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Asynchronous Alterations of Muscle Force and Tendon Stiffness Following 8 Weeks of Resistance Exercise with Whole-Body Vibration in Older Women

Seong-won Han, Dae-yeon Lee, Dong-Sung Choi, Boram Han, Jin-Sun Kim, and Hae-Dong Lee

This study aimed to examine whether muscle force and tendon stiffness in a muscle-tendon complex alter synchronously following 8-week whole-body vibration (WBV) training in older people. Forty older women aged 65 years and older were randomly assigned into control (CON, n = 15) and whole-body vibration (WBV) training groups (exposure time, n = 13; vibration intensity, n = 12). For the training groups, a 4-week detraining period was completed following the training period. Throughout the training/detraining period, force of the medial gastrocnemius (MG) muscle and stiffness of the Achilles tendon were assessed four times (0, 4, 8, and 12 weeks) using a combined system of dynamometer and ultrasonography. While muscle force gradually increased throughout the training period (p < .05), a significant increase in tendon stiffness was observed after 8 weeks (p < .05). These findings indicated that, during the early phase of WBV training, muscle force and tendon stiffness changed asynchronously, which might be a factor in possible musculotendinous injuries.

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Sensitivity of Estimated Muscle Force in Forward Simulation of Normal Walking

Ming Xiao and Jill Higginson

Generic muscle parameters are often used in muscle-driven simulations of human movement to estimate individual muscle forces and function. The results may not be valid since muscle properties vary from subject to subject. This study investigated the effect of using generic muscle parameters in a muscle-driven forward simulation on muscle force estimation. We generated a normal walking simulation in OpenSim and examined the sensitivity of individual muscle forces to perturbations in muscle parameters, including the number of muscles, maximum isometric force, optimal fiber length, and tendon slack length. We found that when changing the number of muscles included in the model, only magnitude of the estimated muscle forces was affected. Our results also suggest it is especially important to use accurate values of tendon slack length and optimal fiber length for ankle plantar flexors and knee extensors. Changes in force production by one muscle were typically compensated for by changes in force production by muscles in the same functional muscle group, or the antagonistic muscle group. Conclusions regarding muscle function based on simulations with generic musculoskeletal parameters should be interpreted with caution.

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Hamstring Stiffness Returns More Rapidly After Static Stretching Than Range of Motion, Stretch Tolerance, and Isometric Peak Torque

Genki Hatano, Shigeyuki Suzuki, Shingo Matsuo, Satoshi Kataura, Kazuaki Yokoi, Taizan Fukaya, Mitsuhiro Fujiwara, Yuji Asai, and Masahiro Iwata

muscles. Therefore, the duration of the effects of static stretching was investigated by comparing SPT, ROM, passive torque (PT) at the onset of pain, passive stiffness, and isometric muscle force before stretching versus 10, 20, and 30 minutes after stretching. The purpose of this study was to elucidate

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Racing an Opponent: Alteration of Pacing, Performance, and Muscle-Force Decline but Not Rating of Perceived Exertion

Marco J. Konings, Jordan Parkinson, Inge Zijdewind, and Florentina J. Hettinga

and by relating these to neuromuscular adjustments in the knee extensors and perceived exertion. We hypothesized that the presence of a virtual opponent would invite a change in pacing and evoke an improvement in performance, leading to a greater decline in voluntary muscle force after a 4-km TT than

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A Comparison between Two Models of Shoulder Muscle Force Estimation

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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Too Early to Explain All Experimental Data with a Single Model

C.C.A.M. Gielen

The target article by Prilutsky gives an excellent overview of the predictions of the Crowninshield and Brand model and about the relevant literature about muscle coordination. However, we do not agree with the claim that the Crowninshield and Brand model can explain the coordination between one-joint and two-joint muscles. In this commentary we will make three claims: (a) The Crowninshield and Brand model cannot explain all aspects of muscle coordination, (b) there is good experimental evidence that different constraints and models may be necessary to explain muscle coordination in different motor tasks, and (c) the reason for the lack of quantitative fits between predictions about muscle force and experimental data is that it is hard to measure muscle force in man. As a compromise one has to rely on EMG activity as a measure of muscle force. Because of the complex relationship between EMG and muscle force, a quantitative test of models is difficult.

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The Effect of Muscle Fatigue on Muscle Force-Couple Activation of the Shoulder

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


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.


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


Pretest, posttest.


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.


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


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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The Relationships Between Muscle Force Steadiness and Visual Steadiness in Young and Old Adults

Rebecca L. Krupenevich, Nick Murray, Patrick M. Rider, Zachary J. Domire, and Paul DeVita

Since vision is used in studies of muscle force control, reduced muscle force control might be related to reduced visual ability. We investigated relationships between steadiness in eye movements and quadriceps muscle torque (a surrogate for force) during isometric contractions of constant and varying torques. Nineteen young adults with an average age of 20.7 years and 18 old adults with an average age of 71.6 years performed three vision tasks, three vision and torque tasks at 40% maximal voluntary contraction (MVC), and three vision and torque tasks at 54 nm. Age groups had identical torque steadiness (CV) in 40%-MVC and 54-nm conditions (p > .05). Old had similar vertical (p > .05) but decreased horizontal visual steadiness (SD) (p < .05) compared with young. Correlations between visual steadiness and muscle torque steadiness failed to show a significant relationship (p > .05). We were unable to identify a substantial relationship between muscle torque steadiness and eye movement, as a component of visual steadiness, and conclude that reduced visual steadiness does not contribute to reduced muscle torque steadiness.

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Some Mechanical Considerations on Muscle Coordination

Rositsa T. Raikova

This commentary emphasizes three points of discussion. (a) The terminology: The terms multifunctional, synergisic, antagonistic muscles, and synergistic and antagonistic coactivations are discussed and the conclusion is drawn that they could not be used without mentioning the particular joint motion. (b) The importance of the external joint moments for activation of the muscles is confirmed on the basis of logical and mechanical considerations. Not all experimental results, however, could be explained by this means. (c) The optimization criterion: Prilutsky's conclusion concerning the predicted muscle force proportionality to the muscle moment arm and PCSA is confirmed using a simple analytical solution of the optimization problem. It is shown, however, that the proportionality to the PCSA is a consequence of the chosen optimization criterion.

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A 3-Dimensional Gait Analysis of the Effects of Fatigue-Induced Reduced Foot Adductor Muscle Strength on the Walking of Healthy Subjects

Rogerio Pessoto Hirata, Alexander W. Erbs, Erik Gadsbøll, Rannvá Winther, Sanne H. Christensen, and Morten Bilde Simonsen

proposed as possible mechanisms behind the decreased tibialis posterior muscle force. However, when evaluating patients with tibialis posterior weakness, it is difficult to understand the isolated effects of tibialis posterior muscle weakness on the impairments in functional tasks such as walking 8 and