Optimization of the levels of grip force, stroke rotation, frequency and grip span for a torqueing task

This study was to investigate the effects of grip force, frequency, stroke rotation and grip-span on discomfort and obtain best posture for hand tool users. Fifteen male participants volunteered in this study. Participants performed combined gripping with torqueing exertions for 5 min for two levels of frequency (10 and 20 exertions/min) at two levels of grip force (50 and 70 N), two levels of stroke rotation (30(○) and 60(○)) and three levels of grip-span (4.7, 6 and 7.3 cm). Therefore, a 2×2×2×3 full factorial design was used. The analysis of variance (ANOVA) showed that frequency, stroke rotation and grip-span were significant on discomfort score. Minimum discomfort and comfortable posture was found to be 90 N grip force with 10 exertions/min for 60° stroke rotation at 6-cm grip-span. The grip force, frequency and stroke rotation were found significant on EMG activity of forearm muscles using multivariate analysis of variance (MANOVA). The extensor muscles were found more activated than flexor muscles during the given task.

The Effect of Pressure Glove Tightness on Forearm Muscle Activity and Psychophysical Responses

OBJECTIVE

The impact of pressure glove tightness on maximum grip force, muscle activity, and psychophysical responses is investigated to facilitate the prescription of a suitable reduction factor (RF) for pressure treatment.

BACKGROUND

The wearing of pressure therapy gloves is often considered to hinder hand performance and cause discomfort, resulting in unsatisfactory treatment adherence during burn rehabilitation.

METHOD

A wear trial was carried out with 10 participants for three custom-made pressure gloves that consist of different RFs-10%, 15% and 20%-as well as for the bare hand. The surface electromyography of three forearm muscles was measured during tasks that involve moving marbles, buttoning a shirt, and typing. The psychophysical responses were also recorded.

RESULTS

The use of pressure gloves results in a reduction in the maximum gripping force. Gloves with tighter pressure contribute to lower perceived comfort and ease of hand motion. Increased glove tightness (with RFs of 15% and 20%) decreases muscle activity as compared to the bare-hand condition when buttoning a shirt. In terms of typing, the forearm muscle activity increases with high glove pressure (RF of 20%).

CONCLUSION

The forearm muscles are significantly affected by glove tightness in performing different daily tasks that required gripping, pinching, and typing. The increase of RF of pressure gloves causes negative impact on psychophysical response and handgrip strength. Glove tightness in relation to hand performance and comfort is important in prescribing an optimal pressure therapy glove for hypertrophic scar treatment.

APPLICATION

The findings give insight into the impacts of pressure glove tightness on muscle activity, thus providing a reference for glove development.

Flexible Control of Safety Margins for Action Based on Environmental Variability

To reduce the risk of slip, grip force (GF) control includes a safety margin above the force level ordinarily sufficient for the expected load force (LF) dynamics. The current view is that this safety margin is based on the expected LF dynamics, amounting to a static safety factor like that often used in engineering design. More efficient control could be achieved, however, if the motor system reduces the safety margin when LF variability is low and increases it when this variability is high. Here we show that this is indeed the case by demonstrating that the human motor system sizes the GF safety margin in proportion to an internal estimate of LF variability to maintain a fixed statistical confidence against slip. In contrast to current models of GF control that neglect the variability of LF dynamics, we demonstrate that GF is threefold more sensitive to the SD than the expected value of LF dynamics, in line with the maintenance of a 3-sigma confidence level. We then show that a computational model of GF control that includes a variability-driven safety margin predicts highly asymmetric GF adaptation between increases versus decreases in load. We find clear experimental evidence for this asymmetry and show that it explains previously reported differences in how rapidly GFs and manipulatory forces adapt. This model further predicts bizarre nonmonotonic shapes for GF learning curves, which are faithfully borne out in our experimental data. Our findings establish a new role for environmental variability in the control of action.

