The effect of torque direction and cylindrical handle diameter on the coupling between the hand and a cylindrical handle

Pheasant and O'Neill's torque model (1975) was modified to account for grip force distributions. The modified model suggests that skin friction produced by twisting an object in the direction of fingertips causes flexion of the distal phalanges and increases grip force and, thus, torque. Twelve subjects grasped a cylindrical object with diameters of 45.1, 57.8, and 83.2 mm in a power grip, and performed maximum torque exertions about the long axis of the handle in two directions: the direction the thumb points and the direction the fingertips point. Normal force on the fingertips increased with torque toward the fingertips, as predicted by the model. Consequently, torque toward the fingertips was 22% greater than torque toward the thumb. Measured torque and fingertip forces were compared with model predictions. Torque could be predicted well by the model. Measured fingertip force and thumb force were, on average, 27% less than the predicted values. Consistent with previous studies, grip force decreased as the handle diameter increased from 45.1 to 83.2 mm. This may be due not only to the muscle length-strength relationship, but also to major active force locations on the hand: grip force distributions suggest that a small handle allows fingertip force and thumb force to work together against the palm, resulting in a high reaction force on the palm, and, therefore, a high grip force. For a large handle, fingertip force and thumb force act against each other, resulting in little reaction force on the palm and, thus, a low grip force.     

Task Dependency of Grip Stiffness—A Study of Human Grip Force and Grip Stiffness Dependency during Two Different Tasks with Same Grip Forces

It is widely known that the pinch-grip forces of the human hand are linearly related to the weight of the grasped object. Less is known about the relationship between grip force and grip stiffness. We set out to determine variations to these dependencies in different tasks with and without visual feedback. In two different settings, subjects were asked to (a) grasp and hold a stiffness-measuring manipulandum with a predefined grip force, differing from experiment to experiment, or (b) grasp and hold this manipulandum of which we varied the weight between trials in a more natural task. Both situations led to grip forces in comparable ranges. As the measured grip stiffness is the result of muscle and tendon properties, and since muscle/tendon stiffness increases more-or-less linearly as a function of muscle force, we found, as might be predicted, a linear relationship between grip force and grip stiffness. However, the measured stiffness ranges and the increase of stiffness with grip force varied significantly between the two tasks. Furthermore, we found a strong correlation between regression slope and mean stiffness for the force task which we ascribe to a force stiffness curve going through the origin. Based on a biomechanical model, we attributed the difference between both tasks to changes in wrist configuration, rather than to changes in cocontraction. In a new set of experiments where we prevent the wrist from moving by fixing it and resting it on a pedestal, we found subjects exhibiting similar stiffness/force characteristics in both tasks.     

The effects of ball impact location and grip tightness on the arm, racquet and ball for one-handed tennis backhand groundstrokes

A torque-driven, 3D computer simulation model of an arm-racquet system was used to investigate the effects of ball impact location and grip tightness on the arm, racquet and ball during one-handed tennis backhand groundstrokes. The stringbed was represented by nine point masses connected to each other and the racquet frame with elastic springs and three torsional spring-dampers between the hand and the racquet were used to represent grip tightness. For each perturbation of nine impact locations and grip tightness, simulations were run for a 50 ms period starting with ball-racquet impact. Simulations showed that during off-centre impacts below the longitudinal axis of the racquet, the wrist was forced to flex up to 16° more with up to six times more wrist extension torque when compared to a centre impact simulation. Perturbing grip tightness had no substantial effect on centre impact simulations. However, for off-centre impacts (below the longitudinal axis of the racquet) a tight grip condition resulted in a substantial decrease in racquet rotation within the hand (less than 2°) and an increase of 6° in wrist flexion angle when compared to the equivalent simulation with a normal grip. In addition there was approximately 20% more wrist extension torque when compared with equivalent off-centre impact simulation with a normal grip. Consequently off-centre impacts below the longitudinal axis of the racquet may be a substantial contributing factor for tennis elbow injuries with a tight grip aggravating the effect due to high eccentric wrist extension torques and forced wrist flexion.     

