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Kathleen Williams, Kathleen Haywood and Ann VanSant

Older adults were tested to clarify findings of an earlier examination of movement responses to shifting task requirements (Williams et al., 1993). Eleven participants (average age = 77 years) were evaluated on form and velocity as they performed overarm throws for force and accuracy. Significant gender and force-accuracy differences occurred for resultant velocity. Although no statistically significant differences occurred for force-accuracy comparisons of movement form, there were trends toward change in most movement components. Additionally, many individuals displayed change in one or more components as they shifted from force to accuracy throws. Results of this study point to the importance of examining developmental status and task requirements simultaneously.

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Joseph P. Stitt and Karl M. Newell

This paper presents the stochastic modeling of isometric force variability in the steady-state time series recorded from the index finger of young adults in the act of attempting to hold different levels of constant force. The isometric force time series were examined by assuming that the stochastic (random) models were linear. System identification techniques were employed to estimate the parameters of each linear model. Once the models were parameterized, the values of the estimated parameters were compared to determine if a single linear time-invariant model was applicable across the entire isometric force range. Although the overall random models were found to be nonlinear functions of the target force level, within a fixed target level, linear modeling provided adequate estimates of the underlying processes thus enabling the use of well-known linear system identification algorithms.

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Max C. Stuelcken, René E.D. Ferdinands, Karen A. Ginn and Peter J. Sinclair

This preliminary study aimed to quantify the magnitude of the peak shoulder distraction force during the bowling action of female cricket fast bowlers. An eight camera Vicon motion analysis system operating at 120 Hz recorded the fast bowling actions of 18 Australian female fast bowlers. A three segment inverse solution model of the bowling arm was used to calculate the shoulder distraction force. A large peak shoulder distraction force was recorded during the early stages of the follow-through of the bowling action. When normalized for body weight, the distraction force was within the range of values reported for baseball and softball pitchers, who are considered to be at high risk of shoulder injury. Therefore, the relative importance of the peak shoulder distraction force in the fast bowling action for the development of shoulder pain in female cricket fast bowlers warrants further investigation.

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Matthew J. Hodgson, David Docherty and E. Paul Zehr

The contractile history of muscle can potentiate electrically evoked force production. A link to voluntary force production, related in part to an increase in reflex excitability, has been suggested.

Purpose:

Our purpose was to quantify the effect of postactivation potentiation on voluntary force production and spinal H-reflex excitability during explosive plantar fexion actions.

Methods:

Plantar flexor twitch torque, soleus H-reflex amplitudes, and the rate of force development of explosive plantar fexion were measured before and after 4 separate conditioning trials (3 × 5 s maximal contractions).

Results:

Twitch torque and rate of force production during voluntary explosive plantar flexion were significantly increased (P < .05) while H-reflex amplitudes remained unchanged. Although twitch torque was significantly higher after conditioning, leading to a small increase in the rate of voluntary force production, this was unrelated to changes in reflex excitability.

Conclusion:

We conclude that postactivation potentiation may result in a minor increase in the rate of voluntary isometric force production that is unrelated to neural excitability.

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Roger J. Paxton, Caitlin Feldman-Kothe, Megan K. Trabert, Leah N. Hitchcock, Raoul F. Reiser II and Brian L. Tracy

Introduction:

The purpose was to determine the effect of peripheral neuropathy (PN) on motor output variability for ankle muscles of older adults, and the relation between ankle motor variability and postural stability in PN patients.

Methods:

Older adults with (O-PN) and without PN (O), and young adults (Y) underwent assessment of standing postural stability and ankle muscle force steadiness.

Results:

O-PN displayed impaired ankle muscle force control and postural stability compared with O and Y groups. For O-PN, the amplitude of plantarflexor force fluctuations was moderately correlated with postural stability under no-vision conditions (r = .54, p = .01).

Discussion:

The correlation of variations in ankle force with postural stability in PN suggests a contribution of ankle muscle dyscontrol to the postural instability that impacts physical function for older adults with PN.

