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Erika Zemková and Michal Jeleň

differentiate the strength of back muscle contractions and its reproduction during repeated trials. However, the erector spinae display muscle fiber characteristics, which are clearly very different from those of other skeletal muscles, and which, with their predominance of relatively large type I (slow twitch

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Nahid Tahan, Amir Massoud Arab, Bita Vaseghi and Khosro Khademi

Context:

Coactivation of abdominal and pelvic-floor muscles (PFM) is an issue considered by researchers recently. Electromyography (EMG) studies have shown that the abdominal-muscle activity is a normal response to PFM activity, and increase in EMG activity of the PFM concomitant with abdominal-muscle contraction was also reported.

Objective:

The purpose of this study was to compare the changes in EMG activity of the deep abdominal muscles during abdominal-muscle contraction (abdominal hollowing and bracing) with and without concomitant PFM contraction in healthy and low-back-pain (LBP) subjects.

Design:

A 2 × 2 repeated-measures design.

Setting:

Laboratory.

Participants:

30 subjects (15 with LBP, 15 without LBP).

Main Outcome Measures:

Peak rectified EMG of abdominal muscles.

Results:

No difference in EMG of abdominal muscles with and without concomitant PFM contraction in abdominal hollowing (P = .84) and abdominal bracing (P = .53). No difference in EMG signal of abdominal muscles with and without PFM contraction between LBP and healthy subjects in both abdominal hollowing (P = .88) and abdominal bracing (P = .98) maneuvers.

Conclusion:

Adding PFM contraction had no significant effect on abdominal-muscle contraction in subjects with and without LBP.

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Walter Herzog, Timothy R. Leonard, Venus Joumaa and Ashi Mehta

According to the cross-bridge theory, the steady-state isometric force of a muscle is given by the amount of actin–myosin filament overlap. However, it has been known for more than half a century that steady-state forces depend crucially on contractile history. Here, we examine history-dependent steady-state force production in view of the cross-bridge theory, available experimental evidence, and existing explanations for this phenomenon. This is done on various structural levels, ranging from the intact muscle to the myofibrillar and isolated contractile protein level, so that advantages and limitations of the various preparations can be fully exploited and overcome. Based on experimental evidence, we conclude that steady-state force following active muscle stretching is enhanced, and this enhancement has a passive and an active component. The active component is associated with the cross-bridge kinetics, and the passive component is associated with a calcium-dependent increase in titin stiffness.

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Catriona O’Dwyer, David Sainsbury and Kieran O’Sullivan

Context:

Functional subdivisions are proposed to exist in the gluteus medius (GM) muscle. Dysfunction of the GM, in particular its functional subdivisions, is commonly implicated in lower limb pathologies. However, there is a lack of empirical evidence examining the role of the subdivisions of the GM.

Objectives:

To compare the activation of the functional subdivisions of the GM (anterior, middle, and posterior) during isometric hip contractions.

Design:

Single-session, repeated-measures observational study.

Setting:

University research laboratory.

Participants:

Convenience sample of 15 healthy, pain-free subjects.

Intervention:

Subjects performed 3 maximal voluntary isometric contractions for hip abduction and internal and external rotation on an isokinetic dynamometer with simultaneous recording of surface electromyography (sEMG) activity of the GM subdivisions.

Main Outcome Measures:

sEMG muscle activity for each functional subdivision of the GM during each hip movement was analyzed using a 1-way repeated-measures ANOVA (post hoc Bonferroni).

Results:

The response of GM subdivisions during the 3 different isometric contractions was significantly different (interaction effect; P = .003). The anterior GM displayed significantly higher activation across all 3 isometric contractions than the middle and posterior subdivisions (main effect; both P < .001). The middle GM also demonstrated significantly higher activation than the posterior GM across all 3 isometric contractions (main effect; P = .027). There was also significantly higher activation of all 3 subdivisions during both abduction and internal rotation than during external rotation (main effect; both P < .001).

Conclusions:

The existence of functional subdivisions in the GM appears to be supported by the findings. Muscle activation was not homogeneous throughout the entire muscle. The highest GM activation was found in the anterior GM subdivision and during abduction and internal rotation. Future studies should examine the role of GM functional subdivisions in subjects with lower limb pathologies.

