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Marco Testa, Tommaso Geri, Alessio Signori and Silvestro Roatta

The assessment of the individual ability of modulating and coordinating the right and left bite force is poorly investigated. The present study describes a methodology for the assessment of the bilateral control of the biting force and evaluates the test-retest reliability in a sample of 13 healthy subjects. By modulating the intensity and the left/right balance of the biting force, the subject was able to drive a cursor on the screen to “reach and hold” targets, randomly generated within the physiological “range of force” of the subject. The average motor performance was evaluated by the mean cursor-target distance = 13 ± 5%, the Offset Error = 9 ± 5% and the standard deviation of the force vector = 17.7 ± 6.1% (expressed as % of the target). Mean distance and standard deviation indices had acceptable reliability. This technique improves the characterization of the mandibular motor function and it may have a relevant role for the assessment and rehabilitation of the neuromusculoskeletal disorders affecting the orofacial system.

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Clayton L. Van Doren

The purpose of this study was to determine whether direct measurements of grasp stiffness agreed with stiffness inferred from the slopes of isovolitional force-span characteristics derived from previous grasp-effort matching data. Grasp stiffness for three-finger pinch was measured as a function of initial force and finger span using step displacements applied in a do-not-intervene paradigm. Subjects pinched a free-floating, motorized manipulandum in each hand and squeezed both with equal effort; one of the hands was perturbed at random. Stiffness was calculated from the initial and final steady-state values of force and span. The effects of step amplitude, rise-time, and initial load stiffness were investigated; grasp stiffness decreased significantly for larger steps, increased slightly for longer rise-times, and was unaffected by load stiffness. Grasp stiffness then was measured as a function of initial force and span using a single set of step parameters. Stiffness increased significantly in proportion to force but was changed only slightly by span. It was concluded that the perturbation and effort-matching measures of stiffness are not equivalent and represent different components of motor behavior.

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Dominic James Farris, Erica Buckeridge, Grant Trewartha and Miranda Polly McGuigan

This study assessed the effects of orthotic heel lifts on Achilles tendon (AT) force and strain during running. Ten females ran barefoot over a force plate in three conditions: no heel lifts (NHL), with 12 mm heel lifts (12HL) and with 18 mm heel lifts (18HL). Kinematics for the right lower limb were collected (200 Hz). AT force was calculated from inverse dynamics. AT strain was determined from kinematics and ultrasound images of medial gastrocnemius (50 Hz). Peak AT strain was less for 18HL (5.5 ± 4.4%) than for NHL (7.4 ± 4.2%) (p = .029, effect size [ES] = 0.44) but not for 12HL (5.8 ± 4.8%) versus NHL (ES = 0.35). Peak AT force was significantly (p = .024, ES = 0.42) less for 18HL (2382 ± 717 N) than for NHL (2710 ± 830 N) but not for 12HL (2538 ± 823 N, ES = 0.21). The 18HL reduced ankle dorsiflexion but not flexion-extension ankle moments and increased the AT moment arm compared with NHL. Thus, 18HL reduced force and strain on the AT during running via a reduction in dorsiflexion, which lengthened the AT moment arm. Therefore, heel lifts could be used to reduce AT loading and strain during the rehabilitation of AT injuries.

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Brian J. O'Connor, H. John Yack and Scott C. White

A strategy is presented for temporally aligning ground reaction force and kinematic data. Alignment of these data requires marking both the force and video records at a common event. The strategy uses the information content of the video signal, which is A/D converted along with the ground reaction force analog signals, to accomplish this alignment in time. The vertical blanking pulses in the video signal, which define the start of each video field, can be readily identified, provided the correct A/D sampling rate is selected. Knowledge of the position of these vertical blanking pulses relative to the synchronization pulse makes it possible to precisely align the video and analog data in time. Choosing an A/D sampling rate of 598 Hz would enable video and analog data to be synchronized to within 1/1,196 s. Minimizing temporal alignment error results in greater accuracy and .reliability in calculations used to determine joint kinetics.

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David A. Aitken and Robert J. Neal

A system was developed to quantify the on-water forces, impulse, and power generated by a kayak paddlet. The system is lightweight (<1 kg), portable (i.e., it can be used in single [Kl], double [K2], and fours [K4] boats), and does not affect the integrity of either the kayak paddle or the boat. Changes in the strain on the kayak paddle were measured by force transducers attached to the shaft of the paddle, and these signals were then recorded on an FM tape recorder located in the boat. The data were then analyzed by the Kayak Data Acquisition and Analysis System software which graphically presented the paddlers' force time curve as well as a printed tabular report on the paddlers' average force, impulse, work, power, and the instantaneous boat velocity.

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Jay L. Alberts, Christopher M. Elder, Michael S. Okun and Jerrold L. Vitek

The aim of this study was to determine the effects of unilateral deep brain stimulation (DBS) on the control and coordination of grasping forces produced by Parkinson's disease (PD) patients. Ten advanced PD patients with unilateral DBS in the globus pallidus (GPi) or the subthalamic nucleus (STN) (5 patients in each group) performed a functional bimanual dexterous manipulation task. Experiments were performed in the “Off” medication state with DBS “On” and “Off.” DBS resulted in (a) significant clinical improvements, (b) greater maximum grip force for both limbs, (c) reduced movement time, and (d) bilateral coupling of grasping forces. There were no significant differences between the GPi and STN groups for any clinical or kinematic measures. DBS of the GPi and STN leads to an improvement in the motor functioning of advanced PD patients. Improvement in force-timing specification during DBS might allow PD patients to employ a feedforward method of force control.