Proximal arm kinematics affect grip force-load force coordination

During object manipulation, grip force is coordinated with load force, which is primarily determined by object kinematics. Proximal arm kinematics may affect grip force control, as proximal segment motion could affect control of distal hand muscles via biomechanical and/or neural pathways. The aim of this study was to investigate the impact of proximal kinematics on grip force modulation during object manipulation. Fifteen subjects performed three vertical lifting tasks that involved distinct proximal kinematics (elbow/shoulder), but resulted in similar end-point (hand) trajectories. While temporal coordination of grip and load forces remained similar across the tasks, proximal kinematics significantly affected the grip force-to-load force ratio (P = 0.042), intrinsic finger muscle activation (P = 0.045), and flexor-extensor ratio (P < 0.001). Biomechanical coupling between extrinsic hand muscles and the elbow joint cannot fully explain the observed changes, as task-related changes in intrinsic hand muscle activation were greater than in extrinsic hand muscles. Rather, between-task variation in grip force (highest during task 3) appears to contrast to that in shoulder joint velocity/acceleration (lowest during task 3). These results suggest that complex neural coupling between the distal and proximal upper extremity musculature may affect grip force control during movements, also indicated by task-related changes in intermuscular coherence of muscle pairs, including intrinsic finger muscles. Furthermore, examination of the fingertip force showed that the human motor system may attempt to reduce variability in task-relevant motor output (grip force-to-load force ratio), while allowing larger fluctuations in output less relevant to task goal (shear force-to-grip force ratio).

Evaluation of repetitive isometric contractions on the heads of triceps brachii muscle during grip force exercise

OBJECTIVES

Normally, surface electromyography electrodes are used to evaluate the activity of superficial muscles during various kinds of voluntary contractions of muscle fiber. The objective of the present study was to investigate the effect of repetitive isometric contractions on the three heads of the triceps brachii muscle during handgrip force exercise.

METHODS

Myoelectric signals were recorded from the lateral, long and medial heads of the triceps brachii muscle in 13 healthy males during maximal isometric contractions for 10 s of concurrent handgrip force and elbow extension. The subjects were asked to perform their contraction task five times with 3 minutes interval between two successive contractions.

RESULTS

Decreasing electromyographic activities were found for the lateral and long heads, and increasing for the medial head throughout the 5 different contractions. Electromyographic activities were found for the lateral head with mean=199.84, SD=7.65, CV=3.83%, the long head with mean=456.76, SD=18.10, CV=3.96%, and the medial head with mean=505.16, SD=8.47, CV=1.68%. Electromyographic activities among the three heads of triceps brachii were significantly different (F=3.82) at the alpha level of (p<0.05).

CONCLUSIONS

These findings support that repetitive isometric contractions decrease the contractile activity in the lateral and long heads, and increases in the medial head of the triceps brachii muscle during handgrip force exercise with full elbow extension, and the electromyographic activity changes are observed to be more significant at the long head as compared to the lateral and medial heads.

The effect of force feedback delay on stiffness perception and grip force modulation during tool-mediated interaction with elastic force fields

During interaction with objects, we form an internal representation of their mechanical properties. This representation is used for perception and for guiding actions, such as in precision grip, where grip force is modulated with the predicted load forces. In this study, we explored the relationship between grip force adjustment and perception of stiffness during interaction with linear elastic force fields. In a forced-choice paradigm, participants probed pairs of virtual force fields while grasping a force sensor that was attached to a haptic device. For each pair, they were asked which field had higher level of stiffness. In half of the pairs, the force feedback of one of the fields was delayed. Participants underestimated the stiffness of the delayed field relatively to the nondelayed, but their grip force characteristics were similar in both conditions. We analyzed the magnitude of the grip force and the lag between the grip force and the load force in the exploratory probing movements within each trial. Right before answering which force field had higher level of stiffness, both magnitude and lag were similar between delayed and nondelayed force fields. These results suggest that an accurate internal representation of environment stiffness and time delay was used for adjusting the grip force. However, this representation did not help in eliminating the bias in stiffness perception. We argue that during performance of a perceptual task that is based on proprioceptive feedback, separate neural mechanisms are responsible for perception and action-related computations in the brain.

