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Andrea Biscarini

We have developed a 2-D analytical biomechanical model for monoarticular open kinetic-chain exercises with lever selectorized equipment, and different relative placement between the joint center of rotation (J) and the center of rotation (C) of the resistance input lever (“off-center” exercises). All the relevant geometrical aspects of such exercises have been characterized: the change with the joint angle of the distance between the resistance pad (P) and J, and of the angle between CP and JP (i.e., the angle between the resistance input lever and the exercising limb). These changes may strongly affect the joint load and the muscle torque in inverse dynamic problems, given the joint kinematics and the mass of the selected weight stack. Therefore, the muscle torque, the shear and axial components of the joint load have been calculated analytically as a function of the relative positioning of C and J, and the length CP, in addition to the parameters that define the joint kinematics, the equipment mechanics, and the external load. From these results we have derived the optimal cam profiles for “off-center” exercises, as well as the geometrical “off-center” setting that minimizes the shear component of the tibiofemoral joint load in leg extension equipment.

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Stan C.A.M. Gielen

EMG recordings are frequently used to obtain a better understanding in the coordination of movements. However, EMG activity is made up by the weighted summation of activity of many motor units with different contractile properties. Recent studies have revealed that different motor units contribute to muscle force in different motor tasks. The flexible recruitment of motor units with various contractile properties allows a flexible tuning of muscle properties, but also complicates the interpretation of EMG activity.

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Marjan A. Admiraal, Martijn J.M.A.M. Kusters, and Stan C.A.M. Gielen

A central problem in motor control relates to the coordination of the arm's many degrees of freedom. This problem concerns the many arm postures (kinematics) that correspond to the same hand position in space and the movement trajectories between begin and end position (dynamics) that result in the same arm postures. The aim of this study was to compare the predictions for arm kinematics by various models on human motor control with experimental data and to study the relation between kinematics and dynamics. Goal-directed arm movements were measured in 3-D space toward far and near targets. The results demonstrate that arm postures for a particular target depend on previous arm postures, contradicting Donders's law. The minimum-work and minimum-torque-change models, on the other hand, predict a much larger effect of initial posture than observed. These data suggest that both kinematics and dynamics affect postures and that their relative contribution might depend on instruction and task complexity.

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Mary D. Klein Breteler, Ruud G.J. Meulenbroek, and Stan C.A.M. Gielen

In the present study we evaluated the minimum-jerk and the minimum torque-change model at the path, trajectory, and movement-cost levels. To date, most evaluations of these models have mainly been restricted to path comparisons. Assessments of the time courses of realized jerk and torque changes are surprisingly lacking. Moreover, the extent to which the presumed optimized parameters change as a function of the duration and other temporal features of aiming movements has never been investigated, most probably because the models presuppose movement time. In order to till mis gap, we analyzed a subset of the data of an earlier experiment in which 12 participants performed leftward and rightward planar pointing movements. Hand displacements and joint excursions were recorded with a 3D motion-tracking system and subsequently evaluated by means of model-based analyses. The results show that despite a good agreement between observed paths and predicted paths, especially by the minimum torque-change model, the time courses of jerk and torque changes of observed and modeled movements differed considerably. These differences could mainly be attributed to asymmetrical properties of the time functions of slow movements. Variations of movement costs as a function of movement lime and skewness of tangential velocity profiles show that, especially at high movement speed, costs increase exponentially with departures of symmetry. It is concluded that trajectory-formation models have limited explanatory power in situations that require demanding information processing during the homing-in phase of goal-directed movements. However. for slow movements, deviations from the optimal timing profiles require little extra costs in terms of jerk or torque change.

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David Giles, Cam Hartley, Hamish Maslen, Josh Hadley, Nicola Taylor, Ollie Torr, Joel Chidley, Tom Randall, and Simon Fryer

Purpose: The fatigue resistance of the finger flexors is known to be a key determinant of climbing performance. This study set out to establish the association between the single all-out assessment of finger flexor critical force (ff-CF) and the impulse above CF (W') on climbing performance (self-reported sport and boulder climbing ability). Methods: A total of 129 subjects completed an assessment of dominant arm ff-CF, comprised of a series of rhythmic isometric maximum voluntary contractions (CF defined as mean end-test force [in kilograms]; W' impulse above CF [in kilogram second]). Results: The ff-CF protocol resulted in the same force decay to a plateau seen in previous isometric critical torque and critical force tests. Linear regression analysis, adjusting for sex, revealed that CF percentage of body mass explained 61% of sport and 26% of bouldering performance and W' per kilogram body mass explained 7% sport and 34% bouldering performance. A combined model of CF as a percentage of body mass and W' per kilogram body mass, after adjustment for sex differences, was able to explain 66% of sport climbing and 44% of bouldering performance. Conclusions: The results illustrate the relevance of the CF threshold in describing the fatigue resistance of the finger flexors of rock climbers. Given ff-CF ability to describe a considerable proportion of variance in sport climbing and bouldering ability, the authors expect it to become a common test used by coaches for understanding exercise tolerance and for determining optimal training prescription.