Functional Corticospinal Projections from Human Supplementary Motor Area Revealed by Corticomuscular Coherence during Precise Grip Force Control

The purpose of the present study was to investigate whether corticospinal projections from human supplementary motor area (SMA) are functional during precise force control with the precision grip (thumb-index opposition). Since beta band corticomuscular coherence (CMC) is well-accepted to reflect efferent corticospinal transmission, we analyzed the beta band CMC obtained with simultaneous recording of electroencephalographic (EEG) and electromyographic (EMG) signals. Subjects performed a bimanual precise visuomotor force tracking task by applying isometric low grip forces with their right hand precision grip on a custom device with strain gauges. Concurrently, they held the device with their left hand precision grip, producing similar grip forces but without any precision constraints, to relieve the right hand. Some subjects also participated in a unimanual control condition in which they performed the task with only the right hand precision grip while the device was held by a mechanical grip. We analyzed whole scalp topographies of beta band CMC between 64 EEG channels and 4 EMG intrinsic hand muscles, 2 for each hand. To compare the different topographies, we performed non-parametric statistical tests based on spatio-spectral clustering. For the right hand, we obtained significant beta band CMC over the contralateral M1 region as well as over the SMA region during static force contraction periods. For the left hand, however, beta band CMC was only found over the contralateral M1. By comparing unimanual and bimanual conditions for right hand muscles, no significant difference was found on beta band CMC over M1 and SMA. We conclude that the beta band CMC found over SMA for right hand muscles results from the precision constraints and not from the bimanual aspect of the task. The result of the present study strongly suggests that the corticospinal projections from human SMA become functional when high precision force control is required.     

Control of Precision Grip Force in Lifting and Holding of Low-Mass Objects

Few studies have investigated the control of grip force when manipulating an object with an extremely small mass using a precision grip, although some related information has been provided by studies conducted in an unusual microgravity environment. Grip-load force coordination was examined while healthy adults (N = 17) held a moveable instrumented apparatus with its mass changed between 6 g and 200 g in 14 steps, with its grip surface set as either sandpaper or rayon. Additional measurements of grip-force-dependent finger-surface contact area and finger skin indentation, as well as a test of weight discrimination, were also performed. For each surface condition, the static grip force was modulated in parallel with load force while holding the object of a mass above 30 g. For objects with mass smaller than 30 g, on the other hand, the parallel relationship was changed, resulting in a progressive increase in grip-to-load force (GF/LF) ratio. The rayon had a higher GF/LF force ratio across all mass levels. The proportion of safety margin in the static grip force and normalized moment-to-moment variability of the static grip force were also elevated towards the lower end of the object mass for both surfaces. These findings indicate that the strategy of grip force control for holding objects with an extremely small mass differs from that with a mass above 30 g. The data for the contact area, skin indentation, and weight discrimination suggest that a decreased level of cutaneous feedback signals from the finger pads could have played some role in a cost function in efficient grip force control with low-mass objects. The elevated grip force variability associated with signal-dependent and internal noises, and anticipated inertial force on the held object due to acceleration of the arm and hand, could also have contributed to the cost function.     

Effects of carpal tunnel syndrome on adaptation of multi-digit forces to object weight for whole-hand manipulation

The delicate tuning of digit forces to object properties can be disrupted by a number of neurological and musculoskeletal diseases. One such condition is Carpal Tunnel Syndrome (CTS), a compression neuropathy of the median nerve that causes sensory and motor deficits in a subset of digits in the hand. Whereas the effects of CTS on median nerve physiology are well understood, the extent to which it affects whole-hand manipulation remains to be addressed. CTS affects only the lateral three and a half digits, which raises the question of how the central nervous system integrates sensory feedback from affected and unaffected digits to plan and execute whole-hand object manipulation. We addressed this question by asking CTS patients and healthy controls to grasp, lift, and hold a grip device (445, 545, or 745 g) for several consecutive trials. We found that CTS patients were able to successfully adapt grip force to object weight. However, multi-digit force coordination in patients was characterized by lower discrimination of force modulation to lighter object weights, higher across-trial digit force variability, the consistent use of excessively large digit forces across consecutive trials, and a lower ability to minimize net moments on the object. Importantly, the mechanical requirement of attaining equilibrium of forces and torques caused CTS patients to exert excessive forces at both CTS-affected digits and digits with intact sensorimotor capabilities. These findings suggest that CTS-induced deficits in tactile sensitivity interfere with the formation of accurate sensorimotor memories of previous manipulations. Consequently, CTS patients use compensatory strategies to maximize grasp stability at the expense of exerting consistently larger multi-digit forces than controls. These behavioral deficits might be particularly detrimental for tasks that require fine regulation of fingertip forces for manipulating light or fragile objects.     