Moving a hand-held object: Reconstruction of referent coordinate and apparent stiffness trajectories

This study used the framework of the referent configuration hypothesis and slow changes in the external conditions during vertical oscillation of a hand-held object to infer the characteristics of hypothetical control variables. The study had two main objectives: (1) to show that hypothetical control variables, namely, referent coordinates and apparent stiffness of vertical hand position and grip force can be measured in an experiment; and (2) to establish relation(s) between these control variables that yield the classic grip-force-load-force coupling. Healthy subjects gripped a handle and performed vertical oscillations between visual targets at one of five metronome-prescribed frequencies. A HapticMaster robot was used to induce slow changes in the vertical force applied to the handle, while the size of the handle was changed slowly leading to changes in the grip aperture. The subjects were instructed not to react to possible changes in the external forces. A linear, second-order model was used to reconstruct the referent coordinate and apparent stiffness values for each phase of the vertical oscillation cycle using across-cycle regressions. The reconstructed time profiles of the referent coordinates and apparent stiffness showed consistent trends across subjects and movement frequencies. To validate the method, these values were used to predict the vertical force and the grip force applied to the handle for movement cycles that were not utilized in the reconstruction process. Analysis of the coupling between the four variables, two referent coordinates and two apparent stiffness values, revealed a single strong constraint reflecting the coupling between the grip force and vertical force. We view these data as providing experimental support for the idea of controlling natural, multi-muscle actions with shifts in a low-dimensional set of referent coordinates.

The brain adjusts grip forces differently according to gravity and inertia: a parabolic flight experiment

In everyday life, one of the most frequent activities involves accelerating and decelerating an object held in precision grip. In many contexts, humans scale and synchronize their grip force (GF), normal to the finger/object contact, in anticipation of the expected tangential load force (LF), resulting from the combination of the gravitational and the inertial forces. In many contexts, GF and LF are linearly coupled. A few studies have examined how we adjust the parameters–gain and offset–of this linear relationship. However, the question remains open as to how the brain adjusts GF regardless of whether LF is generated by different combinations of weight and inertia. Here, we designed conditions to generate equivalent magnitudes of LF by independently varying mass and movement frequency. In a control experiment, we directly manipulated gravity in parabolic flights, while other factors remained constant. We show with a simple computational approach that, to adjust GF, the brain is sensitive to how LFs are produced at the fingertips. This provides clear evidence that the analysis of the origin of LF is performed centrally, and not only at the periphery.

Changes in sensory function and force production in adults with type II diabetes

INTRODUCTION

The purpose of this study was to evaluate the relationship among sensory function, disease severity, and upper extremity force production in adults with type II diabetes (T2D) as compared with healthy age- and gender-matched controls.

METHODS

Ten adults with T2D and 10 healthy age- and gender-matched control subjects underwent a battery of sensory and motor function evaluations. Data on disease severity and duration were also collected.

RESULTS

The T2D group exhibited sensory deficits and altered force production as compared with healthy controls. Sensory function correlated with disease severity, as did signal predictability of kinetic output during submaximal force production tasks. Maximal force production tasks were associated with altered output in T2D, but these data did not correlate with disease severity or sensory dysfunction.

CONCLUSIONS

Some, not all, motor performance deficits in T2D are associated with sensory dysfunction. Mechanisms responsible for these changes in adult-onset T2D are described.

The influence of rice plow handle design and whole-body posture on grip force and upper-extremity muscle activation

A previous job screening study revealed ergonomics risk factors in rice field plowing. This work motivated the present experimental investigation of the influence of plow handle design and farmer whole-body posture on grip force and arm muscle activity. A total of 24 experienced farmers performed a simulated plowing task, including walking on even and uneven ground while rolling a tiller equipped with conventional horizontal and proposed vertical handles. Results revealed the proposed handles, designed to promote neutral wrist posture, to increase upper-arm muscle use between 47% and 70% across ground types, as compared with conventional handles. The ratio of grip force to forearm muscle activity (or efficiency in muscle use) increased from 1.85 when using conventional handles on uneven ground to 2.16 when using the proposed handles with symmetrical body posture on even ground. However, participants perceived higher discomfort when using the proposed handles, as they were accustomed to the conventional design.

