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Hans Jobse, Ruud Schuurhof, Ferenc Cserep, A. Wim Schreurs and Jos J. de Koning

Portable equipment for active measurements of push-off force and ice friction was developed. The equipment consists of a pair of skates with three measuring elements between the shoe and the skate blade to register force in both fore/aft and normal direction. A portable computer samples the friction force and normal force signals during one or more strokes, calculates the mean coefficient of ice friction, and stores the sampled data in memory. The push-off force and ice friction force were measured. The peak push-off forces reach values of up to 140% of body weight. The magnitude of the coefficient of ice friction varies, depending on the weather conditions and preparatory method, generally between 0.003 and 0.007 when skating the straightaway. During the skating of the curves the coefficient of ice friction is 35% higher, most likely due to the different skating technique in the curves.

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Kevin McQuade, Michelle L. Harris-Love and Jill Whitall

The purpose of this study was to determine whether the phenomenon of bilateral deficit in muscular force production observed in healthy subjects and mildly impaired stroke patients also exists in patients with more chronic and greater levels of stroke impairment. Ten patients with chronic hemiparesis resulting from stroke performed unilateral and bilateral maximal voluntary isometric contractions of the elbow flexors. When the total force produced by both arms was compared, 12% less force was produced in the bilateral compared with unilateral condition (p = 0.01). However, studying the effect of task conditions on each arm separately revealed a significant decline in nonparetic (p = 0.01) but not paretic elbow flexor force in the bilateral compared with unilateral condition. Results suggest that a significant bilateral force deficit exists in the nonparetic but not the paretic arm in individuals with chronic stroke. Bilateral task conditions do not seem to benefit or impair paretic arm maximal isometric force production in individuals with moderate-severity chronic stroke.

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Rainer Blank and Joachim Hermsdörfer

Fast force changes with hand-held objects are an important prerequisite for object manipulation in everyday life. This study examines the development of fastest isometric force changes in a precision grip. One hundred sixty-five children (76 girls, 89 boys), 3–14 years, without neurological abnormalities increased and decreased repetitively isometric grip forces as rapidly as possible by their dominant hand using a small cylindrical pinch grip object (20 g). The frequency of repetitive force changes increased in a linear way from the age of 4 years until about 12 years by 0.23 Hz per year (r 2 = .54) without noticeable gender difference. The ratio of the duration of force increase and decrease slightly declined from 1.05 (4-year-olds) to 0.95 (11- to 14-year-olds). The development of force amplitudes and the mean force were more variable. Temporal parameters become less variable with age, whereas force parameters become more variable. In particular, the temporal parameters of fastest isometric force changes are best predictors for developmental changes. Fastest isometric force changes may be an important basic capacity for fast object manipulation, particularly in young children and in children with movement disorders.

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Caroline Giroux, Giuseppe Rabita, Didier Chollet and Gaël Guilhem

Performance during human movements is highly related to force and velocity muscle capacities. Those capacities are highly developed in elite athletes practicing power-oriented sports. However, it is still unclear whether the balance between their force and velocity-generating capacities constitutes an optimal profile. In this study, we aimed to determine the effect of elite sport background on the force–velocity relationship in the squat jump, and evaluate the level of optimization of these profiles. Ninetyfive elite athletes in cycling, fencing, taekwondo, and athletic sprinting, and 15 control participants performed squat jumps in 7 loading conditions (range: 0%–60% of the maximal load they were able to lift). Theoretical maximal power (Pm), force (F 0), and velocity (v 0) were determined from the individual force–velocity relationships. Optimal profiles were assessed by calculating the optimal force (F 0th) and velocity (v 0th). Athletic sprinters and cyclists produced greater force than the other groups (P < .05). F 0 was significantly lower than F 0th, and v 0 was significantly higher than v 0th for female fencers and control participants, and for male athletics sprinters, fencers, and taekwondo practitioners (P < .05). Our study shows that the chronic practice of an activity leads to differently balanced force–velocity profiles. Moreover, the differences between measured and optimal force–velocity profiles raise potential sources of performance improvement in elite athletes.

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Christopher A. Bailey and Patrick A. Costigan

The step-up-and-over test has been used successfully to examine knee function after knee injury. Knee function is quantified using the following variables extracted from force plate data: the maximal force exerted during the lift, the maximal impact force at landing, and the total time to complete the step. For various reasons, including space and cost, it is unlikely that all clinicians will have access to a force plate. The purpose of the study was to determine if the step-up-and-over test could be simplified by using an accelerometer. The step-up-and-over test was performed by 17 healthy young adults while being measured with both a force plate and a 3-axis accelerometer mounted at the low back. Results showed that the accelerometer and force plate measures were strongly correlated for all 3 variables (r = .90–.98, Ps < .001) and that the accelerometer values for the lift and impact indices were 6–7% higher (Ps < .01) and occurred 0.07–0.1 s later than the force plate (Ps < .05). The accelerometer returned values highly correlated to those from a force plate. Compared with a force plate, a wireless, 3-axis accelerometer is a less expensive and more portable system with which to measure the step-up-and-over test.

