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Jos J. de Koning, Ruud W. de Boer, Gert de Groot and Gerrit Jan van Ingen Schenau

In speed skating, performance is related to the product of the amount of work per stroke and the stroke frequency. Work per stroke is dependent on the component of the push-off force in the direction perpendicular to the gliding direction of the skate. The push-off force at different velocities was measured in three trained speed skaters. The results showed that the peak push-off force and mean force do not change at different velocities, and that the stroke time was decreased at higher velocities. It can be concluded that these speed skaters regulate their velocity not by changing the push-off force but by changing their stroke time. The shape of push-off–time curves is dependent on push-off technique and differs during straight lane and curve skating.

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Xiaogang Hu and Karl M. Newell

This study investigated the asymmetry of bilateral interference in relation to the relative difference of force amplitude between hands and the hand dominance. In Experiment 1, one hand produced a fixed constant force of 5% maximum voluntary contraction (MVC) while the other hand produced different constant forces of 5%, 20%, and 50% MVC in blocked conditions. Asymmetric interference in force amplitude alone was evident in that the hand producing the fixed low force showed a stronger interference than the hand performing the higher force. Asymmetric interference in hand dominance was also found in that more interference was observed when the nondominant left hand produced the higher force, a finding that does not support the hemisphere specialization hypothesis. Experiment 2 was performed to rule out the fixed force level interpretation compared with the low force level account and the fixed force was set at 50% MVC. The results were consistent with the findings in Experiment 1 showing asymmetric interference with force amplitude rather than with fixed force level. The findings revealed that without a timing constraint the task demand associated with force amplitude alone can induce the asymmetric bilateral interference. The external task asymmetry and intrinsic asymmetry of the organism interact and influence the bimanual force coordination and control patterns.

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Loren Z.F. Chiu, Brian K. Schilling, Andrew C. Fry and Lawrence W. Weiss

Displacement-based measurement systems are becoming increasingly popular for assessment of force expression variables during resistance exercise. Typically a linear position transducer (LPT) is attached to the barbell to measure displacement and a double differentiation technique is used to determine acceleration. Force is calculated as the product of mass and acceleration. Despite the apparent utility of these devices, validity data are scarce. To determine whether LPT can accurately estimate vertical ground reaction forces, two men and four women with moderate to extensive resistance training experience performed concentric-only (CJS) and rebound (RJS) jump squats, two sessions of each type in random order. CJS or RJS were performed with 30%, 50%, and 70% one-repetition maximum parallel back squat 5 minutes following a warm-up and again after a 10-min rest. Displacement was measured via LPT and acceleration was calculated using the finite-difference technique. Force was estimated from the weight of the lifter-barbell system and propulsion force from the lifter-barbell system. Vertical ground reaction force was directly measured with a single-component force platform. Two-way random average-measure intraclass correlations (ICC) were used to assess the reliability of obtained measures and compare the measurements obtained via each method. High reliability (ICC > 0.70) was found for all CJS variables across the load-spectrum. RJS variables also had high ICC except for time parameters for early force production. All variables were significantly (p < 0.01) related between LPT and force platform methods with no indication of systematic bias. The LPT appears to be a valid method of assessing force under these experimental conditions.

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Toshimasa Yanai, Akifumi Matsuo, Akira Maeda, Hiroki Nakamoto, Mirai Mizutani, Hiroaki Kanehisa and Tetsuo Fukunaga

leg at landing through ball release, 1 , 3 and increased elbow valgus load. 1 , 2 , 4 These observations suggest that the pitching technique used in baseball is a form of throwing uniquely adapted to the height and slope of the mound. Being the only source of external force that could translate the

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Sarah M. Coppola, Philippe C. Dixon, Boyi Hu, Michael Y.C. Lin and Jack T. Dennerlein

comparing external tablet keyboard attachments with the no-travel, on-screen keyboards have demonstrated better performance with attached keyboard use. 2 , 3 However, the effects of these new short-travel key designs on upper-extremity muscle activity and typing force are unknown. Keyboard design

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Edward C. Frederick, Jeremy J. Determan, Saunders N. Whittlesey and Joseph Hamill