PRACTITIONER SUMMARY

The findings of this work may be used to educate farmers on the potential for hand and arm injury in rice cultivation activities. Results may also provide a basis for redesign of existing tiller handles to promote neutral wrist posture, greater efficiency in muscle use and machine control.

Precision grip in congenital and acquired hemiparesis: similarities in impairments and implications for neurorehabilitation

Background: Patients with congenital and acquired hemiparesis incur long-term functional deficits, among which the loss of prehension that may impact their functional independence. Identifying, understanding, and comparing the underlying mechanisms of prehension impairments represent an opportunity to better adapt neurorehabilitation.

Objective: The present review aims to provide a better understanding of precision grip deficits in congenital and acquired hemiparesis and to determine whether the severity and type of fine motor control impairments depend on whether or not the lesions are congenital or acquired in adulthood.

Methods: Using combinations of the following key words: fingertip force, grip force, precision grip, cerebral palsy, stroke, PubMed, and Scopus databases were used to search studies from 1984 to 2013.

Results: Individuals with both congenital and acquired hemiparesis were able to some extent to use anticipatory motor control in precision grip tasks, even if this control was impaired in the paretic hand. In both congenital and acquired hemiparesis, the ability to plan efficient anticipatory motor control when the less-affected hand is used provides a possibility to remediate impairments in anticipatory motor control of the paretic hand.

Conclusion: Surprisingly, we observed very few differences between the results of studies in children with congenital hemiplegia and stroke patients. We suggest that the underlying specific strategies of neurorehabilitation developed for each one could benefit the other.

Functional Brain Activity Relates to 0-3 and 3-8 Hz Force Oscillations in Essential Tremor

It is well-established that during goal-directed motor tasks, patients with essential tremor have increased oscillations in the 0-3 and 3-8 Hz bands. It remains unclear if these increased oscillations relate to activity in specific brain regions. This study used task-based functional magnetic resonance imaging to compare the brain activity associated with oscillations in grip force output between patients with essential tremor, patients with Parkinson's disease who had clinically evident tremor, and healthy controls. The findings demonstrate that patients with essential tremor have increased brain activity in the motor cortex and supplementary motor area compared with controls, and this activity correlated positively with 3-8 Hz force oscillations. Brain activity in cerebellar lobules I-V was reduced in essential tremor compared with controls and correlated negatively with 0-3 Hz force oscillations. Widespread differences in brain activity were observed between essential tremor and Parkinson's disease. Using functional connectivity analyses during the task evidenced reduced cerebellar-cortical functional connectivity in patients with essential tremor compared with controls and Parkinson's disease. This study provides new evidence that in essential tremor 3-8 Hz force oscillations relate to hyperactivity in motor cortex, 0-3 Hz force oscillations relate to the hypoactivity in the cerebellum, and cerebellar-cortical functional connectivity is impaired.

Assessment of hand function through the coordination of contact forces in manipulation tasks

Exploration of force coordination has been one of the most often used approaches in studies of hand function. When holding and manipulating a hand-held object healthy individuals are typically able to highly coordinate the perpendicular (grip force; GF) with the tangential component of the contact force (load force; LF). The purpose of this review is to present the findings of our recent studies of GF-LF coordination. Regarding the mechanical factors affecting GF-LF coordination, our data suggest that both different hand segments and their particular skin areas could have markedly different friction properties. It also appears that the absolute, rather than relative safety margin (i.e., how much the actual GF exceeds the minimum value that prevents slipping) should be a variable of choice when assessing the applied magnitude of GF. The safety margin could also be lower in static than in free holding tasks. Regarding the involved neural factors, the data suggest that the increased frequency, rather than an increased range of a cyclic LF could have a prominent detrimental effect on the GF-LF coordination. Finally, it appears that the given instructions (e.g., ‘to hold’ vs. ‘to pull’) can prominently alter GF-LF coordination in otherwise identical manipulation tasks. Conversely, the effects of handedness could be relatively week showing only slight lagging of GF in the non-dominant, but not in the dominant hand. The presented findings reveal important aspects of hand function as seen through GF-LF coordination. Specifically, the use of specific hand areas for grasping, calculation of particular safety margins, the role of LF frequency (but not of LF range) and the effects of given instructions should be all taken into account when conducting future studies of manipulation tasks, standardizing their procedures and designing routine clinical tests of hand function.

