Fast Force Changes Research Articles

Decoding corticospinal excitability changes during a force tracking task

Corticospinal excitability (CSE) has been extensively used as a means to probe the involvement of the primary motor cortex (M1) in a given task while subjects are at rest. For motor tasks, to prevent confounding biases due to ongoing muscle activity, CSE readouts have mainly been quantified during the preparatory period, i.e. before movement onset. Therefore, what is encoded in CSE during ongoing muscle contraction remains unclear. We addressed this issue by using a force tracking task where subjects had to generate a force output using a pinch grip in order to maintain a cursor on a target moving at different speeds, with or without explicit visual feedback about the force target. During the task, transcranial magnetic stimulation (TMS) was applied over M1 to probe CSE. Crucially, we always triggered TMS at the same applied force, so as to discard any low level effect on CSE during force generation. We found that CSE coded for the direction and speed of the force output. CSE was higher when subjects had to increase their forces compared to force decreases. In addition, fast force changes were associated with higher CSE than slower changes. Finally, CSE did not depend on the available visual feedback, suggesting that CSE is not sensitive to either explicit or implicit force requirements. Our findings suggest that probing CSE is a useful technique to investigate the role of M1 not only during movement preparation, but also during ongoing muscle contraction. Furthermore, our results show what dynamic processes occur in M1 when fingertip forces need to be continuously controlled.

Implementation and Use of a Laser-Ultrasonic System in a Deformation- and Quenching Dilatometer

State of the art of in-situ analysis on grain structure of metals during thermal and stress treatment is done by observation of the probe in a thermomechanical treatment system. Potential analysis methods are high energy x-ray scattering (e.g. in a synchrotron) or laser-ultrasonics (LUS). The most commonly used thermomechanical system, is the so called “Gleeble” from Dynamic Systems Inc., which is able to heat and load the material in a quite fast manner with extremely high heating rates, very high forces and fast force changes. There is a wide area of research and applications, though, where these capabilities are not fully required, a less complex deformation-and quenching dilatometer would often be sufficient. In this paper we will show the implementation of a LUS system in such a dilatometer and compare it to the “all inclusive” Gleeble system, pointing out benefits and downsides on different aspects, like the technical specifications, the needed footprint and more. A sketch of the full system and the beam path will show the general idea on the implementation of the LUS system into the dilatometer. We will also present first results of a thermal treatment on a metal sample suited for grain structure and phase transition analysis.

Impaired performance of patients with writer’s cramp in complex fine motor tasks

ObjectiveWriter’s cramp (WC) is a task-specific focal dystonia. WC is characterized by involuntary contractions of muscles of the hand and arm during handwriting, resulting in impaired writing with exaggerated finger forces. The generalization of symptoms to other fine motor tasks is widely discussed. The aim of the study was to determine affected fine motor aspects with an extensive testing battery. MethodsTwelve people with WC and twelve healthy controls were examined. Performance in the Jebsen-Taylor Hand Function Test, Nine-Hole-Peg Test and 2-point discrimination was evaluated. To analyze object manipulation skills, we examined grip forces, temporal measures and other aspects of force control during (1) lifting actions with variations of weight and surface (2) cyclic movements (3) visuomotor tracking (4) fast force changes and (5) grip strength. In addition, correlation between the dependent variables of the fine motor tasks and the handwriting deficits was assessed. ResultsWC patients had impaired performance in the visuomotor tracking task (root mean square error (RMSE), p = 0.03 and time lag, p = 0.05) and the fast force changes (frequency, p = 0.01). There were no statistically significant group differences in the other tasks. We found a correlation between the RMSE of the tracking task and the time needed to write the test sentence (r = 0.643, p = 0.01). ConclusionWC patients revealed abnormalities in complex fine motor performance in tasks with high demands on coordination and visual components, specifically in tracking and fast force changes. SignificanceThis suggests a deficit in visuomotor integration, coordination and cognitive aspects related to movement processing particularly with respect to low forces. These insights may prove useful in the development of targeted training approaches.