Learned Manipulation at Unconstrained Contacts Does Not Transfer across Hands

Recent studies about sensorimotor control of the human hand have focused on how dexterous manipulation is learned and generalized. Here we address this question by testing the extent to which learned manipulation can be transferred when the contralateral hand is used and/or object orientation is reversed. We asked subjects to use a precision grip to lift a grip device with an asymmetrical mass distribution while minimizing object roll during lifting by generating a compensatory torque. Subjects were allowed to grasp anywhere on the object’s vertical surfaces, and were therefore able to modulate both digit positions and forces. After every block of eight trials performed in one manipulation context (i.e., using the right hand and at a given object orientation), subjects had to lift the same object in the second context for one trial (transfer trial). Context changes were made by asking subjects to switch the hand used to lift the object and/or rotate the object 180° about a vertical axis. Therefore, three transfer conditions, hand switch (HS), object rotation (OR), and both hand switch and object rotation (HS+OR), were tested and compared with hand matched control groups who did not experience context changes. We found that subjects in all transfer conditions adapted digit positions across multiple transfer trials similar to the learning of control groups, regardless of different changes of contexts. Moreover, subjects in both HS and HS+OR group also adapted digit forces similar to the control group, suggesting independent learning of the left hand. In contrast, the OR group showed significant negative transfer of the compensatory torque due to an inability to adapt digit forces. Our results indicate that internal representations of dexterous manipulation tasks may be primarily built through the hand used for learning and cannot be transferred across hands.     

Carpal tunnel syndrome caused by hand injuries.

Although carpal tunnel syndrome associated with injury to the wrist is common, it is possible to overlook symptoms of median nerve compression caused by an ascending tenosynovitis secondary to trauma distal to the wrist. One should look for these symptoms in such patients who complain of pain and weakness of the hand, and release the carpal tunnel if nerve compression is suspected.     

Quantifying deficits in the 3D force capabilities of a digit caused by selective paralysis: application to the thumb with simulated low ulnar nerve palsy

We present the development of a vision-feedback method to characterize how selective paralysis distorts the three-dimensional (3D) volume representing digit-tip force production capability and its application to healthy individuals producing thumb-tip force with and without simulated low ulnar nerve palsy (LUNP). Subjects produced maximal static voluntary force spanning the transverse, sagittal and frontal planes of the thumb (16, 15 and 10 subjects, respectively). Subjects produced thumb-tip force tasks in guided and self-selected directions. The envelope (convex hull) of extreme forces in each plane approximated that cross-section of the 3D volume of force capability. Some subjects repeated the tasks with a temporary ulnar nerve block applied at the wrist to simulate complete acute LUNP. Three geometric properties of the force convex hull characterized each cross-section's shape: the ratios of its principal moments of inertia (RPMIs), the orientation of its principal axis (OPA), and its centroid location. Our results show that force production in the thumb's sagittal plane may be a reproducible and objective test to grade motor impairment in LUNP: paired t-tests of the larger RPMI in this plane best distinguished the nerve-blocked cases from the control cases in the guided task (p = 0.012), and Discriminant Analysis of the centroid location for the self-selected task in this plane correctly classified 94.7% of subjects into the control and ulnar nerve-blocked groups. We show that our method measures and detects changes in a digit's force production capabilities. Towards a clinical test of motor impairment in LUNP, this biomechanical study dictates which 3D thumb-tip forces to measure (those in the sagittal plane) and how to measure them (using the self-selected task).     

