbar extensor muscles were neurally "relaxed" (i.e., myoelectric silence), substantial elastic forces would assist the passive tissues in extensor moment support. On average, subjects sustained almost 3 kN in compressive load on the lumbar spine and about 755 N of anterior shear during full flexion with only 8 kg held in the hands. CONCLUSIONS: The "relaxation" of lumbar extensor muscles appeared to occur only in an electrical sense because they generated substantial force elastically through stretching. Loading of the interspinous and supraspinous ligaments, in particular, was high relative to their failure tolerance.     

The effect of keyboard keyswitch make force on applied force and finger flexor muscle activity

The design of the force-displacement characteristics or 'feel' of keyboard keyswitches has been guided by preference and performance data; there has been very little information on how switch 'feel' alters muscle activity or applied force. This is a laboratory-based repeated measures design experiment to evaluate the effect of computer keyboard keyswitch design on applied finger force and muscle activity during a typing task. Ten experienced typists typed on three keyboards which differed in keyswitch make force (0.34, 0.47 and 1.02 N) while applied fingertip force and finger flexor electromyograms were recorded. The keyboard testing order was randomized and subjects typed on each keyboard for three trials, while data was collected for a minimum of 80 keystrokes per trial. No differences in applied fingertip force or finger flexor EMG were observed during typing on keyboards with switch make force of 0.34 or 0.47 N. However, applied fingertip force increased by approximately 40% (p < 0.05) and EMG activity increased by approximately 20% (p < 0.05) when the keyswitch make force was increased from 0.47 to 1.02 N. These results suggest that, in order to minimize the biomechanical loads to forearm tendons and muscles of keyboard users, keyswitches with a make force of 0.47 N or less should be considered over switches with a make force of 1.02 N.     

A prospective study of flexor tendon repair in zone 5

A prospective study of postoperative mobilization of flexor tendon repairs in zone 5 was conducted over a 2-year period between 1994 and 1996 using a controlled active motion (active extension-active flexion) regimen of mobilization. Fifty-two patients, who had a total of 151 flexor digitorum superficialis (FDS) and 103 flexor digitorum profundus (FDP) divisions, were available for review at a mean follow-up of 10 months. Of the 161 fingers with division of one or both flexor tendons, 66% exhibited independent FDS function and 90% achieved good or excellent results of digital range of motion. No rupture of an FDP tendon repair occurred during the study period. The data allowed us to define a new method of classifying the results of treatment of these injuries in terms of the injured wrists as a whole and not simply as a series of isolated observations for each individual finger with divided flexor tendons. The results of recovery of independent FDS action and range of finger movement achieved for injuries in which the flexors of all four fingers had been divided indicate a statistically significant interdependence of injuries of finger flexors of adjacent fingers at the wrist. Multivariate analysis showed the presence of a "spaghetti wrist" injury to have a significant adverse effect on the recovery of the independent FDS action but not on the recovery of the digital range of motion.