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Mark Holten Mora-Jensen, Pascal Madeleine and Ernst Albin Hansen

Index finger tapping is a relatively simple motor task that is related to various everyday activities such as computer work and playing musical instruments. Furthermore, the task is widely applied in studies of both healthy individuals ( Hammond & Gunasekera, 2008 ; Hansen & Ohnstad, 2008

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Laurent Vigouroux, Mathieu Domalain and Eric Berton

The objective of this study was to identify the impact of modifying the object width on muscle and joint forces while gripping objects. The experimental protocol consisted to maintain horizontally five objects of different widths (3.5, 4.5, 5.5, 6.5, and 7.5 cm) with a thumb–index finger grip. Subjects were required to grasp spontaneously the object without any instruction regarding the grip force (GF) to apply. A biomechanical model of thumb–index finger pinch was developed to estimate muscle and joint forces. This model included electromyography, fingertip force, and kinematics data as inputs. The finger joint postures and the GF varied across the object widths. The estimated muscle forces also varied significantly according to the object width. Interestingly, we observed that the muscle force/GF ratios of major flexor muscles remain particularly stable with respect to the width whereas other muscle ratios differed largely. This may argue for a control strategy in which the actions of flexors were preserved in spite of change in joint postures. The estimated joint forces tended to increase with object width and increased in the distal–proximal sense. Overall, these results are of importance for the ergonomic design of handheld objects and for clinical applications.

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Adriana V. Savescu, Mark L. Latash and Vladimir M. Zatsiorsky

This article proposes a technique to calculate the coefficient of friction for the fingertip– object interface. Twelve subjects (6 males and 6 females) participated in two experiments. During the first experiment (the imposed displacement method), a 3-D force sensor was moved horizontally while the subjects applied a specified normal force (4 N, 8 N, 12 N) on the surface of a sensor covered with different materials (sandpaper, cotton, rayon, polyester, and silk).The normal force and the tangential force (i.e., the force due to the sensor motion) were recorded. The coefficient of frictiond) was calculated as the ratio between the tangential force and the normal force. In the second experiment (the beginning slip method), a small instrumented object was gripped between the index finger and the thumb, held stationary in the air, and then allowed to drop. The weight (200 g, 500 g, and 1,000 g) and the surface (sandpaper, cotton, rayon, polyester, and silk) in contact with the digits varied across trials. The same sensor as in the first experiment was used to record the normal force (in a horizontal direction) and the tangential force (in the vertical direction). The slip force (i.e., the minimal normal force or grip force necessary to prevent slipping) was estimated as the force at the moment when the object just began to slip. The coefficient of friction was calculated as the ratio between the tangential force and the slip force. The results show that (1) the imposed displacement method is reliable; (2) except sandpaper, for all other materials the coefficient of friction did not depend on the normal force; (3) the skin–sandpaper coefficient of friction was the highest µd = 0.96 ± 0.09 (for 4-N normal force) and the skin–rayon rayon coefficient of friction was the smallest µd = 0.36 ± 0.10; (4) no significant difference between the coefficients of friction determined with the imposed displacement method and the beginning slip method was observed. We view the imposed displacement technique as having an advantage as compared with the beginning slip method, which is more cumbersome (e.g., dropped object should be protected from impacts) and prone to subjective errors owing to the uncertainty in determining the instance of the slip initiation (i.e., impeding sliding).

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David A. Rosenbaum, Ruud J.G. Meulenbroek, Jonathan Vaughan and Catherine Elsinger

The hypothesis introduced by Smeets and Brenner concerning the perpendicular approach of the thumb and index finger during grasping has heuristic value, but it also has limitations. Among the limitations are the following: (a) the approach parameter is not directly testable and it is unclear how the values of deceleration at contact and movement time are set theoretically; (b) it is questionable that motion of the thumb and index finger are independent; (c) reliance on the minimum-jerk account ignores critiques of that account; and (d) the model begs the question of how the effectors proximal to the index finger and thumb are controlled. We briefly review an alternative model that can handle these challenges.

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S. L. Hong, M-H. Lee and K.M. Newell

This experiment examined the magnitude and structure of force variability in isometric index finger force production tasks at 5, 15, 25, 35, 45, 55, 65, 75, 85, and 95% of maximal force in two different finger orientations. In the finger flexion task, the participants generated a downward isometric force through index finger flexion. In the finger abduction task, isometric force was generated by adducting the index finger (mediolateral motion of the middle finger and forearm were restricted). The task-related, normal force (Fz) and tangential forces (Fx and Fy) were collected with a three-dimensional force transducer. The standard deviation (SD) of the task-related force output (Fz) increased exponentially with force level. With increasing force level, approximate entropy (ApEn, a measure of irregularity) of Fz followed an inverted-U function for finger flexion, but decreased linearly in finger abduction. However, changes in the ApEn of the tangential forces were generally opposite to that of Fz, revealing compensations in the irregularity of force output between force dimensions. The findings provide evidence that force variability is related to muscle force-length characteristics (Feldman, 1966; Gottlieb & Agarwal, 1988).