An Electromagnetic Tweezers for Studying Fast Protein Folding Dynamics

Protein folding under force does mechanical work, however, for that work to be physiologically relevant, it must be delivered fast enough to compete with the speed of molecular motors. For instance, it is not known if titin domains can fold fast enough to follow the myosin motors, thus contributing to the mechanical power during muscle contraction. Accurate measurement of the initial folding velocity requires applying fast force changes in order to resolve the initial folding events. Yet, most force-spectroscopy instruments change the force by moving mechanical components, introducing a time lag-at least 100 ms in the case of magnetic tweezers-where these early molecular events are lost. Here, we present a newly developed electromagnetic tweezers instrument, where the magnetic field is generated by an electromagnetic head. The force acting on the superparamagnetic beads depends on the electric current flowing through the head, and can therefore be changed as fast as current can be supplied. Based on Karlqvist approximation of the field created by an electromagnetic head, we work out a closed analytical expression that relates the force on the superparamagnetic beads with the electric current and the vertical distance of the head to the bead. We calibrate our instrument by relating this force expression with the step-sizes of protein L unfolding/refolding events, allowing to reach forces of 50 pN in 5 ms, when working at a distance of 200 μm and an electric current of 600 mA. This force range can potentially be increased up to 500 pN by using thinner fluid chambers, approaching the AFM force range, but with the exquisite stability of magnetic tweezers. Our instrument development offers a unique tool for studying how proteins respond mechanically to force perturbations as those experienced in vivo.

Fine motor skills predict performance in the Jebsen Taylor Hand Function Test after stroke

ObjectiveTo determine factors characterizing the differences in fine motor performance between stroke patients and controls. To confirm the relevance of the factors by analyzing their predictive power with regard to the Jebsen Taylor Hand Function Test (JTHFT), a common clinical test of fine motor control. MethodsTwenty-two people with slight paresis in an early chronic phase following stroke and twenty-two healthy controls were examined. Performance on the JTHFT, Nine-Hole Peg Test and 2-point discrimination was evaluated. To analyze object manipulation skills, grip forces and temporal measures were examined during (1) lifting actions with variations of weight and surface (2) cyclic movements (3) predictive/reactive catching tasks. Three other aspects of force control included (4) visuomotor tracking (5) fast force changes and (6) grip strength. ResultsBased on 9 parameters which significantly distinguished fine motor performance in the two groups, we identified three principal components (factors): grip force scaling, motor coordination and speed of movement. The three factors are shown to predict JTHFT scores via linear regression (R2=0.687, p<0.001). ConclusionsWe revealed a factor structure behind fine motor impairments following stroke and showed that it explains JTHFT results to a large extend. SignificanceThis result can serve as a basis for improving diagnostics and enabling more targeted therapy.

Design and Evaluation of a Fiber-Optic Grip Force Sensor with Compliant 3D-Printable Structure for (f)MRI Applications

Grip force sensors compatible with magnetic resonance imaging (MRI) are used in human motor control and decision-making research, providing objective and sensitive behavioral outcome measures. Commercial sensors are expensive, cover limited force ranges, rely on pneumatic force transmission that cannot detect fast force changes, or are electrically active, which increases the risk of electromagnetic interference. We present the design and evaluation of a low-cost, 3D-printed, inherently MRI-compatible grip force sensor based on a commercial intensity-based fiber-optic sensor. A compliant monobloc structure with flexible hinges transduces grip force to a linear displacement captured by the fiber-optic sensor. The structure can easily be adapted for different force ranges by changing the hinge thickness. A prototype designed for forces up to 800 N was manufactured and showed a highly linear behavior (nonlinearity of 2.37%) and an accuracy of 1.57% in a range between zero and 500 N. It can be printed and assembled within one day and for less than $300. Accurate performance was confirmed, both inside and outside a 3 T MRI scanner within a pilot study. Given its simple design allowing for customization of sensing properties and ergonomics for different applications and requirements, the proposed grip force handle offers researchers a valuable scientific tool.