Seven top amateur or professional skateboarders (BW = 713 N ± 83 N) performed Ollie maneuvers onto and off an elevated wooden platform (45.7 cm high). We recorded ground reaction force (GRF) data for three Ollie Up (OU) and Ollie Down (OD) trials per participant. The vertical GRF (VGRF) during the OU has a characteristic propulsive peak (M = 2.22 body weight [BW] ± 0.22) resulting from rapidly rotating the tail of the board into the ground to propel the skater and board up and forward. The anterior-posterior (A-P) GRF also shows a pronounced peak (M = 0.05 ± 0.01 BW) corresponding with this propulsive VGRF peak. The initial phase of landing in the OD shows an impact peak in VGRF rising during the first 30 to 80 ms to a mean of 4.74 ± 0.46 BW. These impact peaks are higher than expected given the relatively short drop of 45.7 cm and crouched body position. But we observed that our participants intentionally affected a firm landing to stabilize the landing position; and the Ollie off the platform raised the center of mass, also contributing to higher forces.

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Stacey L. DeJong, Rebecca L. Birkenmeier and Catherine E. Lang

In animal models, hundreds of repetitions of upper extremity (UE) task practice promote neural adaptation and functional gain. Recently, we demonstrated improved UE function following a similar intervention for people after stroke. In this secondary analysis, computerized measures of UE task performance were used to identify movement parameters that changed as function improved. Ten people with chronic poststroke hemiparesis participated in high-repetition UE task-specific training 3 times per week for 6 weeks. Before and after training, we assessed UE function with the Action Research Arm Test (ARAT), and evaluated motor performance using computerized motion capture during a reach-grasp-transport-release task. Movement parameters included the duration of each movement phase, trunk excursion, peak aperture, aperture path ratio, and peak grip force. Group results showed an improvement in ARAT scores (p = .003). Although each individual changed significantly on at least one movement parameter, across the group there were no changes in any movement parameter that reached or approached significance. Changes on the ARAT were not closely related to changes in movement parameters. Since aspects of motor performance that contribute to functional change vary across individuals, an individualized approach to upper extremity motion analysis appears warranted.

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Christopher A. Knight, Adam R. Marmon and Dhiraj H. Poojari

Subjects learned to produce brief isometric force pulses that were 10% of their maximal voluntary contraction (MVC) force. Subjects became proficient at performing sets of 10 pulses within boundaries of 8–12% MVC, with visual feedback and without (kinesthetic sense). In both the control (Con, n = 10) and experimental (Exp, n = 20) groups, subjects performed two sets of 10 kinesthetically guided pulses. Subjects then either performed a 10-s MVC (Exp) or remained at rest (Con) between sets. Following the MVC, Exp subjects had force errors of +30%, whereas performance was maintained in Con. There was evidence for both muscular and neural contributions to these errors. Postactivation potentiation resulted in a 40% gain in muscle contractility (p = .003), and there was a 26% increase in the neural stimulation of muscle (p = .014). Multiple regression indicated that the change in neural input had a stronger relationship with force errors than the increased contractility.

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Ramón Marcote-Pequeño, Amador García-Ramos, Víctor Cuadrado-Peñafiel, Jorge M. González-Hernández, Miguel Ángel Gómez and Pedro Jiménez-Reyes

linear sprint). Jump height and sprint time are the 2 performance variables most commonly used to evaluate vertical jump and linear sprint capabilities, respectively. 10 , 11 However, a new testing methodology based on the force–velocity (FV) relationship has recently emerged with the expectation of

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Aisha Khan and Stacey L. Gorniak

Previous studies of fine motor control have focused on the ability of participants to match their grip force production to a visually provided template. We investigated differences exhibited in pinch force control during variable force production templates, including sine-, sawtooth-, and square-wave templates. Our results indicate that increased force requirements are associated with increased error rates and a noisier frequency spectrum, consistent with previous studies. Our results also indicate that visual feedback, in the form of template shape, directly affect pinch force production features and motor unit firing patterns, despite the use of consistent baseline force requirements, amplitude changes, and visual signal frequency. This suggests that CNS modulation of motor unit responses can be triggered by basic changes in visual feedback unrelated to force requirements. The potential implications of error compensation based on this study due to aging are also discussed.