Coordination of precision grip in 2-6 years-old children with autism spectrum disorders compared to children developing typically and children with developmental disabilities

Impaired motor coordination is prevalent in children with Autism Spectrum Disorders (ASD) and affects adaptive skills. Little is known about the development of motor patterns in young children with ASD between 2 and 6 years of age. The purpose of the current study was threefold: (1) to describe developmental correlates of motor coordination in children with ASD, (2) to identify the extent to which motor coordination deficits are unique to ASD by using a control group of children with other developmental disabilities (DD), and (3) to determine the association between motor coordination variables and functional fine motor skills. Twenty-four children with ASD were compared to 30 children with typical development (TD) and 11 children with DD. A precision grip task was used to quantify and analyze motor coordination. The motor coordination variables were two temporal variables (grip to load force onset latency and time to peak grip force) and two force variables (grip force at onset of load force and peak grip force). Functional motor skills were assessed using the Fine Motor Age Equivalents of the Vineland Adaptive Behavior Scale and the Mullen Scales of Early Learning. Mixed regression models were used for all analyses. Children with ASD presented with significant motor coordination deficits only on the two temporal variables, and these variables differentiated children with ASD from the children with TD, but not from children with DD. Fine motor functional skills had no statistically significant associations with any of the motor coordination variables. These findings suggest that subtle problems in the timing of motor actions, possibly related to maturational delays in anticipatory feed-forward mechanisms, may underlie some motor deficits reported in children with ASD, but that these issues are not unique to this population. Further research is needed to investigate how children with ASD or DD compensate for motor control deficits to establish functional skills.

Motor Consciousness during Intention-Based and Stimulus-Based Actions: Modulating Attention Resources through Mindfulness Meditation

Mindfulness-Based Stress Reduction meditation (MBSR) may offer optimal performance through heightened attention for increased body consciousness. To test this hypothesis, MBSR effects were assessed on the simple task of lifting an object. A dual task paradigm was included to assess the opposite effect of a limited amount of attention on motor consciousness. In a stimulus-based condition, the subjects’ task was to lift an object that was hefted with weights. In an intentional-based condition, subjects were required to lift a light object while imagining that the object was virtually heavier and thus, adjust their grip voluntarily. The degree of motor consciousness was evaluated by calculating correlation factors for each participant between the grip force level used during the lift trial (“lift the object”) and that used during its associated reproduce trial (“without lifting, indicate the force you think you used in the previous trial”). Under dual task condition, motor consciousness decreased for intention- and stimulus-based actions, revealing the importance of top-down attention for building the motor representation that guides action planning. For MBSR-experts, heightened attention provided stronger levels of motor consciousness; this was true for both intention and stimulus-based actions. For controls, heightened attention decreased the capacity to reproduce force levels, suggesting that voluntary top-down attention interfered with the automatic bottom-up emergence of body sensations. Our results provide strong arguments for involvement of two types of attention for the emergence of motor consciousness. Bottom-up attention would serve as an amplifier of motor-sensory afferences; top-down attention would help transfer the motor-sensory content from a preconscious to a conscious state of processing. MBSR would be a specific state for which both types of attention are optimally combined to provide experts with total experiences of their body in movement.

