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Results for “Isometric Contraction”

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PubMed2005

Long-term repeatability of force, endurance time and muscle activity during isometric contractions.

We determined the repeatability and correlations between force, endurance and muscle activity during isometric contractions over three years. Twenty-six subjects, with and without complaints of the shoulder and neck, performed standardized maximal and submaximal shoulder-abduction contractions and wrist extension-contractions at yearly intervals from 1997 to 1999. Peak forces developed during maximal contraction and the endurance times of submaximal contractions during shoulder abduction and wrist extension were measured. Electromyography (EMG) of muscle activity was recorded bilaterally from the upper trapezius, middle deltoid, and forearm extensor muscles. Root mean square EMG amplitudes were calculated. We found statistically significant associations between peak forces developed during wrist extension and shoulder abduction, and between endurance times of submaximal wrist extension and shoulder abduction. No statistically significant changes in peak force and EMG(peak) were found over the measurement years. The responses were not statistically significantly influenced by gender, or neck and shoulder pain. However, we observed considerable intra-individual variation in the inter-year measurements particularly for the responses to submaximal contraction. Such large variations represent a challenge when attempting to use the responses to interpret the effects of therapies.

Adult↗

PubMed1993

Frequency of acoustic myography during isometric contraction of fresh and fatigued muscle and during dynamic contractions.

The frequency of the acoustic myographic (AMG) signal was examined during fresh and fatigued isometric contractions of quadriceps and during dynamic contractions of biceps brachii (BB) in healthy subjects. Recordings were obtained from quadriceps over a range of forces between 10% and 100% maximal voluntary contraction prior to, and 15 minutes after, a fatiguing exercise. Recordings from BB were obtained over a range of submaximal forces (0-8.5 kg) during concentric and eccentric contractions. The mean power frequency (MPF) of the AMG signal was analyzed during each of these contractions by fast-Fourier transform (FFT). The MPF was not significantly different (P > 0.05) during fresh and fatigued contractions of quadriceps and increased quadratically with force in both states (r = 0.81, fresh; r = 0.77, fatigued). During concentric contractions of BB the MPF initially increased with force, but then decreased at the heavier loads (> 5.5 kg). The MPF of eccentric contractions did not significantly (P > 0.05) alter with force. The AMG MPF was within a similar low frequency range for both muscles, during different types of contraction, and was unaltered with fatigue.

Adolescent↗

PubMed1994

Myofibrillar ATPase activity during isometric contraction and isomyosin composition in rat single skinned muscle fibres.

1. Myofibrillar ATPase activity, isometric tension (Po) and unloaded shortening velocity (Vo) were determined in single skinned fibres isolated from rat hindlimb muscles during maximal calcium activation at 12 degrees C. In each fibre, myosin heavy chain (MHC) isoforms were identified using electrophoresis and immunocytochemistry. ATPase activity was determined spectrophotometrically from NADH oxidation in a coupled enzyme assay. 2. On the basis of their MHC isoform composition, the fibres (n = 102) were divided into five groups containing the slow isoform, I MHC, or one of the fast isoforms, IIB MHC, IIA MHC, IIX MHC, or a mixture of the latter three. ATPase activity was significantly higher in IIB than in 2X and IIA fibres (0.230 +/- 0.010, 0.178 +/- 0.023 and 0.168 +/- 0.026 nmol mm-3 s-1, respectively). Mixed fibres had intermediate values. ATPase activity in slow fibres was considerably less (0.045 +/- 0.006 nmol mm-3 s-1). 3. The ratio between ATPase activity and Po, i.e. tension cost, was found to be 2.90 +/- 0.09, 2.56 +/- 0.14, 1.89 +/- 0.22, 1.52 +/- 0.13 and 0.66 +/- 0.004 pmol ATP nM-1 mm-1 s-1 in IIB, mixed, IIX, IIA and slow fibres, respectively. All the differences were statistically significant except that between IIA and IIX fibres. 4. Within each group of fibres with the same MHC composition, ATPase activity was found to correlate with Po, but not Vo. However, ATPase activity was found to correlate with Vo when all the fibre types were pooled together. 5. In thirty-seven fast fibres the MLC ratio, i.e. the proportion of the fast alkali light chain isoform, MLC3f, to the amount of the regulatory light chain, MLC2f, was determined. IIB fibres had the highest proportion of MLC3f and IIA fibres, the lowest. 6. A multiple regression analysis, used to distinguish between the effects of MHC and MLC composition, showed that ATPase activity was insensitive to the MLC ratio, whereas it had a significant impact on Vo. 7. The results obtained in this study indicate that in rat skeletal muscle fibres: (a) ATPase activity during isometric contractions and tension cost are strongly dependent on MHC isoform composition, and (b) there is no evidence that the alkali MLC ratio is a determinant of ATPase activity.