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Timothy J. Henry, Scott M. Lephart, Jorge Giraldo, David Stone and Freddie H. Fu

Context:

Muscle fatigue is an important concept in regard to the muscle function of the shoulder joint. Its effect on the muscle force couples of the glenohumeral joint has not been fully identified.

Objective:

To examine the effects of muscle fatigue on muscle force-couple activation in the normal shoulder.

Design:

Pretest, posttest.

Patients:

Ten male subjects, age 18–30 years, with no previous history of shoulder problems.

Main Outcome Measures:

EMG (area) values were assessed for the anterior and middle deltoid, subscapularis, and infraspinatus muscles during 4 dynamic stabilizing exercises before and after muscle fatigue. The exercises examined were a push-up, horizontal abduction, segmental stabilization, and rotational movement on a slide board.

Results:

No significant differences were observed for any of the muscles tested.

Conclusions:

The results of our study indicate that force-couple coactivation of the glenohumeral joint is not significantly altered after muscle fatigue.

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Jurjen Bosga and Ruud G. J. Meulenbroek

In this study we investigated redundancy control in joint action. Ten participantpairs (dyads) performed a virtual lifting task in which isometric forces needed to be generated with two or four hands. The participants were not allowed to communicate but received continuous visual feedback of their performance. When the task had to be performed with four hands, participants were confronted with a redundant situation and between-hand force synergies could, in principle, be formed. Performance timing, success rates, cross-correlations, and relative phase analyses of the force-time functions were scrutinized to analyze such task-dependent synergies. The results show that even though the dyads performed the task slower and less synchronized in the joint than in the solo conditions, the success rates in these conditions were identical. Moreover, correlation and relative phase analyses demonstrated that, as expected, the dyads formed between-participant synergies that were indicative of force sharing in redundant task conditions.

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Pedro Jiménez-Reyes, Pierre Samozino, Fernando Pareja-Blanco, Filipe Conceição, Víctor Cuadrado-Peñafiel, Juan José González-Badillo and Jean-Benoît Morin

Purpose:

To analyze the reliability and validity of a simple computation method to evaluate force (F), velocity (v), and power (P) output during a countermovement jump (CMJ) suitable for use in field conditions and to verify the validity of this computation method to compute the CMJ force–velocity (Fv) profile (including unloaded and loaded jumps) in trained athletes.

Methods:

Sixteen high-level male sprinters and jumpers performed maximal CMJs under 6 different load conditions (0–87 kg). A force plate sampling at 1000 Hz was used to record vertical ground-reaction force and derive vertical-displacement data during CMJ trials. For each condition, mean F, v, and P of the push-off phase were determined from both force-plate data (reference method) and simple computation measures based on body mass, jump height (from flight time), and push-off distance and used to establish the linear Fv relationship for each individual.

Results:

Mean absolute bias values were 0.9% (± 1.6%), 4.7% (± 6.2%), 3.7% (± 4.8%), and 5% (± 6.8%) for F, v, P, and slope of the Fv relationship (SFv), respectively. Both methods showed high correlations for Fv-profile-related variables (r = .985–.991). Finally, all variables computed from the simple method showed high reliability, with ICC >.980 and CV <1.0%.

Conclusions:

These results suggest that the simple method presented here is valid and reliable for computing CMJ force, velocity, power, and Fv profiles in athletes and could be used in practice under field conditions when body mass, push-off distance, and jump height are known.

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Jean-Benoît Morin, George Petrakos, Pedro Jiménez-Reyes, Scott R. Brown, Pierre Samozino and Matt R. Cross

Background:

Sprint running acceleration is a key feature of physical performance in team sports, and recent literature shows that the ability to generate large magnitudes of horizontal ground-reaction force and mechanical effectiveness of force application are paramount. The authors tested the hypothesis that very-heavy loaded sled sprint training would induce an improvement in horizontal-force production, via an increased effectiveness of application.

Methods:

Training-induced changes in sprint performance and mechanical outputs were computed using a field method based on velocity–time data, before and after an 8-wk protocol (16 sessions of 10- × 20-m sprints). Sixteen male amateur soccer players were assigned to either a very-heavy sled (80% body mass sled load) or a control group (unresisted sprints).

Results:

The main outcome of this pilot study is that very-heavy sled-resisted sprint training, using much greater loads than traditionally recommended, clearly increased maximal horizontal-force production compared with standard unloaded sprint training (effect size of 0.80 vs 0.20 for controls, unclear between-groups difference) and mechanical effectiveness (ie, more horizontally applied force; effect size of 0.95 vs –0.11, moderate between-groups difference). In addition, 5-m and 20-m sprint performance improvements were moderate and small for the very-heavy sled group and small and trivial for the control group, respectively.

Practical Applications:

This brief report highlights the usefulness of very-heavy sled (80% body mass) training, which may suggest value for practical improvement of mechanical effectiveness and maximal horizontal-force capabilities in soccer players and other team-sport athletes.

Results:

This study may encourage further research to confirm the usefulness of very-heavy sled in this context.