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Tarkeshwar Singh, Vladimir M. Zatsiorsky and Mark L. Latash

The effects of muscle fatigue on the stability of precision grasps are not well known. The purpose of the current study was to investigate the effects of exercise-induced fatigue of a digit on prehension synergies in a static precision grasp. One group of participants performed the fatiguing exercise using the thumb (group-thumb) and the second group performed the exercise using the index finger (group-index). Grasp force and load-resisting force-stabilizing synergies were weaker during fatigue for group-thumb and showed no significant change for group-index. These results indicate that fatiguing the thumb compromises the stability of the precision grasp more than when the index finger is fatigued. Our results support the idea of hierarchical organization of prehension control. We proffer an explanation of our results based on two control constructs: a) Principle of superposition. This principle states that prehension can be viewed as a superposition of two independent processes controlling the slip and the tilt of the object respectively; and b) The referent configuration hypothesis. According to this hypothesis, the neural control of actions is associated with defining a set of referent values for task-related coordinates (given an external force field) defined as the referent configuration.

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Jacob J. Sosnoff, Sae Young Jae, Kevin Heffernan and Bo Fernhall

The purpose of the current investigation was to examine the relation between cardioballistic impulse and the fluctuations in continuous isometric force production. Subjects produced isometric force via index finger flexion to constant force targets (0.5, 1 and 2 N) with and without visual feedback while beat to beat blood pressure of their middle finger was recorded. Force fluctuations were quantified using distributional statistics. The association between blood pressure oscillations and fluctuations in force output were quantified with coherence analysis. Overall, it was found that force variability (i.e., SD) increased with force level and removal of visual feedback. Coherence values between blood pressure oscillations and force fluctuations were significant and the greatest in the 8–12 Hz bandwidth. There was no effect of force magnitude on the coupling strength between blood pressure oscillations and force production. This coupling was greater in the visual condition. These data suggest that peripheral alterations in blood pressure are related to fluctuations in isometric force production independent of force level and that this interaction is influenced by visual feedback.

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Jérémy Rossi, Benjamin Goislard De Monsabert, Eric Berton and Laurent Vigouroux

The objectives of this study were to investigate the effect of handle shape on the grip force distribution in the hand and on the muscle forces during maximal power grip tasks. Eleven subjects maximally grasped 3 handles with different external shapes (circular, elliptic, and double-frustum). A handle dynamometer, equipped with both a force sensor and a pressure map, was used to record the forces exerted at the hand/handle interface. The finger and wrist joint postures were also computed from synchronized kinematic measurement. These processed data were then used as input of a biomechanical hand model to estimate muscle forces. The results showed that handle shape influences the maximal grip force, the grip force distribution, and the finger joint postures. Particularly, we observed that the elliptical shape resulted in a 6.6% lower maximal grip force compared with the circular and double-frustum handle. Concomitantly, the estimated muscle forces also varied significantly according to the handle shape, with up to 48% differences for the flexor digitorum superficialis muscle for example. Interestingly, different muscle coordination strategies were observed depending on the handle shape, therefore suggesting a potential influence of these geometrical characteristics on pathological risks such as tendonitis.

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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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Kathye E. Light and Waneen W. Spirduso

Unlike stimulus–response compatibility, which has been explored for aging effects, the motor behavior issue of response–response (R–R) compatibility has not been addressed in the gerontological literature. R–R compatibility refers to the ease with which two responses can be prepared together either simultaneously or as choice alternatives. In the present study, young, middle-aged, and elderly adult female subjects were tested in a two-choice reaction-time (RT) paradigm involving four types of finger movements paired in every possible choice combination, creating different levels of R–R compatibility. Significant age differences increased as R–R compatibility decreased. The practical significance of this study is to establish R–R compatibility as an important factor influencing task difficulty to which older adults are particularly sensitive and to encourage recognition of this factor when prescribing progressive motor-skill training in elderly clients.