The effect of mild whole-body cold stress on isometric force control during hand grip and key pinch tasks

Prolonged exposure to cold can impair manual performance, which in turn can affect work task performance. We investigated whether mild whole-body cold stress would affect isometric force control during submaximal hand grip and key pinch tasks. Twelve male participants performed isometric hand grip and key pinch tasks at 10% and 30% of maximal voluntary contraction (MVC) for 30 and 10 s respectively, in cold (8 °C) and control (25 °C) conditions. Finger temperature decreased significantly by 18.7 ± 2.1 °C and continuous low-intensity shivering in the upper trunk increased significantly in intensity and duration during cold exposure. Rectal temperature decreased similarly for the 8 °C and 25 °C exposures. Force variability (FCv) was <2% for the hand grip tasks, and <3% for the key pinch tasks. No significant changes in FCv or force accuracy were found between the ambient temperatures. In conclusion, isometric force control during hand grip and key pinch tasks was maintained when participants experienced mild whole-body cold stress compared with when they were thermally comfortable.     

Radial force distribution changes associated with tangential force production in cylindrical grasping, and the importance of anatomical registration

Radial force (F(r)) distributions describe grip force coordination about a cylindrical object. Recent studies have employed only explicit F(r) tasks, and have not normalized for anatomical variance when considering F(r) distributions. The goals of the present study were (i) to explore F(r) during tangential force production tasks, and (ii) to examine the extent to which anatomical registration (i.e. spatial normalization of anatomically analogous structures) could improve signal detectability in F(r) data. Twelve subjects grasped a vertically oriented cylindrical handle (diameter=6 cm) and matched target upward tangential forces of 10, 20, and 30 N. F(r) data were measured using a flexible pressure mat with an angular resolution of 4.8°, and were registered using piecewise-linear interpolation between five manually identified points-of-interest. Results indicate that F(r) was primarily limited to three contact regions: the distal thumb, the distal fingers, and the fingers' metatacarpal heads, and that, while increases in tangential force caused significant increases in F(r) for these regions, they did not significantly affect the F(r) distribution across the hand. Registration was found to substantially reduce between-subject variability, as indicated by both accentuated F(r) trends, and amplification of the test statistic. These results imply that, while subjects focus F(r) primarily on three anatomical regions during cylindrical grasp, inter-subject anatomical differences introduce a variability that, if not corrected for via registration, may compromise one's ability to draw anatomically relevant conclusions from grasping force data.     

Effect of Tendon Vibration on Hemiparetic Arm Stability in Unstable Workspaces

Sensory stimulation of wrist musculature can enhance stability in the proximal arm and may be a useful therapy aimed at improving arm control post-stroke. Specifically, our prior research indicates tendon vibration can enhance stability during point-to-point arm movements and in tracking tasks. The goal of the present study was to investigate the influence of forearm tendon vibration on endpoint stability, measured at the hand, immediately following forward arm movements in an unstable environment. Both proximal and distal workspaces were tested. Ten hemiparetic stroke subjects and 5 healthy controls made forward arm movements while grasping the handle of a two-joint robotic arm. At the end of each movement, the robot applied destabilizing forces. During some trials, 70 Hz vibration was applied to the forearm flexor muscle tendons. 70 Hz was used as the stimulus frequency as it lies within the range of optimal frequencies that activate the muscle spindles at the highest response rate. Endpoint position, velocity, muscle activity and grip force data were compared before, during and after vibration. Stability at the endpoint was quantified as the magnitude of oscillation about the target position, calculated from the power of the tangential velocity data. Prior to vibration, subjects produced unstable, oscillating hand movements about the target location due to the applied force field. Stability increased during vibration, as evidenced by decreased oscillation in hand tangential velocity.     