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Ziad D. Alkurdi and Yazan M. Dweiri

The present work examined the handgrip force at different anatomical positions for both hands. Anthropometrics, handgrip force, and fatigue were obtained from a representative sample of 20 males randomly selected from the German Jordanian University students. The hand dynamometer first was calibrated with respect to the volunteer’s maximal grip strength, and he was then asked to squeeze maximally until the grip force decreased to 50% of its maximal due to fatigue; this test was performed for both hands at different anatomical positions with 2 min of rest for recovery of muscle function. The results showed differences in the handgrip force between subjects of the same anatomical positions and for the different anatomical positions, differences in the time for 50% of the force maximal for both right hand and left hand, higher time required to achieve 50% of maximal handgrip force for the nondominant hand, and maximal handgrip force was obtained when arm adduction with 90 degrees forward at the elbow joint. Recommendations for future work are to measure fatigue time at different percentages, 25%, 50%, 60%, and 75% of maximal force and to investigate the factors affecting handgrip force over a larger sample.

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Beatriz B. Gomes, Nuno V. Ramos, Filipe A.V. Conceição, Ross H. Sanders, Mário A.P. Vaz and João Paulo Vilas-Boas

In sprint kayaking the role that paddling technique plays in optimizing paddle forces and resultant kayak kinematics is still unclear. The aim of this study was to analyze the magnitude and shape of the paddle force–time curve at different stroke rates, and their implications for kayak performance. Ten elite kayak paddlers (5 males and 5 females) were analyzed while performing 2000-m on-water trials, at 4 different paces (60, 80, and 100 strokes per minute, and race pace). The paddle and kayak were instrumented with strain gauges and accelerometers, respectively. For both sexes, the force–time curves were characterized at training pace by having a bell shape and at race pace by a first small peak, followed by a small decrease in force and then followed by a main plateau. The force profile, represented by the mean force/peak force ratio, became more rectangular with increasing stroke rate (F[3,40] = 7.87, P < .01). To obtain a rectangular shape to maximize performance, kayak paddlers should seek a stronger water phase with a rapid increase in force immediately after blade entry, and a quick exit before the force dropping far below the maximum force. This pattern should be sought when training at race pace and in competition.

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Gregory W. Heath, Ross C. Brownson, Judy Kruger, Rebecca Miles, Kenneth E. Powell, Leigh T. Ramsey and the Task Force on Community Preventive Services

Background:

Although a number of environmental and policy interventions to promote physical activity are being widely used, there is sparse systematic information on the most effective approaches to guide population-wide interventions.

Methods:

We reviewed studies that addressed the following environmental and policy strategies to promote physical activity: community-scale urban design and land use policies and practices to increase physical activity; street-scale urban design and land use policies to increase physical activity; and transportation and travel policies and practices. These systematic reviews were based on the methods of the independent Task Force on Community Preventive Services. Exposure variables were classified according to the types of infrastructures/policies present in each study. Measures of physical activity behavior were used to assess effectiveness.

Results:

Two interventions were effective in promoting physical activity (community-scale and street-scale urban design and land use policies and practices). Additional information about applicability, other effects, and barriers to implementation are provided for these interventions. Evidence is insufficient to assess transportation policy and practices to promote physical activity.

Conclusions:

Because community- and street-scale urban design and land-use policies and practices met the Community Guide criteria for being effective physical activity interventions, implementing these policies and practices at the community-level should be a priority of public health practitioners and community decision makers.

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Simon R. Goodman, Mark L. Latash, Sheng Li and Vladimir M. Zatsiorsky

This study involved an optimization, numerical analysis of a network for two-hand multi-finger force production, analogous in its structure to the double-representation mirror image (DoReMi) network suggested earlier based on neurophysiological data on cortical finger representations. The network accounts for phenomena of enslaving (unintended finger force production), force deficit (smaller force produced by a finger in multi-finger tasks as compared to its single-finger task), and bilateral deficit (smaller forces produced in two-hand tasks as compared to one-hand tasks). Matrices of connection weights were computed, and the results of optimization were compared to the experimental data on finger forces during one- and two-hand maximal force production (MVC) tasks. The network was able to reproduce the experimental data in two-hand experiments with high accuracy (average error was 1.2 N); it was also able to reproduce findings in one-hand multi-finger MVC tasks, which were not used during the optimization procedure, although with a somewhat higher error (2.8 N). Our analysis supports the feasibility of the DoReMi network. It suggests that within-a-hand force deficit and bilateral force deficit are phenomena of different origins whose effects add up. Is also supports a hypothesis that force deficit and enslaving have different neural origins.

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Timothy C. Sell, Jonathan S. Akins, Alexis R. Opp and Scott M. Lephart

Proximal anterior tibia shear force is a direct loading mechanism of the anterior cruciate ligament (ACL) and is a contributor to ACL strain during injury. Measurement of this force during competition may provide insight into risk factors for ACL injury. Accelerometers may be capable of measuring tibial acceleration during competition. The purpose of this study was to examine the relationship between acceleration measured by a tibia-mounted accelerometer and proximal anterior tibia shear force as measured through inverse dynamics and peak posterior ground reaction forces during two leg stop-jump tasks. Nineteen healthy male subjects performed stop-jump tasks across increasing jump distances. Correlation coefficients were calculated to determine if a relationship exists between accelerometer data and proximal anterior tibia shear force and peak posterior ground reaction force. An analysis of variance was performed to compare these variables across jump distance. Significant correlations were observed between accelerometer data and peak posterior ground reaction force, but none between accelerometer data and proximal anterior tibia shear force. All variables except peak proximal anterior tibia shear force increased significantly as jump distance increased. Overall, results of this study provide initial, positive support for the use of accelerometers as a useful tool for future injury prevention research.