A dynamic model of the forearm including fatigue

Exhaustion of Skeletal Muscle Fibers Within Seconds: Incorporating Phosphate Kinetics Into a Hill-Type Model

Initiated by neural impulses and subsequent calcium release, skeletal muscle fibers contract (actively generate force) as a result of repetitive power strokes of acto-myosin cross-bridges. The energy required for performing these cross-bridge cycles is provided by the hydrolysis of adenosine triphosphate (ATP). The reaction products, adenosine diphosphate (ADP) and inorganic phosphate (P i ), are then used-among other reactants, such as creatine phosphate-to refuel the ATP energy storage. However, similar to yeasts that perish at the hands of their own waste, the hydrolysis reaction products diminish the chemical potential of ATP and thus inhibit the muscle's force generation as their concentration rises. We suggest to use the term "exhaustion" for force reduction (fatigue) that is caused by combined P i and ADP accumulation along with a possible reduction in ATP concentration. On the basis of bio-chemical kinetics, we present a model of muscle fiber exhaustion based on hydrolytic ATP-ADP-P i dynamics, which are assumed to be length- and calcium activity-dependent. Written in terms of differential-algebraic equations, the new sub-model allows to enhance existing Hill-type excitation-contraction models in a straightforward way. Measured time courses of force decay during isometric contractions of rabbit M. gastrocnemius and M. plantaris were employed for model verification, with the finding that our suggested model enhancement proved eminently promising. We discuss implications of our model approach for enhancing muscle models in general, as well as a few aspects regarding the significance of phosphate kinetics as one contributor to muscle fatigue.

Finger Tendon Travel Associated with Sequential Trigger Nail Gun Use

BACKGROUND

Pneumatic nail guns used in wood framing are equipped with one of two triggering mechanisms. Sequential actuation triggers have been shown to be a safer alternative to contact actuation triggers because they reduce traumatic injury risk. However, the sequential actuation trigger must be depressed for each individual nail fired as opposed to the contact actuation trigger, which allows the trigger to be held depressed as nails are fired repeatedly by bumping the safety tip against the workpiece. As such, concerns have been raised about risks for cumulative trauma injury, and reduced productivity, due to repetitive finger motion with the sequential actuation trigger.

PURPOSE

This study developed a method to predict cumulative finger flexor tendon travel associated with the sequential actuation trigger nail gun from finger joint kinematics measured in the trigger actuation and productivity standards for wood-frame construction tasks.

METHODS

Finger motions were measured from six users wearing an instrumented electrogoniometer glove in a simulation of two common framing tasks-wall building and flat nailing of material. Flexor tendon travel was calculated from the ensemble average kinematics for an individual nail fired.

RESULTS

Finger flexor tendon travel was attributable mostly to proximal interphalangeal and distal interphalangeal joint motion. Tendon travel per nail fired appeared to be slightly greater for a wall-building task than a flat nailing task. The present study data, in combination with construction industry productivity standards, suggest that a high-production workday would be associated with less than 60 m/day cumulative tendon travel per worker (based on 1700 trigger presses/day).

CONCLUSION AND APPLICATIONS

These results suggest that exposure to finger tendon travel from sequential actuation trigger nail gun use may be below levels that have been previously associated with high musculoskeletal disorder risk.

The effects of protracted graphomotor tasks on tripod pinch strength and handwriting performance in children with dysgraphia

PURPOSE

To examine the impact of prolonged graphomotor tasks on tripod-pinch strength and on handwriting process and product measures of children with dysgraphia and typical peers.

METHOD

Participants were 51 children in third to fifth grades, divided into two groups: 23 children with dysgraphia and 28 typical peers, as determined by the Handwriting Proficiency Screening Questionnaire. The procedure included two sessions, with a 15-min break between sessions. In each session, the participants performed two tasks: the visual-motor control subtest of Bruininks-Oseretsky and a handwriting copying task, both performed on an electronic tablet as part of the Computerised Penmanship Evaluation Tool. Tripod pinch strength was evaluated before and after each session.

RESULTS

Significantly lower tripod-pinch strength was found among children with dysgrphia in comparison to typically developed peers. This deterioration in tripod-pinch strength was associated with a significant deterioration in handwriting process and product measures along the protracted task.

CONCLUSIONS

Pinch force that is required for mastering the handwriting tool needs to be considered in the evaluation process of children with dysgraphia. Combining the pinch factor with an evaluation of handwriting's process and product may supply a better insight about the child's deficits and assist in focusing treatment objectives.