Adenosine Triphosphate↗

PubMed1998

A simple model of force generation by skeletal muscle during dynamic isometric contractions.

The force that an isometric skeletal muscle will produce in response to time-varying stimulation ("dynamic isometric" force) is important both for understanding muscle function and for designing neuroprostheses. This paper reports a model for predicting the force produced by an isometric skeletal muscle at rest length in response to a wide range of stimulation patterns. The model consists of two linear, first-order systems separated by a static nonlinearity. The rate constant of the second first-order system varies with force level. The model was validated using three cat soleus and three cat plantaris muscles. The following whole-nerve stimulation trains were used: single pulses (twitches), 2-4 pulses, constant rates, triangularly modulated interpulse intervals, and randomly modulated interpulse intervals. The model reproduced most responses accurately. The model shows that a force-dependent rate constant is essential for model validity, and could be used in the control of neuroprostheses.

Analog-Digital Conversion↗

PubMed1980

Systolic blood pressure responses during isometric contractions of large and small muscle groups.

The purpose of this study was to test for a difference between the systolic blood pressure responses to voluntary contractions of large and small muscle groups of the upper extremity. Systolic blood pressure was measured at approximately 20 sec intervals during sustained isometric contractions of the index finger adductors and handgrip muscles contracting at 40% of maximal voluntary contraction (MVC). Contractions were terminated when EMG activity exceeded 10% of maximal activity of selected accessory muscles or when the 40% MVC tension could no longer be held constant (+/-10%). The slopes of the regression lines representing systolic blood pressure responses to sustained isometric contraction of a small and large muscle group of the upper extremity were significantly different (p less than .001). Older theories that pressor responses are determined only by the % MVC and not by the mass of contracting muscle need to be reexamined.

Adult↗

PubMed2006

Influence of structure on the tissue dynamics of the human soleus muscle observed in MRI studies during isometric contractions.

This article investigates how the internal structure of muscle and its relationship with tendon and even skeletal structures influence the translation of muscle fiber contractions into movement of a limb. Reconstructions of the anatomy of the human soleus muscle from the Visible Human Dataset (available from the National Library of Medicine), magnetic resonance images (MRI), and cadaver studies revealed a complex 3D connective tissue structure populated with pennate muscle fibers. The posterior aponeurosis and the median septum of the soleus form the insertion of the muscle and are continuous with the Achilles tendon. The distal extremities of the pennate muscle fibers attach to these structures. The anterior aponeurosis is located intramuscularly, between the posterior aponeurosis and the median septum. It forms the origin of the muscle and contacts the proximal extremities of the soleus muscle fibers. MRI measurements of in vivo tissue velocities during isometric contractions (20% and 40% maximum voluntary contractions) revealed a similarly complex 3D distribution of tissue movements. The distribution of velocities was similar to the distribution of major connective tissue structures within the muscle. During an isometric contraction, muscle fiber contractions move the median septum and posterior aponeurosis proximally, relative to the anterior aponeurosis. The pennate arrangement of muscle fibers probably amplifies muscle fiber length changes but not sufficiently to account for the twofold difference in muscle fiber length changes relative to excursion of the calcaneus. The discrepancy may be accounted for by an additional gain mechanism operating directly on the Achilles tendon by constraining the posterior movement of the tendon, which would otherwise occur due to the increasingly posterior location of the calcaneus in plantarflexeion.

Achilles Tendon↗

PubMed1986

Electrical and mechanical changes in human soleus muscle during sustained maximum isometric contractions.

Experiments were designed to evaluate changes in the electrical activation and force generating capabilities of human soleus muscle during sustained, maximum isometric contractions. Eighteen experiments were conducted on 7 healthy subjects. Surface EMG, and in select cases, intramuscular fine wire EMG recordings, were made to assess the electrical activation of soleus. Subjects performed maximum isometric plantarflexion contractions of 1-3 min during which time supramaximal electrical pulses were delivered to the tibial nerve at 5-s intervals to elicit maximum M waves. M wave areas were assessed for evidence of neuromuscular junction failure. The results revealed that, on average, maximum force declined to 80% of unfatigued maximum by 60 s of effort, 74% by 90 and 120 s, and 70% by 180 s. M waves were stable for efforts up to 3 min, thereby providing little evidence for neuromuscular junction failure. In 3 experiments, total spike counts from intramuscular recordings displayed a 50% reduction in firing by 30 s of effort, with little additional slowing for up to 3 min. Although all of the fatigue-induced electrical and mechanical alterations in muscle activation reported earlier for intrinsic hand and foot muscles were verified in these experiments on soleus, the magnitudes and time courses of these changes were quite different. All changes were consistent with a muscle designed to optimally resist fatigue.