Consensus paper: roles of the cerebellum in motor control--the diversity of ideas on cerebellar involvement in movement

Considerable progress has been made in developing models of cerebellar function in sensorimotor control, as well as in identifying key problems that are the focus of current investigation. In this consensus paper, we discuss the literature on the role of the cerebellar circuitry in motor control, bringing together a range of different viewpoints. The following topics are covered: oculomotor control, classical conditioning (evidence in animals and in humans), cerebellar control of motor speech, control of grip forces, control of voluntary limb movements, timing, sensorimotor synchronization, control of corticomotor excitability, control of movement-related sensory data acquisition, cerebro-cerebellar interaction in visuokinesthetic perception of hand movement, functional neuroimaging studies, and magnetoencephalographic mapping of cortico-cerebellar dynamics. While the field has yet to reach a consensus on the precise role played by the cerebellum in movement control, the literature has witnessed the emergence of broad proposals that address cerebellar function at multiple levels of analysis. This paper highlights the diversity of current opinion, providing a framework for debate and discussion on the role of this quintessential vertebrate structure.

Dependence of safety margins in grip force on isometric push force levels in lateral pinch

This study examined the relationship between safety margin and force level during an isometric push task in a lateral pinch posture. Ten participants grasped an object with an aluminium- or rubber-finished grip surface using a lateral pinch posture and exerted 20%, 40%, 60%, 80% and 100% of maximum push force while voluntary grip force was recorded. Then minimum required grip force was measured for each push force level. Mean safety margin, the difference between voluntary and minimum required grip forces, was 25% maximum voluntary contraction (MVC) when averaged for all push levels. Safety margin significantly increased with increasing push force for both grip surfaces. Grip force used during maximum push exertion was only 74% lateral pinch grip MVC. Possible underlying mechanisms for increasing safety margin with increasing push force are discussed as well as the implication of this finding for ergonomic analysis. This study demonstrates that ergonomic analyses of push tasks that involve friction force should account for safety margin and reduced grip strength during the push. Failure to consider these can result in overestimation of people's push capability.

A novel device to measure power grip forces in squirrel monkeys

Understanding the neural bases for grip force behaviors in both normal and neurologically impaired animals is imperative prior to improving treatments and therapeutic approaches. The present paper describes a novel device for the assessment of power grip forces in squirrel monkeys. The control of grasping and object manipulation represents a vital aspect of daily living by allowing the performance of a wide variety of complex hand movements. However, following neurological injury such as stroke, these grasping behaviors are often severely affected, resulting in persistent impairments in strength, grip force modulation and kinematic hand control. While there is a significant clinical focus on rehabilitative strategies to address these issues, there exists the need for translational animal models. In the study presented here, we describe a simple grip force device designed for use in non-human primates, which provides detailed quantitative information regarding distal grip force dynamics. Adult squirrel monkeys were trained to exceed a specific grip force threshold, which was rewarded with a food pellet. One of these subjects then received an infarct of the M1 hand representation area. Results suggest that the device provides detailed and reliable information on grip behaviors in healthy monkeys and can detect deficits in grip dynamics in monkeys with cortical lesions (significantly longer release times). Understanding the physiological and neuroanatomical aspects of grasping function following neurological injury may lead to more effective rehabilitative interventions.

Wrist strength is dependent on simultaneous power grip intensity

The effect of grip activities on wrist flexion/extension strength was examined. Twelve healthy subjects performed maximum wrist flexion/extension exertions with one of five levels of simultaneous grip effort: minimum effort; preferred effort; 30%, 60% and 100% maximum voluntary contraction. As grip force increased from the minimum to the maximum effort, average wrist flexion strength increased 34% and average wrist extension strength decreased 10%. It appears that the finger flexor tendons on the volar aspect of the wrist act agonistically in wrist flexion and act antagonistically to wrist extension. When an object gripped by the hand is fragile or uncomfortable, the reduced finger flexor activity will limit wrist flexion strength. Gripping a slippery object that requires high grip effort will result in reduced wrist extension strength. Grip force should be controlled during measurement of wrist flexion or extension strength. When analysing a task that involves both grip and wrist exertions, use of grip/wrist strength values that were measured during grip exertions only, or wrist exertions only, may incorrectly estimate the true grip/wrist strength, as grip and wrist activities significantly interact with each other as demonstrated in this paper.