Muscle contribution to elbow joint valgus stability

Repetitive valgus stress of the elbow can result in excessive strain or rupture of the native medial ulnar collateral ligament (MUCL). The flexor-pronator mass (FPM) may be particularly important for elbow valgus stability in overhead-throwing athletes. The aim of this study was to identify the relative contribution of each muscle of the FPM--that is, the flexor carpi ulnaris (FCU), flexor digitorum superficialis (FDS), flexor carpi radialis (FCR), and pronator teres (PT)--and of the extensor-supinator mass, including the extensor carpi ulnaris (ECU), extensor digitorum communis (EDC), extensor carpi radialis longus and brevus, and brachioradialis, to elbow valgus stability at 45 degrees and 90 degrees of elbow flexion angles. Eight fresh-frozen elbow specimens (mean age at death, 73.75 +/- 14.07 years) were tested. With the skin and subcutaneous tissue removed but all muscles left intact, each individual muscle of the FPM and extensor-supinator mass was loaded at 3 levels of force. During loading, strain on the MUCL and the kinematics of the elbow were measured simultaneously. Kinematic measurements were later repeated when the MUCL was fully cut. At 45 degrees and 90 degrees of elbow flexion, individual loading of the FCU, FDS, and FCR caused significant relief to the MUCL whereas the PT produced no significant change. Furthermore, of these flexor muscles, the FCU provided the greatest MUCL relief at both 45 degrees and 90 degrees . In contrast, loading of the ECU at 45 degrees of elbow flexion produced a significant increase in MUCL strain. All FPM muscles caused significant elbow varus movement at both 45 degrees and 90 degrees when loaded individually. At 90 degrees , the FCU created more motion than both the FCR and PT but not the FDS, and the FDS created more motion than the PT. The EDC and ECU created significant valgus movement at 45 degrees and 90 degrees , which became insignificant when the MUCL was transected. Our study suggested that the FCU, FDS, and FCR may function as dynamic stabilizers, with the FCU being the primary stabilizer for elbow valgus stability, incorporating with the MUCL for all tested joint configurations. Our findings also suggest that the ECU and EDC increased MUCL strain and elbow valgus movement at both 45 degrees and 90 degrees .

Dielectric elastomers as actuators for upper limb prosthetics: challenges and opportunities

Recent research has indicated that consumers of upper limb prostheses desire lighter-weight, anthropomorphic devices. The potential of dielectric elastomer (DE) actuators to better meet the design priorities of prosthesis users is explored. Current challenges are critically reviewed with respect to (1) durability, (2) precision control, (3) energy consumption, and (4) anthropomorphic implementation. The key points arising from the literature review are illustrated with empirical examples of the strain performance and durability of one of the most popular DEs, VHB 4910. Practical application of DE actuators in powered upper extremity prosthetics is at present impeded by poor durability and susceptibility to air-borne contaminants, unreliable control owing to viscoelasticity, hysteresis, stress relaxation and creep mechanisms, high voltage requirements, and insufficient stress and strain performance within the confines of anthropomorphic size, weight, and function. Our review suggests that the implementation of DE actuators in powered upper extremity prosthetics is not feasible at present but worthy of reevaluation as the materials advance.

Effect of simulated muscle activity on distal radioulnar joint loading in vitro

This study investigated the relationship between simulated forearm muscle loads and the joint reaction force in the distal radioulnar joint using an in vitro model. Seven fresh frozen cadaveric specimens were mounted in an upper extremity joint simulator capable of applying pneumatic loads to various (muscle) tendons while restraining the forearm in the three positions of pronation, supination, and neutral rotation. Loads were applied to model four forearm muscles (biceps, pronator teres, pronator quadratus, and supinator) in 10 N increments ranging from 10 N to 80 N for the biceps and pronator teres and in 10 N increments from 10 N to 50 N for the pronator quadratus and the supinator. Distal ulnar arthroplasty was performed on each specimen with a custom instrumented ulnar head replacement implant that quantified loads (via strain gauge instrumentation). The relationship between increasing muscle load and joint load was found to be positive and quasilinear in most cases. The biceps had the greatest influence on the distal radioulnar joint reaction force with a joint force in the range of 8% to 33% of the applied muscle load. The pronator teres, supinator, and pronator quadratus were less influential with a joint reaction force ranging between 6% to 19%, 4% to 9% and 2% to 10% of the applied muscle load, respectively.

Biomechanics of the flexor tendons

This article examines basic tendon biomechanics, the anatomy and mechanics of digital flexor tendons, and the digital flexor pulley system. It also explores the various models that have tried to simulate the motion of the flexor tendons and several testing modalities that have been used. Finally, clinical applications are considered, including the biomechanics of flexor tendon repairs and tendon transfers. As we reach limits in the care of flexor tendon injuries, research into molecular, biochemical, and micromechanical methods of tendon repair will become the forefront of future investigation.

Grip forces of the fingertips

OBJECTIVE

A model has been developed for the power grip on a cylindrical handle. The model gives estimates of the forces acting on the fingertips as functions of total grip force, diameter of the handle, and hand size.

DESIGN

The data in the literature were used to estimate the parameters of the model. An experiment was conducted to verify that the distribution of the forces is independent of the force level.

BACKGROUND

In the authors' opinion, the outcomes of the various experiments described in the literature can be represented in a compact model without too much information loss.

METHODS

The forces acting on the fingertips of 10 subjects were measured with three different handle diameters and five grip force levels.

RESULTS

The force sharing turned out to be independent of the grip force and the diameter of the handle.

CONCLUSIONS

It is advantageous to shape the handle so that the local diameters at the fingers are proportional to the finger lengths.

RELEVANCE

Knowing the force distribution of the power grip is important in design of handles. Experimental data presented in a concise form is needed also in the development of biomechanical models.