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John H. Hollman, Tyler A. Berling, Ellen O. Crum, Kelsie M. Miller, Brent T. Simmons and James W. Youdas

cues to promote gluteal and quadriceps muscle contractions during supine bridging exercises may facilitate gluteal recruitment and inhibit hamstring recruitment. His hypothesis, however, has not been experimentally confirmed. The purpose of this study was to examine whether performing the supine

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Markus Tilp, Simon Steib, Gudrun Schappacher-Tilp and Walter Herzog

Force enhancement following muscle stretching and force depression following muscle shortening are well-accepted properties of skeletal muscle contraction. However, the factors contributing to force enhancement/depression remain a matter of debate. In addition to factors on the fiber or sarcomere level, fiber length and angle of pennation affect the force during voluntary isometric contractions in whole muscles. Therefore, we hypothesized that differences in fiber lengths and angles of pennation between force-enhanced/depressed and reference states may contribute to force enhancement/depression during voluntary contractions. The purpose of this study was to test this hypothesis. Twelve subjects participated in this study, and force enhancement/depression was measured in human tibialis anterior. Fiber lengths and angles of pennation were quantified using ultrasound imaging. Neither fiber lengths nor angles of pennation were found to differ between the isometric reference contractions and any of the force-enhanced or force-depressed conditions. Therefore, we rejected our hypothesis and concluded that differences in fiber lengths or angles of pennation do not contribute to the observed force enhancement/depression in human tibialis anterior, and speculate that this result is likely true for other muscles too.

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Hakan Gür and Selçuk Küçükoglu

Fifteen young and 17 older females (29.5 ± 4.6 and 58.2 ± 6.1 years of age) and 14 young and 15 older males (27.2 ± 3.3 and 56.5 ± 5.7 years) volunteered as subjects for this study to investigate age and gender effects on reciprocal moment ratios in knee muscle groups. Subjects completed 4 consecutive maximal contractions of dominant knee extensor and flexor muscles at 60°/s for the concentric (CON) and eccentric (ECC) torque tests and 20 consecutive maximal contractions at 180°/s and 1207s for the CON and ECC torque-work tests, respectively. Subjects also performed 5-s isometric (ISM) contractions at a 60° angle and 07s velocity for knee extensors. The results demonstrated that (a) ECC strength, particularly in males, was less influenced than ISM and CON strength by aging factors, (b) older groups, particularly females, possessed greater capacity for ECC muscle action than CON compared to other groups, and (c) older groups, particularly females, had a lower ratio of CON extensors:ECC flexors and a greater ratio of ECC extensors:CON flexors than young groups.

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Bonnie L. Van Lunen, Clayton Carroll, Kristen Gratias and Doug Straley

Context:

Rehabilitation.

Objective:

To determine the effects of a 20-min ice treatment on pain tolerance and peak torque.

Design:

A 2 × 2 × 6 factorial with repeated measures on 1 factor.

Setting:

Outpatient rehabilitation clinic.

Participants:

20 men and 15 women.

Intervention:

The participants were randomly assigned to an experimental (ice bag) or control (no ice bag) group.

Main Outcome Measures:

Peak electrical-stimulation output intensity (ESOI) was recorded in mV, and isokinetic peak torque (IPT), in N · m, every 4 min for 20 min.

Results:

ESOI and IPT increased over time. ESOI for the experimental condition was greater than for the control and within the experimental condition at 12, 16, and 20 min. No other differences were found for the IPT measures. There were no differences for ESOI and IPT between genders.

Conclusions:

Cryotherapy enables patients to tolerate greater output intensities but does not result in increased peak torque

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Wan X. Yao

The purpose of this study was to examine the effect of motor-unit recruitment on force variability by using computer simulated isometric contractions of a hand muscle (i.e., first dorsal interosseus). The force was simulated at 10 levels of excitation, ranging from 10 to 100% of maximum. Two recruitment conditions were simulated to compare the relative effect of motor-unit recruitment (MUR) on the relationship of force variability and level of force. One condition (40%MUR) recruited all motor units at 40% of the maximum excitation level, and the other (50%MUR) recruited all motor units at 50% of the maximum. The 40%MUR condition had a greater number of motor units than the 50%MUR group before the excitation level reached 50% of the maximum. The results showed that force variability increased at a faster rate before the completion of motor-unit recruitment and, thereafter, increased at a slower rate. In addition, the 40%MUR group showed greater force variability than the 50%MUR group. These data suggest that motor-unit recruitment is an important factor in causing force variability.

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James M. Wakeling, Meghan Jackman and Ana I. Namburete

The velocity at which a muscle fascicle will shorten, and hence the force that it can develop, depends on its gearing within the muscle belly. Muscle fascicle length depends on both its pennation and the thickness of the muscle. It was expected that external compression would reduce the muscle thickness and pennation and thus cause a reduction to the gearing of the fascicles relative to the muscle belly. Structural properties of the medial gastrocnemius muscle were visualized using B-mode ultrasound in six subjects. Measurements were taken during cyclical isotonic contractions at three different ankle torques and with the application of no, one, or two elastic compression bandages to the lower leg. Ankle torques and angular velocities were unaffected by the external compression. External compression did, however, reduce the muscle thickness and the fascicle pennation and resulted in a decrease in the gearing within the muscle belly. Reductions in gearing would result in an increase in the muscle fascicle shortening velocity that would reduce the force-generating potential of the fascicles. It is suggested that externally applied compression should not be considered a way to enhance muscle performance when based on the structural mechanics.