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Steven Morrison and Karl M. Newell

The relation between limb stiffness and postural tremor in the upper arm was investigated during a pointing task. The task goal was to minimize the amount of motion (tremor) at the index finger under levels of increasing limb stiffness. This study investigated the influence of increasing limb stiffness on the pattern of intra- and interlimb dynamics. The frequency profile of the tremor for all limb segments across all conditions displayed two peaks, one between 2-4 Hz and another between 8-12 Hz. A third, higher frequency component (20-22 Hz) was present in the index finger. Increasing limb stiffness through voluntary co-contraction of antagonistic muscle pairs effectively constrained the segments of the upper limb to increasingly operate as a single biomechanical degree of freedom. Higher levels of limb stiffness typically led to an increase in the frequency and power of the 2-4 and 8-12 Hz peaks. There was also a decrease in the frequency of the 20-22 Hz component of finger tremor. The act of reducing the effective degrees of freedom in joint space through voluntarily stiffening of the upper limbs also resulted in decreased performance as determined by an increase in finger tremor. In the preferred, natural level of limb stiffness, specific intralimb segment relations were observed but there was no significant interlimb coupling. The intralimb segment correlations were characterized by compensatory (out of phase) coupling between the upper arm/forearm and hand/index finger segment pairs of each limb that were organized about the action of the wrist joint. Increasing the degree of limb stiffness led to a decrease in the level of intralimb coupling. The findings suggest that the most efficient mechanism for reducing tremor at the periphery is that of compensatory coupling between relevant intralimb segments with a low level of limb stiffness.

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Jin Qin, Matthieu Trudeau, Bryan Buchholz, Jeffrey N. Katz, Xu Xu and Jack T. Dennerlein

Upper extremity kinematics during keyboard use is associated with musculoskeletal health among computer users; however, specific kinematics patterns are unclear. This study aimed to determine the dynamic roles of the shoulder, elbow, wrist and metacarpophalangeal (MCP) joints during a number entry task. Six subjects typed in phone numbers using their right index finger on a stand-alone numeric keypad. The contribution of each joint of the upper extremity to the fingertip movement during the task was calculated from the joint angle trajectory and the Jacobian matrix of a nine-degree-of-freedom kinematic representation of the finger, hand, forearm and upper arm. The results indicated that in the vertical direction where the greatest fingertip movement occurred, the MCP, wrist, elbow (including forearm) and shoulder joint contributed 10.2%, 55.6%, 27.7% and 6.5%, respectively, to the downward motion of the index finger averaged across subjects. The results demonstrated that the wrist and elbow contribute the most to the fingertip vertical movement, indicating that they play a major role in the keying motion and have a dynamic load beyond maintaining posture.

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Waneen W. Spirduso, Britta G. Schoenfelder-Zohdi, Jonghwan Choi and Susan M. Jay

This study investigated age-related differences in tapping speed with respect to warm-up and fatigue effects and also with respect to task complexity. An additional purpose was to determine the site of age-related slowing in stationary tapping. Adult females from three different age groups were asked to tap as fast as possible for 25 s with a specified digit combination by depressing microswitches on one or two metal boxes that were mounted on a data acquisition board. All groups showed a warm-up period during the first block, reached their peak tapping speed during the second block, and then gradually fatigued, as indicated by a decreasing number of taps. These findings suggest that to assess true tapping speed, a trial should not last more than 15 s, or the results may be confounded by fatigue effects. It was found that tapping with the thumb and index finger simultaneously is more difficult than tapping with one or both index fingers, regardless of age.

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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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Justin W.L. Keogh, Steve Morrison and Rod Barrett

The current study investigated the effect of 2 different types of unilateral resistance training on the postural tremor output of 19 neurologically healthy men age 70–80 yr. The strength- (n = 7) and coordination-training (n = 7) groups trained twice a week for 6 wk, performing dumbbell biceps curls, wrist flexions, and wrist extensions, while the control group (n = 5) maintained their normal activities. Changes in index-finger tremor (RMS amplitude, peak, and proportional power) and upper limb muscle coactivation were assessed during 4 postural conditions that were performed separately with the trained and untrained limbs. The 2 training groups experienced significantly greater reductions in mean RMS tremor amplitude, peak, and proportional tremor power 8–12 Hz and upper limb muscle coactivation, as well as greater increases in strength, than the control group. These results further demonstrate the benefits of resistance training for improving function in older adults.