Surface EMG pattern recognition for real-time control of a wrist exoskeleton

Background  

Surface electromyography (sEMG) signals have been used in numerous studies for the classification of hand gestures and movements and successfully implemented in the position control of different prosthetic hands for amputees. sEMG could also potentially be used for controlling wearable devices which could assist persons with reduced muscle mass, such as those suffering from sarcopenia. While using sEMG for position control, estimation of the intended torque of the user could also provide sufficient information for an effective force control of the hand prosthesis or assistive device. This paper presents the use of pattern recognition to estimate the torque applied by a human wrist and its real-time implementation to control a novel two degree of freedom wrist exoskeleton prototype (WEP), which was specifically developed for this work.     

A 3D biomechanical model of the hand for power grip

A three-dimensional scalable biomechanical model of the four fingers of the hand to evaluate power grip is proposed. The model has been validated by means of reproducing an experiment in which the subjects exerted the maximal voluntary grasping force over cylinders of different diameters. The model is used to simulate the cylinder grip for two hand sizes and for five different handle diameters. The reduction of the muscle forces using different handle diameters has been studied. The model can be applied to the design and evaluation of handles for power grip and to the study of power grasp for normal and abnormal hands.     

Wrist splint effects on muscle activity and force during a handgrip task

Wrist splints are commonly prescribed to limit wrist motion and provide support at night and during inactive periods but are often used in the workplace. In theory, splinting the wrist should reduce wrist extensor muscle activity by stabilizing the joint and reducing the need for co-contraction to maintain posture. Ten healthy volunteers underwent a series of 24 10-s gripping trials with surface electromyography on 6 forearm muscles. Trials were randomized between splinted and nonsplinted conditions with three wrist postures (30 degrees flexion, neutral, and 30 degrees extension) and four grip efforts. Custom-made Plexiglas splints were taped to the dorsum of the hand and wrist. It was found that when simply holding the dynamometer, use of a splint led to a small (<1% MVE) but significant reduction in activity for all flexor muscles and extensor carpi radialis (all activity <4% maximum). At maximal grip, extensor muscle activity was significantly increased with the splints by 7.9-23.9% MVE. These data indicate that splinting at low-to-moderate grip forces may act to support the wrist against external loading, but appears counterproductive when exerting maximal forces. Wrist bracing should be limited to periods of no to light activity and avoided during tasks that require heavy efforts.     

Neuromechanical control of the forearm muscles during gripping with sudden flexion and extension wrist perturbations

The purpose of this study was to investigate how gripping modulates forearm muscle co-contraction prior to and during sudden wrist perturbations. Ten males performed a sub-maximal gripping task (no grip, 5% and 10% of maximum) while a perturbation forced wrist flexion or extension. Wrist joint angles and activity from 11 muscles were used to determine forearm co-contraction and muscle contributions to wrist joint stiffness. Co-contraction increased in all pairs as grip force increased (from no grip to 10% grip), corresponding to a 36% increase in overall wrist joint stiffness. Inclusion of individual muscle contributions to wrist joint stiffness enhanced the understanding of forearm co-contraction. The extensor carpi radialis longus (ECRL) and brevis had the largest stiffness contributions (34.5 ± 1.3% and 20.5 ± 2.3%, respectively), yet muscle pairs including ECRL produced the lowest co-contraction. The muscles contributing most to wrist stiffness were consistent across conditions (ECRL for extensors; Flexor Digitorum Superficialis for flexors), suggesting enhanced contributions rather than muscular redistribution. This work provides investigation of the neuromuscular response to wrist perturbations and gripping demands by considering both co-contraction and muscle contributions to joint stiffness. Individual muscle stiffness contributions can be used to enhance the understanding of forearm muscle control during complex tasks.     

Interfering effects of multitasking on muscle activity in the upper extremity.