Investigation of grip force, normal force, contact area, hand size, and handle size for cylindrical handles

OBJECTIVE

To investigate relationships among grip forces, normal forces, contact area for cylindrical handles, handle diameter, hand size, and volar hand area.

BACKGROUND

Data describing those relationships are needed to predict thrust forces and torque capability.

METHOD

Additional analyses were performed retrospectively on data collected in two previous studies in which participants performed maximum grip exertions on cylinders (diameter 38-83 mm) while grip force, normal force, and contact area were recorded. The length, width, and volar area of the hand were measured.

RESULTS

Average total normal force on cylinders was 2.3 times greater than grip force measured using a split cylinder (R2 = 65%), regardless of the handle diameter examined. The ratio of handle diameter to hand length explained 62%, 57%, and 71% of the variances in grip force, normal force, and contact area, respectively. Estimated hand area (hand length x width) had a linear relationship with measured hand area (using photographs; R2 = 91%), although it was 8% less than the measured area.

CONCLUSION

This work describes the relationship between normal force and grip force independent of handle size (for handle diameters from 38 to 83 mm). Normal force and contact area can be explained by the interaction between handle size and hand size. Hand area can be estimated by hand length times width.

APPLICATION

The quantitative relationships described in this paper can be used in the design of objects and hand tools to determine optimal handle sizes for maximizing grip force, total normal force, or contact area.

Similar motion of a hand-held object may trigger nonsimilar grip force adjustments

The tight coupling between load (L) and grip (G) forces during voluntary manipulation of a hand-held object is well established. The current study is to examine grip-load force coupling when motion of the hand with an object was either self-generated (voluntary) or externally generated. Subjects performed similar cyclic movements of different loads at various frequencies with three types of manipulations: 1) voluntary oscillation, 2) oscillating the right arm via the pulley system by the left leg (self-driven oscillation), and 3) oscillating the arm via the pulley system by another person (other-driven oscillation). During the self-generated movements: 1) the grip forces were larger and 2) grip-load force modulation was more pronounced than in the externally generated movements. The G-L adjustments are not completely determined by the mechanics of object motion; nonmechanical factors related to movement performance, for instance perceptual factors, may affect the G-L coupling.

A biomechanical analysis of applied pinch force during periodontal scaling

One of the factors associated with the high prevalence of upper extremity musculoskeletal disorders, such as carpal tunnel syndrome, among dental practitioners is the repeated high pinch force applied during periodontal scaling. The goal of this study was to determine the relationship between the pinch force applied during periodontal scaling and the forces generated at the tip of the tool. A linear biomechanical model that incorporated tool reaction forces and a calculated safety margin was created to predict the pinch force applied by experienced and inexperienced dentists during periodontal scaling. Six dentists and six dental students used an instrumented scaling tool while performing periodontal scaling on patients. Thumb pinch force was measured by a pressure sensor, while the forces developed at the instrument tip were measured by a six-axis load cell. A biomechanical model was used to calculate a safety factor and to predict the applied pinch force. For experienced dentists, the model was moderately successful in predicting pinch force (R(2)=0.59). For inexperienced dentists, the model failed to predict peak pinch force (R(2)=0.01). The mean safety margin was higher for inexperienced (4.88+/-1.58) than experienced (3.35+/-0.55) dentists, suggesting that students apply excessive force during scaling.