Action Potentials↗

PubMed1974

Isometric contraction of the abductor digiti minimi muscle in man.

Isometric contraction of the abductor digiti minimi muscle (ADM) has been studied in six normal subjects. Twitch contraction times of ADM ranged from 60 to 68 ms and twitch torque ranged from 2·33 to 6·24 × 10(-3) Nm. In three subjects torque declined by an average of 31% after tetanization at 50 Hz for 30 seconds but there was no similar diminution in the evoked muscle action potential suggesting that the fatigue arose from intrinsic muscular mechanisms. A marked decline in tetanic torque occurred with continued tetanization in two subjects for a total of five minutes, but this change was accompanied by a decrease in the muscle action potential. In six subjects threshold stimulation to the ulnar nerve at the wrist and to various sites over the motor point of ADM allowed 55 threshold twitch contractions to be identified after averaging. A unimodal range of contraction times ranging from 40-100 ms was found and this was confirmed by additional experiments in two subjects in whom 30 threshold twitch contractions were identified using a needle electrode to stimulate various sites in the motor point. Tetanization at 50 Hz was performed using threshold stimulus levels. Nine threshold tetanic contractions were evoked in two subjects. In eight tetanic torque progressively fatigued to between 14 and 20% within 60-90 seconds, but, in one tetanic contraction, torque proved relatively fatigue resistant. These results suggest that there is a homogeneous group of motor units in ADM (with respect to contraction time) and that this group contains what are probable fast twitch fatigue sensitive and fatigue resistant motor units. No evidence of a distinct group of slow twitch units was found.

Adult↗

PubMed1990

Time-course of force production by fast isometric contraction of the knee extensor in young and elderly subjects.

Rapid force production by isometric contraction of the knee extensor was examined in a wide range of force output for 12 healthy elderly (65-86 years) and 12 young (20-35 years) subjects. Time-course of tension development and duration of first burst EMG activities (AG-1) of the vastus medialis muscle were compared between both groups. Significant increase in the elderly as compared with the young was found in duration from the onset of the EMG to the rise of tension (TLT), but not in time from the rise to peak of force (FTmax), although FTmax tended to be longer in the elderly than the young at three different levels of force output. AG-1 duration was also prolonged in the elderly but the difference was not significant. The prolongation of TLT in the elderly suggests that spatio-temporal recruitments of the motor units and/or the percentage of fast twitch fibers decreases with aging.

Adult↗

PubMed1994

X-ray evidence for the elongation of thin and thick filaments during isometric contraction of a molluscan smooth muscle.

The elongation of thin and thick filaments during isometric contraction of a molluscan smooth muscle was studied by measuring spacing changes of meridional reflections in the medium-angle X-ray diffraction pattern. X-ray patterns from the anterior byssus retractor muscle of Mytilus edulis in the resting, active, and catch states were taken from the same part of a muscle bundle at a fixed specimen-to-detector distance, using imaging plates and 10 s exposure to synchrotron radiation. The third-order reflection (9.2 A) of the axial period of actin, and the fourteenth-order reflection (10.4 A) of the axial subunit-repeat of the thick filament are increased in spacing in the active and catch states. From accurately measured changes in the axial distance of the 9.2 A layer line from the origin, thin filament elongations in the active and catch states are estimated to be 0.48 and 0.32%, respectively, in a muscle that maintains a tension of 12.2 kg cm-2 in the active state and 9.8 kg cm-2 in the catch state. Thick filament elongations in the active and catch states are similarly estimated to be 0.33, and 0.28%, respectively, based on the axial shift of the 10.4 A reflection. The 0.48% elongation of the thin filament in the active state agrees with an elongation that is presumed by White and Thorson (1973) to estimate the lower limit of the thin-filament stiffness. It seems that in the catch state the activated and resting thin filament structures are intermixed. The activated parts of the thin filament are probably more elongated than the apparent value, 0.32%.

Actin Cytoskeleton↗

PubMed2001

Evaluation of grip strength with a sustained maximal isometric contraction for 6 and 10 seconds.

