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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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Shigetada Kudo, Yuji Matsuda, Yoshihisa Sakurai, and Yasushi Ikuta

are generated around the object than around a moving object without acceleration, and the vortices induce a large magnitude of hydrodynamic forces on the object. 4 , 5 With regard to a hand during swimming, Rouboa et al 6 and Samson et al 7 reported the unsteady effect on hydrodynamic forces acting

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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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Mark L. Latash

trials patients can perform. Although accuracy of performance is defined by V ORT with no effects from V UCM , stability is not synonymous with accuracy and cannot be estimated based on V ORT only. Consider the following example. Imagine that you walk along the hallway with a mug of coffee in hand

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Masanori Sakamoto and Hirotoshi Ifuku

a rake), to wield the stick with their hand and hook small boxes in order to pick up or move them for 15 min. After using the stick, the participants’ perceived midpoint of the forearm moved toward the distal. Using a similar stick for dozens of minutes also induced an increase in perceived distance

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Valters Abolins and Mark L. Latash

When we want to move only one finger, other fingers of the hand move unintentionally. A similar effect can be observed in force production tasks: When one finger presses against a stop purposefully, the other fingers of the hand also produce pressing force. Similar phenomena are also observed when

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Mark L. Latash, Shirin Madarshahian, and Joseph M. Ricotta

synergies stabilizing salient variables across different tasks ranging from accurate hand and finger actions to whole-body actions performed while standing by patients with early stage Parkinson’s disease (PD; reviewed in Latash & Huang, 2015 ). Those studies have revealed that significant changes in