Sensorimotor memory for fingertip forces: evidence for a task-independent motor memory

When repetitively lifting an object with randomly varying mechanical properties, the fingertip forces reflect the previous lift. We examined the specificity of this "sensorimotor memory" by observing the effects of an isolated pinch on the subsequent lift of a known object. In this case, the pinch force was unrelated to the fingertip forces necessary to grip the object efficiently. The peak grip force used to lift the test object (4 N weight) depended on the preceding task. Compared with repetitively lifting the 4 N test object, the peak grip force was 2 N greater when a lift of the same object was preceded by a lift in which a hidden mass was attached to the object to increase the weight to 8 N. This 2 N increase in grip force also occurred when subjects lifted the 4 N test object after pinching a force transducer with a force of 8 N. Thus, similar grip forces were stored in sensorimotor memory for both tasks, and reflected subjects' use of 7.9 +/- 1.1 N to lift the 8 N object. Similar effects occurred when the preceding pinch or lift was performed with the opposite hand. The peak lift force was unaffected by the isolated pinch, suggesting that a generalized increase in fingertip and limb forces did not occur. We conclude that the sensorimotor memory is not specific for lifting an object. It is doubtful that this particular memory stores the physical properties of objects or reflects a forward internal model for predictively controlling fingertip forces.

Pathophysiological tissue changes associated with repetitive movement: a review of the evidence

Work-related musculoskeletal disorders (WMSDs) represent approximately one third of workers' compensation costs in US private industry, yet estimates of acceptable exposure levels for forceful and repetitive tasks are imprecise, in part, due to lack of measures of tissue injury in humans. In this review, the authors discuss the scope of upper-extremity WMSDs, the relationship between repetition rate and forcefulness of reaching tasks and WMSDs, cellular responses to injury in vivo and in vitro, and animal injury models of repetitive, forceful tasks. The authors describe a model using albino rats and present evidence related to tissue injury and inflammation due to a highly repetitive reaching task. A conceptual schematic for WMSD development and suggestions for further research are presented. Animal models can enhance our ability to predict risk and to manage WMSDs in humans because such models permit the direct observation of exposed tissues as well as motor behavior.

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.

Predicting the consequences of our own actions: the role of sensorimotor context estimation

During self-generated movement it is postulated that an efference copy of the descending motor command, in conjunction with an internal model of both the motor system and environment, enables us to predict the consequences of our own actions (von Helmholtz, 1867; Sperry, 1950; von Holst, 1954; Wolpert, 1997). Such a prediction is evident in the precise anticipatory modulation of grip force seen when one hand pushes on an object gripped in the other hand (Johansson and Westling, 1984; Flanagan and Wing, 1933). Here we show that self-generation is not in itself sufficient for such a prediction. We used two robots to simulate virtual objects held in one hand and acted on by the other. Precise predictive grip force modulation of the restraining hand was highly dependent on the sensory feedback to the hand producing the load. The results show that predictive modulation requires not only that the movement is self-generated, but also that the efference copy and sensory feedback are consistent with a specific context; in this case, the manipulation of a single object. We propose a novel computational mechanism whereby the CNS uses multiple internal models, each corresponding to a different sensorimotor context, to estimate the probability that the motor system is acting within each context.

The role of internal models in motion planning and control: evidence from grip force adjustments during movements of hand-held loads

We investigated the issue of whether or not the CNS makes use of an internal model of the motor apparatus in planning and controlling arm movements. In particular, we tested the ability of subjects to predict different hand-held loads by examining grip force adjustments used to stabilize the load in the hand during arm movements. Subjects grasped a manipulandum using a precision grip with the tips of the thumb and index finger on either side. The grip force (normal to the contact surfaces) and the load force (tangential to the surfaces) were measured, along with the trajectory of the hand. The manipulandum was attached to two servo-controlled linear motors used to create inertial and viscous loads as well as a composite load, including inertial, viscous, and elastic components. The form of the hand trajectory was independent of load for some subjects but varied systematically across load conditions in others. Nevertheless, under all load conditions and in all subjects, grip force was modulated in parallel with, and thus anticipated, fluctuations in load force despite the marked variation in the form of the load function. This indicates that the CNS is able to predict the load force and the kinematics of hand movement on which the load depends. We suggest this prediction is based on an internal model of the motor apparatus and external load and is used to determine the grip forces required to stabilize the load.