The grip strength in which subjects sustained a maximal isometric contraction for 6 seconds (the 6-second test) was compared with that for 10 seconds (the 10-second test) to clarify a reliable sustained grip strength test procedure. Fifty healthy students (22+/-4 years, 25 men and 25 women) participated in this study. Strength was measured continuously by the Dexter (Cedaron Medical Inc.). The maximal grip strength, the peak time, and the momentary strength every second during a trial were evaluated. For both tests, the reliable maximal strength and a typical strength-time curve consisting of an early peak time and a decrease in the strength after the peak gradually over time were obtained. In the 6-second test, the momentary strength after 5 seconds that was 82+/-10 approximately 87+/-7% of the peak strength showed good reliability. This variable may be effective in assessing the ability to sustain maximal grip strength.

Adolescent↗

PubMed1997

Mechanical behavior of skeletal muscle during intermittent voluntary isometric contractions in humans.

Changes in contractile speed and force-fusion properties were examined during repetitive isometric contractions with the knee extensors at three different target force levels. Seven healthy subjects were studied at target force levels of 30, 45, and 60% of their maximal voluntary contraction (MVC) force. Repeated 6-s contractions followed by 4-s rest were continued until exhaustion. Contractile speed was determined for contractions elicited by electrical stimulation at 1-50 Hz given during exercise and a subsequent 27-min recovery period. Contraction time remained unchanged during exercise and recovery, except for an initial rapid shift in the twitch properties. Half relaxation time (RT1/2) decreased gradually by 20-40% during exercise at 30 and 45% of MVC. In the recovery period, RT1/2 values were not fully restored to preexercise levels. During exercise at 60% MVC, the RT1/2 decreased for twitches and increased for the 50-Hz stimulation. In the recovery period after 60% MVC, RT1/2 values declined toward those seen after the 30 and 45% MVC exercise. The force oscillation amplitude in unfused tetani relative to the mean force increased during exercise at 30 and 45% MVC but remained unaltered during the 60% MVC exercise. This altered force-fusion was closely associated with the changes in RT1/2. The faster relaxation may at least partly explain the increased energy cost of contraction reported previously for the same type of exercise.

Adult↗

PubMed1982

Demonstration of the significance of isometric contraction for the formation of stress fibres in chick embryo fibroblasts.

Observations of the cytoskeleton visualized with Coomassie brilliant blue R250 were carried out on chick embryo fibroblasts emigrating from cell aggregates onto clean or siliconized cover glasses. Conditions for persistent isometric contraction were created in cells attached strongly to glass and to the aggregate. These cells showed a presence of abundant stress fibres in their cytoplasm. Upon induction of lamellipodia retraction, the stress fibres transformed into typical retraction fibres. In cells spreading onto siliconized glass, the fibres were rare. Upon cell contraction their lamellipodia detached, folded and fibres, if originally present, disintegrated. These observations extend the conclusions of Wohlfarth-Bottermann to tissue culture cells, formulated on the basis of extensive study on the structure and behaviour of cytoplasmic fibrils in slime mold plasmodia, that bundles of parallely oriented actomyosin filaments arrange as a result of the persistent isometric contraction of cytoplasm. The present result point out the significance of cell surface adhesive properties for the organization of cell cytoskeleton.

Animals↗

PubMed1982

Ultrastructure of clots during isometric contraction.

We explored the retraction or contraction of platelet-fibrin clots under isometric conditions. In the presence of micromolar calcium clots of normal platelet-rich plasma developed tension at an initial rate of 0.1 to 0.2 g/min per cm2 (initial cross-sectional area). Electron microscopy of clots fixed after attaining a force of 1.6 g/cm2 revealed platelets with elongated bodies and pseudopods in close apposition to fibrin strands which were oriented in cablelike fashion in the direction of tension. The development of tension could not be explained simply on the basis of platelet-platelet association and interaction alone. First, factor XIII-dependent cross-linking of fibrin fibers was critical to normal isometric contraction. Second, tension decreased linearly, rather than exponentially, when the platelet count in the platelet-fibrin clot was decreased, suggesting that platelets must be interacting with another component (i.e. fibrin). Thrombasthenic platelets, deficient in fibrinogen receptors, failed to develop tension or to align fibrin strands or pseudopods in the clot. Platelet-fibrin clots treated with vincristine to disassemble microtubules or cytochalasin B to disrupt microfilaments failed to develop tension and relaxed if these agents were added after tension had developed. Relaxation under these conditions, however, was not associated with loss of orientation of fibrin strands. Our findings suggest that platelet-fibrin interaction in clots under isometric conditions leads to orientation of fibrin strands and platelets in the direction of force generation. Tension develops as platelets simultaneously attach to and spread along fibrin strands, and contract. The contraction draws some fibrin into platelet-fibrin clumps and aligns other strands in the long axis of tension. The achievement and maintenance of maximum tension appears to depend on the development of platelet-fibrin attachments and extension of platelet bodies and long pseudopods containing bundles of microfilaments and microtubules along the oriented fibrin fibers.

