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Kelly J. Cole

In the target article Mark Latash has argued that there is but a single bona-fide theory for hand motor control (referent configuration theory). If this is true, and research is often phenomenological, then we must admit that the science of hand motor control is immature. While describing observations under varying conditions is a crucial (but early) stage of the science of any field, it is also true that the key to maturing any science is to vigorously subject extant theories and budding laws to critical experimentation. If competing theories are absent at the present time is it time for scientists to focus their efforts on maturing the science of hand motor control through critical testing of this long-standing theory (and related collections of knowledge such as the uncontrolled manifold)?

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Ross H. Sanders

The main purpose of this study was to develop a model for calculating forces produced by a swimmer’s hand, with the thumb adducted, accelerating in the direction of flow. The model included coefficients to account for the velocity and acceleration of the hand. These coefficients were designed to calculate forces in the direction opposite the motion (drag) and two components of lift orthogonal to the direction of motion. To determine the coefficients, three-dimensional forces acting on a resin cast of a swimmer’s hand were recorded while accelerating the hand from rest to 0.45 m · s−1 and 0.6 m · −1 in a towing tank. The hand orientation was varied throughout the entire range at 5° increments. Three-dimensional surfaces describing the magnitude of the coefficients as functions of pitch and sweepback angle were produced. It was found that acceleration coefficients as well as velocity coefficients are required for accurate modeling of the forces produced by the hand in swimming. The forces generated by the hand are greatest when pitch angles approach 90° due to the large contribution by the drag component. However, at pitch angles near 45° and sweepback angles near 45° and 135°, lift forces contribute substantially.

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Josje van Houwelingen, Sander Schreven, Jeroen B.J. Smeets, Herman J.H. Clercx, and Peter J. Beek

In this paper, a literature review is presented regarding the hydrodynamic effects of different hand and arm movements during swimming with the aim to identify lacunae in current methods and knowledge, and to distil practical guidelines for coaches and swimmers seeking to increase swimming speed. Experimental and numerical studies are discussed, examining the effects of hand orientation, thumb position, finger spread, sculling movements, and hand accelerations during swimming, as well as unsteady properties of vortices due to changes in hand orientation. Collectively, the findings indicate that swimming speed may be increased by avoiding excessive sculling movements and by spreading the fingers slightly. In addition, it appears that accelerating the hands rather than moving them at constant speed may be beneficial, and that (in front crawl swimming) the thumb should be abducted during entry, catch, and upsweep, and adducted during the pull phase. Further experimental and numerical research is required to confirm these suggestions and to elucidate their hydrodynamic underpinnings and identify optimal propulsion techniques. To this end, it is necessary that the dynamical motion and resulting unsteady effects are accounted for, and that flow visualization techniques, force measurements, and simulations are combined in studying those effects.

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Zong-Ming Li, Shouchen Dun, Daniel A. Harkness, and Teresa L. Brininger

The purpose of the current study was to examine motion enslaving characteristics of multiple fingers during isolated flexion of the distal interphalangeal joints. Because the distal interphalangeal joints are flexed by multiple tendons of the single flexor digitorum profundus, the current experimental design provided a unique advantage to understand inter-finger enslaving effects due to the flexor digitorum profundus. Eight subjects were instructed to flex the distal inter-phalangeal joint of each individual finger from the fully extended position to the fully flexed position as quickly as possible. Maximal angular displacements, velocities, or accelerations of individual fingers were used to calculate the enslaving effects. An independence index, defined as the ratio of the maximal displacement of a master finger to the sum of the maximal displacements of the master and slave fingers, was used to quantify relative independence of each finger. The angular displacements of the index, middle, ring, and little fingers were 68.6° (±7.7), 68.1° (±10.1), 68.1° (±9.7), and 74.7° (±13.3), respectively. The motion of a master finger was invariably accompanied by motion of 1 or 2 slave fingers. Angular displacements of master and slave fingers increased to maximum values with time monotonically. Velocity curves demonstrated bell-shaped profile, and the acceleration curves were sinusoidal. Enslaving effects were generated mainly on the neighboring fingers. The amount of enslaving on the middle and ring fingers exceeded more than 60% of their own maximum angular displacements when a single adjacent finger moved. The index finger had the highest level of independence as indicated by the lowest enslaving effects on other fingers or by other fingers. The independence indices of the index, middle, ring, and little fingers were 0.812 (±0.070), 0.530 (±0.051), 0.479 (±0.099), and 0.606 (±0.148), respectively. In all tasks, motion of slave fingers always lagged with respect to the master finger. Time delays, on average, ranged from 7.8 (±5.0) to 35.9 (±22.1) ms. Our results suggest that there exist relatively large enslaving effects among the compartments of the flexor digitorum profundus, and functional independence of fingers in daily activities is likely enhanced through synergistic activities of multiple muscles, including flexors and extensors.

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Niranjan Chakrabhavi and Varadhan SKM

The hand is considered to be the most dexterous part of the human body, yet it comes with its limitations. A task involving a movement of an instructed finger often involves a movement of noninstructed fingers. This phenomenon is known as finger interdependence 1 or enslavement effect. 2 The

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Monica A. Perez

Most of our daily actions involve movements of the hand. The neuronal pathway contributing to the control of hand movements are complex and not yet completely understood. Recent studies highlight how task-dependent changes in cortical and subcortical pathways driven by contralateral and ipsilateral influences may open avenues to further understand the complexity of hand actions in healthy and disease. In the following section studies using transcranial magnetic and electrical stimulation in healthy subjects and in individuals with chronic incomplete spinal cord injury will be highlighted to further understand neuronal pathways involved in the control of voluntary activity by hand muscles.

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Duane V. Knudson

This study examined the pattern of forces and peak loads on the hands of six advanced and six intermediate level male tennis players as they performed one-handed backhand drives. Two miniature load cells were mounted on a midsized graphite racket. The force on the thenar and hypothenar eminences of the hand were sampled at 1000 Hz. Forces on the thenar eminence in preparation for impact were significantly larger and less variable for the advanced subjects. Postimpact peak forces did not differ across skill level and were smaller than the loads reported for forehand drives. The significantly lower thenar forces the intermediate subjects used in preparation for impact may provide less resistance to the acceleration of the racket created by ball impact. A large impact acceleration may be related to a rapid stretch of the wrist extensors, which has been hypothesized to be the cause of tennis elbow.

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Tomoko Aoki and Koji Kadota

A decline in manual dexterity is commonly observed in older adults. They often encounter difficulty in daily activities such as opening medicine bottles ( Keram & Williams, 1988 ) and removing coins from a wallet ( Shiffman, 1992 ). It has been reported that a broad range of hand functions required

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Daniela Corbetta, Rebecca F. Wiener, Sabrina L. Thurman, and Emalie McMahon

significant transition in the early life of the infant and triggers a cascade of new behaviors in subsequent weeks and months. Objects that were predominantly examined visually prior to this moment can now be touched, explored manually, and eventually grasped and manipulated. In turn, those hand

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Marzie Balali, Shahab Parvinpour, and Mohsen Shafizadeh

is required (e.g., perturbations). In other words, the motor system, through synergy, learns how to covary (share) the components effectively to stabilize the performance outcome ( Gelfand & Latash, 1998 ). For example, ball catching with one or two hands requires a combination of coordination and