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Mark L. Latash, Jae Kun Shim, Fan Gao and Vladimir M. Zatsiorsky

We review a series of studies that show stabilization of the moment of a couple produced by a set of digits in many maximal and submaximal accurate force production tasks that have no requirements for the moment. In particular, an unusual and novel multi-digit force production task shows stabilization of the total moment while the total force requires extensive practice to be stabilized. Similar results were obtained in persons with Down syndrome during easier tasks. During prehension, changes in digit forces and coordinates of their points of application suggest the presence of two multi-digit synergies whose purpose is to assure a certain grip force and a certain total moment, respectively. Elderly persons show impaired production of both maximal and submaximal moments that goes beyond their documented loss of muscle force. We conclude that moment production (keeping rotational equilibrium) is a central constraint in a variety of multi-digit tasks that has received little attention. Analysis of digit interaction for moment production during handwriting could signify a major step towards understanding the control of this action.

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Daniel Cury Ribeiro, Joelly Mahnic de Toledo, Roberto Costa Krug and Jefferson Fagundes Loss

Shoulder injuries are often related to rotator cuff muscles. Although there are various models for muscle force estimation, it is difficult to ensure that the results obtained with such models are reliable. The aim of the current study was to compare two models of muscle force estimation. Eight subjects, seven male and one female (mean age of 24 yr; mean height of 1.83 m), performed five isokinetic maximum concentric contractions of internal and external shoulder rotation. Two models with different algorithms were used. In both, the input data consisted of the measured internal rotation moment. Comparisons were made between the difference and the average results obtained with each model of muscle force estimation. There was reasonable agreement among the results for force between the two models for subscapularis, pectoralis major, and anterior deltoideus muscles results. Conversely, poor correlation was found for the latissimus dorsi, teres major, and middle deltoid. These results suggest that the algorithm structure might have a strong effect on muscle force estimation results.

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John H. Challis

This study examined the influence of force plate targeting, via stride length adjustments, on the magnitude and consistency of ground reaction force and segment angle profiles of the stance phase of human running. Seven male subjects (height, 1.77 m ± 0.081; mass, 72.4 kg ± 7.52; age range, 23 to 32 years) were asked to run at a mean velocity of 3.2 m · s–1 under three conditions (normal, short, and long strides). Four trials were completed for each condition. For each trial, the ground reaction forces were measured and the orientations of the foot, shank, and thigh computed. There were no statistically significant differences (p > .05) between the coefficients of variation of ground reaction force and segment angle profiles under the three conditions, so these profiles were produced consistently. Peak active vertical ground reaction forces, their timings, and segment angles at foot off were not significantly different across conditions. In contrast, significant differences between conditions were found for peak vertical impact forces and their timings, and for the three lower limb segment angles at the start of force plate contact. These results have implications for human gait studies, which require subjects to target the force plate. Targeting may be acceptable depending on the variables to be analyzed.

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Peter F. Vint and Richard N. Hinrichs

Isometric knee extension force and average integrated EMG of the vastus lateralis muscle were obtained from 27 healthy subjects using a maximum effort, ramp and hold protocol. In each of the 125 total trials mat were included in the analysis, a 2-s plateau region was extracted and divided into two adjacent 1000-ms bins. Variability and reliability of bin-to-bin measurements of force and EMG were then evaluated across 14 different integration intervals ranging from 10 to 1000 ms. Statistical analyses of bin-to-bin variability measures demonstrated that integration intervals of 250 ms and longer significantly reduced variability and improved reliability of average integrated EMG values during maximum effort isometric exertions. Bin-to-bin EMG reliability increased from .728 at 10 ms to .991 at 1000 ms. Force parameters appeared less sensitive to changes in length of the integration interval. It was suggested that longer intervals might also improve the validity of the EMG-force relationship during maximum effort isometric exertions by reducing problems associated with electromechanical delay.

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Ali Jalalvand and Mehrdad Anbarian

decreases in muscle force producing capacity, but it also affects the muscles’ reaction time, movement coordination, precision of motor control, and change in proprioception performance, which resulted in impaired skeletal muscle function and a major risk factor for LEI. 15 Although few studies have focused