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Eric G. James

The HKB model and its variants characterize bimanual coordination with fixedpoint dynamics and predict stationarity of the mean and variance of relative phase in stable coordinative states. In the current study, participants performed in-phase and antiphase coordination modes in rhythmic bimanual finger and elbow flexionextension tasks. The results of runs tests revealed that discrete relative phase was nonstationary in 49.25%, 50.25%, and 54% of time-series in the 10, 20, and 30 box runs tests, respectively. In all individual Task conditions >38% of time-series were nonstationary. These findings contradicted model predictions that the mean and variance of relative phase are stationary in bimanual coordination and distinguish the concept of dynamical stability from statistical stationarity. The findings indicated that relative phase was not attracted to a stationary fixed-point and that fluctuations in relative phase are not Gaussian white noise as in existing models of bimanual coordination.

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Bouwien C.M. Smits-Engelsman, Gerard P. Van Galen and Jacques Duysens

Ninety-four participants (age 5–93 years) performed isometric force production tasks at five different levels of their maximum voluntary contraction (MVC) with either one or two index fingers. Research questions were whether variability measures in the bimanual task condition were different compared to the unimanual condition and whether this difference showed a developmental trend. Results showed that force regulation was more demanding during bimanual tasks (33% increase in error). During development signal-to-noise ratio (SNR) increased threefold from 5–12 years of age and again 60% from 12 years to adulthood. SNR for the elderly was comparable to values of 9 to 10-year-olds. SNR decreased in the bimanual task, particularly for the older persons. For adults and elderly, optimal SNR levels were observed around 36% of their MVC. In younger children, however, the inverted U-shape in the SNR over the full range of forces was not yet present.

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Blanka Hejduková, Nasser Hosseini, Bo Johnels, Pall E. Ingvarsson, Goran Steg and Torsten Olsson

During transport of an object using the precision grip with thumb and index finger, a modulation of the grip force is needed in response to the forces evoked by the movement. We measured the grip force (GF) and the load force (LF) in 10 healthy participants moving a 640-g object forward and upward. The task was repeated with various speeds. There were considerable changes with speed of the LF trajectory but not of the GF trajectory. A loss of synergy between GF and LF appeared in fast lifts. This is in contrast to the close coupling between load force and grip force repeatedly demonstrated during simple lifts. We suggest that (a) speed should be considered as an input parameter for movement planning, and (b) regulation of GF and of LF are independent under certain conditions. We discuss whether the grip-load force synergy should be considered a special case rather than a more general principle.

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Joseph P. Stitt and Karl M. Newell

This paper presents the stochastic modeling of isometric force variability in the steady-state time series recorded from the index finger of young adults in the act of attempting to hold different levels of constant force. The isometric force time series were examined by assuming that the stochastic (random) models were linear. System identification techniques were employed to estimate the parameters of each linear model. Once the models were parameterized, the values of the estimated parameters were compared to determine if a single linear time-invariant model was applicable across the entire isometric force range. Although the overall random models were found to be nonlinear functions of the target force level, within a fixed target level, linear modeling provided adequate estimates of the underlying processes thus enabling the use of well-known linear system identification algorithms.

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Takashi Kinugasa, Hiroshi Nagasaki, Taketo Furuna and Hajime Itoh

The goal of this study was to identify methods for characterizing high-functioning older adults living in the community. The subjects were 495 older adults from the Longitudinal Interdisciplinary Study on Aging conducted by the Tokyo Metropolitan Institute of Gerontology. Physical performance measures included grip strength, walking at preferred and maximum speeds, one-leg standing with eyes open, and finger tapping rate. Performance scores were created by summing each categorical score. Consistent differences were found among age groups and genders. Scores were lower in subjects who had stroke or diabetes than in those without these conditions. These results suggest that physical performance measures have both discriminant validity and construct validity, which make them useful methods for characterizing high-functioning older persons.