Blood Platelets↗

PubMed1983

Electromyography reliability in maximal and submaximal isometric contractions.

The purpose of this study was to determine the reliability of average surface electromyography (EMG) within and between days. Nine subjects were tested on three different days. Five maximum and 10 submaximum (five at each of two different levels) isometric contractions were performed each day. The submaximum levels were determined by the 30% and 50% maximum voluntary contraction (MVC) on day one and kept constant over days two and three. The myoelectric signal from the triceps was full wave rectified and low-pass filtered at 1 Hz to yield a relatively noise-free average EMG that corresponded in time to the force signal. The scores were normalized to the force level. The reliability was estimated with intraclass correlations coefficients (R). The magnitude of score variation was also expressed as a ratio, coefficient of variation, CV = standard deviation/mean X 100%. R values were significantly greater for the submaximal levels than the 100% MVC; no significant difference existed between the two submaximal levels. The within-day CV values were similar for all three levels, ranging from 8% to 10%. The between-days variability was higher than that within days, ranging from 12% to 16%. The results suggest that submaximal isometric contractions are more reliable. Measurement error is substantial in this technique and should be included in statistical designs.

Arm↗

PubMed1996

No change in insulin mediators in human skeletal muscle during isometric contraction or recovery.

The rapid activation of glycogen synthase in human skeletal muscle during recovery from isometric contraction is dependent on an intact circulation, which suggests the requirement of an activating humoral factor. To determine whether the activating factor is insulin, muscle biopsies were obtained from subjects at rest, at fatigue, 3 min postexercise with an intact circulation, and 3 min postexercise during which circulation to the muscle was occluded. Two inositol phosphoglycan mediators of insulin action were isolated from the biopsies, and bioactivity was measured by determining the effects of the isolated mediators on the activities of purified cyclic AMP-dependent protein kinase, pyruvate dehydrogenase phosphatase and glycogen synthase phosphatase in vitro. Bioactivity was not altered by any condition compared with rest. These data suggest that changes in inositol phosphoglycans are not responsible for the circulation-dependent activation of glycogen synthase during recovery from exercise.

Adult↗

PubMed1993

The first thin filament layer line decreases in intensity during an isometric contraction of frog skeletal muscle.

During isometric contraction/activation of full-overlap and non-overlap live frog skeletal muscles, the intensity of the first thin filament layer line at the axial spacing of approximately 1/37 nm-1, when separated from the partially overlapping first thick filament layer-line at approximately 1/43 nm-1, remained unchanged in the inner radial region (0.02-0.08 nm-1) where a large intensity increase is observed in the rigor state. The intensity decreased in the outer radial region (0.08-0.18 nm-1) where this layer line is expected to peak in the resting state. The intensity decrease in the outer region became larger with increasing filament overlap; on activation of the non-overlap muscle, it was about half that of the full-overlap muscle. Thus the first thin filament layer line decreases in intensity and any indication of the rigor-like intensification is not observed at all during contraction. This intensity decrease can be attributed to the same structural changes giving rise to the intensity increase of the second thin filament layer line. The results indicate that the configuration of the myosin heads interacting with actin during contraction differs significantly from that of the rigor state. Four-fold rotational symmetry of the thin filament structure as a whole becomes more pronounced during isometric contraction of the overlap muscle.

Actin Cytoskeleton↗

PubMed1986

Changes in human alpha-motoneuron excitability during sustained maximum isometric contractions.

Experiments were conducted to evaluate the change in alpha-motoneuron excitability during sustained maximum isometric contractions of human triceps surae. A test H-reflex was used to assess motoneuron excitability 10 ms after a conditioning reflex was generated. The test reflex was compared to a reference H-reflex; both test and reference reflexes were of approximately equal amplitudes at the onset of the sustained maximum efforts. Both reflexes were assumed to be influenced by similar descending and peripheral inputs. In addition, the test reflex was influenced by the conditioning reflex. For the 4 subjects tested, the test reflex decreased in amplitude within the first 30-40 s of effort, while the reference reflex remained roughly constant or increased in amplitude. The decline of the test reflex relative to the reference was indicative of an inhibitory effect due to the conditioning reflex. In that the conditioning reflex was always generated 10 ms prior to the test reflex, the two factors most likely responsible for the inhibition would be recurrent inhibition and summation of motoneuron afterhyperpolarization. A combination of these two factors could also account for the associated slowing of motoneuron firing during sustained maximum efforts.

Action Potentials↗