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Tom G. Welter, Maarten F. Bobbert, Bauke M. van Bolhuis, Stan C.A.M. Gielen, Leonard A. Rozendaal, and Dirkjan H.E.J. Veeger

We have investigated whether differences in EMG activity in mono- and bi-articuiar muscles for concentric and eccentric contractions (van Bolhuis, Gielen, & van Ingen Schenau, 1998) have to be attributed to a specific muscle coordination strategy or whether they are merely a demonstration of adaptations necessary to adjust for muscle contractile properties. Slow, multi-joint arm movements were studied in a horizontal plane with an external force applied at the wrist. Kinematics and electromyography data from 10 subjects were combined with data from a 3-D model of the arm and a Hill-type muscle model Data for both mono- and bi-articular muscles revealed a higher activation in concentric than in eccentric contractions. The model calculations indicated that the measured difference in activation (20%) was much larger than expected based on the force-velocity relationship (predicting changes of ~5%). Although these findings eliminate the force-velocity relationship as the main explanation for changes in EMG, it cannot be ruled out that other muscle contractile properties, such as history dependence of muscle force, determine muscle activation levels in the task that was studied.

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Allison J. Nelson, Patrick T. Hall, Katherine R. Saul, and Dustin L. Crouch

include (A) the cam wheel with concentric variable radius (A1) and constant radius (A2) wheels, (B) elastic spring, (C) force-transmitting shoulder cable, (D–F)cylindrical wrapping surfaces to route the shoulder cable over the superolateral aspect of the shoulder, (G) cylindrical wrapping surface to

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Joachim Kimmerle, Kim-Kristin Gerbing, Ansgar Thiel, and Ulrike Cress

This research note provides an explorative analysis of sport-related knowledge exchange about Complementary and Alternative Medicine (CAM) on the Internet. Data are taken from a qualitative content analysis of the largest German-speaking Internet sport portal. Knowledge exchange about CAM in these Internet fora is characterized by the following phenomena: Users expected CAM to improve their performance and discussed a great variety of treatments based on primarily anecdotal knowledge. In addition, two main types of users (helpers and help-seekers) dominated the exchanges. The main reasons for seeking alternative medical help on the Internet were cases of prolonged illness and dissatisfaction with biomedical care.

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Ralph W. Smith and Tammy J. Buckley

This pilot study examined the accessibility and usability of three weight training systems for athletes with spinal cord injuries. The systems were Universal Centurion II, Nautilus, and Keiser Cam II. Accessibility factors were direction of transfer, ease of transfer, and access of machine components. Usability includes use with/without assistance, necessary adaptations, and adjustability of resistance level and seat location. Also examined were minimum resistance level and feasibility of use without application of bilateral force. One 25-year-old male athlete with paraplegia (T5-6) served as the demonstrator for all evaluations. A matrix of accessibility and usability was constructed for 10 upper-body exercises that could be performed on at least two of the three weight training systems. Overall, the Keiser Cam II system was judged superior on accessibility and usability for spinal cord-injured athletes; however, the Nautilus system provided the most comprehensive upper-body workout.

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Tom A. Ranger, Wendy M. Braybon, Craig R. Purdam, and Jill L. Cook

Context:

Lateral epicondylalgia, pain at the lateral elbow, is commonly associated with extensor carpi radialis brevis tendinopathy. The radial head, which abuts the extensor tendons and is elliptical in shape, may affect the extensor tendons during pronation of the forearm. Cadaverous studies have shown that the radial head may act as a cam in pronation, to offer a mechanical advantage to the common extensor tendon and to mitigate load on the origin of the extensor carpi radialis brevis tendon.

Objective:

To determine the effect of radial-head position on the wrist-extensor tendons in vivo.

Design:

Repeated-measures design.

Setting:

Laboratory.

Participants:

22 participants (12 male, 2 left-handed).

Main Outcome Measures:

Distance (mm) from subcutaneous fascia to radial head measured by ultrasound.

Results:

The radial head in supination was significantly deeper than either pronation or midprone, indicating a smaller cam effect in supination.

Conclusion:

The authors recommend that the effect of radial-head position and its relationship to the area of tendon pathology be considered clinically in the rehabilitation of patients suffering from lateral epicondylalgia.