The Development of the Fastest Isometric Grip Force Changes and Clinical Relevance

Fast force changes with hand-held objects are an important prerequisite for object manipulation in everyday life. This study examines the development of fastest isometric force changes in a precision grip. One hundred sixty-five children (76 girls, 89 boys), 3-14 years, without neurological abnormalities increased and decreased repetitively isometric grip forces as rapidly as possible by their dominant hand using a small cylindrical pinch grip object (20 g). The frequency of repetitive force changes increased in a linear way from the age of 4 years until about 12 years by 0.23 Hz per year (r2 = .54) without noticeable gender difference. The ratio of the duration of force increase and decrease slightly declined from 1.05 (4-year-olds) to 0.95 (11- to 14-year-olds). The development of force amplitudes and the mean force were more variable. Temporal parameters become less variable with age, whereas force parameters become more variable. In particular, the temporal parameters of fastest isometric force changes are best predictors for developmental changes. Fastest isometric force changes may be an important basic capacity for fast object manipulation, particularly in young children and in children with movement disorders.

Approaches to analysis of handwriting as a task of coordinating a redundant motor system

We consider problems of motor redundancy associated with handwriting using the framework of the uncontrolled manifold (UCM) hypothesis. Recent studies of finger coordination during force production tasks have demonstrated that the UCM-hypothesis provides a fruitful framework for analysis of multi-finger actions. In particular, it has been shown that during relatively fast force changes, finger force variance across trials is structured such that a time pattern of total moment produced by the fingers with respect to a point between the two most lateral fingers involved in the task is stabilized while the time pattern of total force may be destabilized. The findings of selective moment stabilization have been interpreted as being conditioned by the experience with everyday motor tasks that commonly pose more strict requirements to stabilization of total moment than to stabilization of total force. We discuss implications of these findings for certain features of handwriting seen in elderly, children, patients with neurological disorders, and forgers.

Conditions for excitatory or inhibitory masseteric reflexes elicited by tooth pressure in man

The reflex responses evoked by slowly rising pressure (‘push’) stimuli to an upper lateral incisor tooth in human masseter muscle were studied. Factors such as the preload (the static force applied to the tooth by the stimulus probe before the start of the push stimulus) and the shape of the stimulus wave; affected the outcome of the reflex response. When the stimulating probe did not apply preload to the tooth before the push stimulus took place, the force profile exhibited a large fast component as the probe took up the ‘slack’ in the periodontium. The fast component in the force profile was found to be responsible for inducing the inhibitory reflex (sole inhibitory reflex). When preload was applied, the size of the fast component in the force profile was reduced and the change in force rate became slower and smoother. This slower stimulus profile induced the sole excitatory reflex significantly more often (58% vs 21%) and the sole inhibitory reflex significantly less often (15% vs 52%) than in the experiments that used no preload. The shape of the stimulus wave that drove the stimulus probe was also of importance. Provided that a 0.5-N preload was applied to the tooth, the smoothest stimulus wave induced the sole excitatory reflex most often. Fitting a rubber attachment to the tip of the probe made the push-force profile even smoother and thence more successful in inducing the sole excitatory reflex. Furthermore, the rubber tip reduced the possibility of the probe slipping off the tooth due to small and unavoidable movements of the participant's head. It is concluded that the periodontal mechanoreceptors can induce both excitatory and inhibitory reflexes on the jaw closers. The excitatory reflex becomes dominant when a smooth force is applied with preload. The inhibitory reflex becomes dominant when fast-force changes are applied on the tooth and/or no preload is used.

Residual control of isometric finger forces in hemiparetic patients. Evidence for dissociation of performance deficits

Dissociation of performance in the control of isometric finger forces was observed in 6 patients with left-sided unilateral lesions involving sensorimotor areas. In a precision task subjects had to maintain, under visual feedback, a low constant force (2.5 N) while holding a force transducer between thumb and index finger. Performance was compared with that in a speed task, where changes had to be produced, as rapidly as possible, between two prescribed forces (6.25 and 18.75 N). While there were severe deficits in fast force changes corresponding to the functional impairment of the right hand in all patients, right hand maintenance of low isometric force remained intact in the same patients.