Multitasking, where workers are required to perform multiple physical tasks with various levels of cognitive load is common in today's workplace. Simultaneous physical and mental demands are thought to cause task interference and likely increase muscle activity. To test the interfering effects of multitasking, 16 healthy participants performed hand and shoulder exertions with combinations of four grip conditions (no grip, 30% grip with low precision, 30% grip with high precision, and maximal grip) and three shoulder conditions at 90 degrees abduction (maintaining posture, 40% force-controlled moment, 40% posture-controlled moment), with and without the Stroop test while surface EMG was recorded from eight upper extremity muscles. Both 40% MVC shoulder moments increased extrinsic forearm muscle activity by 2-4% MVE (p<0.01). Grip exertion at 30% MVC reduced anterior and middle deltoid activity by 2% MVE (p<0.01). Exerting a constant force against the transducer (force-controlled) required 3-4% MVE greater middle and posterior deltoid activity (p<0.001) compared to supporting an equivalent inertial load at the same shoulder angle (posture-controlled). Performing the mental task (Stroop test) concurrently with either 40% MVC shoulder moments significantly increased trapezius activity by nearly 2% MVE (p<0.05). Interestingly, the Stroop test also reduced all deltoid activity by 1% MVE (p<0.05). The addition of both the Stroop test and force-control shoulder exertion independently reduced maximal grip force by 7% and 10% MVC, respectively. These results suggest that more complex workplace tasks may act to increase muscle load or interfere with task performance. These small but significant findings may play a role in the development of long-term musculoskeletal disorders in the workplace.     

Median nerve neuropathy in the forearm due to recurrence of anterior wrist ganglion that originates from the scaphotrapezial joint: Case Report

Background

Median nerve neuropathy caused by compression from a tumor in the forearm is rare. Cases with anterior wrist ganglion have high recurrence rates despite surgical treatment. Here, we report the recurrence of an anterior wrist ganglion that originated from the Scaphotrapezial joint due to incomplete resection and that caused median nerve neuropathy in the distal forearm.

Case presentation

A 47-year-old right-handed housewife noted the appearance of soft swelling on the volar aspect of her left distal forearm, and local resection surgery was performed twice at another hospital. One year after the last surgery, the swelling reappeared and was associated with numbness and pain in the radial volar aspect of the hand. Magnetic resonance imaging revealed that the multicystic lesion originated from the Scaphotrapezial joint and had expanded beyond the wrist. Exploration of the left median nerve showed that it was compressed by a large ovoid cystic lesion at the distal forearm near the proximal end of the carpal tunnel. We resected the cystic lesion to the Scaphotrapezial joint. Her symptoms disappeared 1 week after surgery, and complications or recurrent symptoms were absent 13 months after surgery.

Conclusions

A typical median nerve compression was caused by incomplete resection of an anterior wrist ganglion, which may have induced widening of the cyst. Cases with anterior wrist ganglion have high recurrence rates and require extra attention in their treatment.     

Cognitive-Motor Interference While Grasping,Lifting and Holding Objects

In daily life, object manipulation is usually performed concurrently to the execution of cognitive tasks. The aim of the present study was to determine which aspects of precision grip require cognitive resources using a motor-cognitive dual-task paradigm. Eighteen healthy participants took part in the experiment, which comprised two conditions. In the first condition, participants performed a motor task without any concomitant cognitive task. They were instructed to grip, lift and hold an apparatus incorporating strain gauges allowing a continuous measurement of the force perpendicular to each contact surface (grip force, GF) as well as the total tangential force applied on the object (load force, LF). In the second condition, participants performed the same motor task while concurrently performing a cognitive task consisting in a complex visual search combined with counting. In the dual-task condition, we found a significant increase in the duration of the preload phase (time between initial contact of the fingers with the apparatus and onset of the load force), as well as a significant increase of the grip force during the holding phase, indicating that the cognitive task interfered with the initial force scaling performed during the preload phase and the fine-tuning of grip force during the hold phase. These findings indicate that these aspects of precision grip require cognitive resources. In contrast, other aspects of the precision grip, such as the temporal coupling between grip and load forces, were not affected by the cognitive task, suggesting that they reflect more automatic processes. Taken together, our results suggest that assessing the dynamic and temporal parameters of precision grip in the context of a concurrent cognitive task may constitute a more ecological and better-suited tool to characterize motor dysfunction in patients.