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PubMed1997

Cortical control of human motoneuron firing during isometric contraction.

We recorded whole scalp magnetoencephalographic (MEG) signals simultaneously with the surface electromyogram from upper and lower limb muscles of six healthy right-handed adults during voluntary isometric contraction. The 15- to 33-Hz MEG signals, originating from the anterior bank of the central sulcus, i.e., the primary motor cortex, were coherent with motor unit firing in all subjects and for all muscles. The coherent cortical rhythms originated in the hand motor area for upper limb muscles (1st dorsal interosseus, extensor indicis proprius, and biceps brachii) and close to the foot area for lower limb muscles (flexor hallucis brevis). The sites of origin corresponding to different upper limb muscles did not differ significantly. The cortical signals preceded motor unit firing by 12-53 ms. The lags were shortest for the biceps brachii and increased systematically with increasing corticomuscular distance. We suggest that the motor cortex drives the spinal motoneuronal pool during sustained contractions, with the observed cortical rhythmic activity influencing the timing of efferent commands. The cortical rhythms could be related to motor binding, but the rhythmic output may also serve to optimize motor cortex output during isometric contractions.

Adult↗
PubMed2006

Activation of nitric oxide modulator effect by isometric contraction in rat resistance arteries.

The participation of nitric oxide (NO) as a modulator of the isometric contraction, as well as the underlying mechanism, was investigated in rat small mesenteric arteries. In the presence of a functional endothelium, L-NAME and L-NA similarly increased the contractions induced by phenylephrine, dependently on the level of contraction. However, no effect was observed in the absence of a functional endothelium. In the presence of selective inhibitors of protein kinase C (PKC) and phosphatidyl-inositol 3-kinase (PI3K), calphostin-C and wortmannin, respectively, the effect of L-NAME was not modified. The same observation was done in the presence of the tyrosine kinase inhibitors, genistein and tyrphostin A-23. However, in the presence of a Ca2+-independent tyrosine kinase inhibitor, erbstatin-A, and on the presence of a non-selective kinase inhibitor, staurosporine, a strong reduction was observed. Our results suggest that protein kinases are involved in the activation of nitric oxide modulator effect on isometric contractions in resistance arteries.

Animals↗
PubMed1986

Modeling the contractility of urinary bladder smooth muscle using isometric contractions.

In the course of developing a clinical contractility index, isometric contractions of pig urinary bladder smooth muscle were analyzed in terms of phase plots (a plot of the rate of change of a variable, in this case force, as a function of the variable itself). The straight line describing the major part of the phase plots is characterized by the two parameters Fiso (the horizontal intercept) and c (the slope) of the line. It was found that changes in the geometry of the tissue sample and changes in extracellular calcium grossly influence Fiso, but not c, whereas changes in prestimulus rest time influence c, but not Fiso. It is concluded that Fiso is related to the number of available contractile units, and c is related to the limiting rate constant in the excitation-contraction coupling, which is probably determined by the release of intracellular calcium. The existence of the calcium paradox and the interpretation of the concept of facilitation or potentiation in this type of smooth muscle are discussed.

Animals↗
PubMedDate not supplied

[Cardiovascular effects of isometric contraction of the masticatory muscles in humans].

The cardiovascular responses to static exercise do not include, until now, studies about masticatory muscles. In this work the cardiocirculatory effects of the isometric contraction of masticatory muscles were compared with those evoked by the handgrip exercise. Non invasive and, in one case, invasive (aortic catheterism) monitoring of arterial blood pressure was performed. The findings show that mean blood pressure and especially heart rate do not increase during static exercise of masticatory muscles as reported in the literature for other muscular groups: it is suggested that the peculiar architecture of masticatory muscles may explain the absence of significant cardiovascular response to their isometric contraction.

Blood Pressure↗
PubMed1979

Relation between integrated electromyographic activity and biting force during voluntary isometric contraction in human masticatory muscles.

The relation between integrated electromyographic activity and computed biting force during voluntary isometric contraction was evaluated in the masticatory muscles of healthy subjects. The slopes of the curves relating integrated electromyographic activity to computed biting force in masseter muscles were steeper on the non-preferred chewing side than on the preferred chewing side, and they progressively became steeper during the course of continuous isometric contraction of a given biting force.

Action Potentials↗
PubMed1978

Muscular compliance during isometric contraction.

The changes in compliance during the development of maximal isometric contraction were studied at 0 degree C in isolated frog (Rana temporaria) sartorii. Three types of method were compared: controlled release from the plateau of tetanic tension (P0); controlled release at various times during the development of tension; and evaluation of the elastic stretching by means of an additional compliance. The length variations were performed at constant velocity under the control of an electromagnetic ergometer. It was confirmed that muscular compliance decreased with increase in developed tension. Furthermore, for a given value of tension, the compliance determined during the elastic lengthening was found to be significantly higher than the compliance measured during the elastic shortening. It was concluded that the variation in compliance during stretching and releasing seems to be due to a difference in behaviour of the elastic material.

Animals↗
PubMed1994

Relationship between force and electromyographic activity during rapid isometric contraction in power grip.

Force response and surface electromyographic (EMG) activity of extrinsic extensors and flexors of the hand were measured under 6 target force conditions during rapid pulse isometric contractions (power grip) targeted using an oscilloscope display of exerted and target forces. For target forces ranging from 16.7% to 50% of maximum voluntary contraction (MVC), the rate of force rise increased with the peak force, while the time to peak force remained almost constant. However, at target forces between 66.7% and 100.0% MVC, the rate of force rise leveled off and the time to peak force was prolonged. In association with these changes in force trajectories, modulation of the EMG activity of the flexor digitorum superficialis muscle was observed. At the lowest target force (16.7 MVC), the EMG of this muscle showed a single initial activity; the activity increased linearly up to the 50% MVC target force, while the duration was relatively constant. However, at target forces above 50% MVC, no further increase of the initial activity was observed, while the amplitude and duration of an additional activity progressively increased. These results indicate that the neural control of rapid isometric contraction at target forces at and below 50% MVC differs from that operating at larger target force levels.

Adolescent↗
PubMed1984

Kinetic model for isometric contraction in smooth muscle on the basis of myosin phosphorylation hypothesis.

A kinetic model was proposed to simulate an isometric contraction curve in smooth muscle on the basis of the myosin phosphorylation hypothesis. The Ca2+-calmodulin-dependent activation of myosin light-chain kinase and the phosphorylation-dephosphorylation reaction of myosin were mathematically treated. Solving the kinetic equations at a steady state, we could calculate the relationship between the Ca2+ concentration and the myosin phosphorylation. Assuming that two-head-phosphorylated myosin has an actin-activated Mg2+-ATPase activity and that this state corresponds to an active state, we computed the time courses of the myosin phosphorylation and the active state for various Ca2+ transients. The time course of the active state was converted into that of isometric tension by use of Sandow's model composed of a contractile element and a series elastic component. The model could simulate not only the isometric contraction curves for any given Ca2+ transient but also the following experimental results: the calmodulin-dependent shift of the Ca2+ sensitivity of isometric tension observed in skinned muscle fibers, the disagreement between the Ca2+ sensitivity of myosin phosphorylation and that of isometric tension at a steady state, and the disagreement between the time course of myosin phosphorylation and that of isometric tension development.

Animals↗
PubMed2006

Neural activation after maximal isometric contractions at different muscle lengths.

PURPOSE: To investigate i) whether neural activation dependence on muscle length is preserved with neuromuscular fatigue and ii) whether fatigue induced by a maximal isometric exercise is muscle length dependent. METHODS: Twelve male subjects performed two fatiguing quadriceps muscle exercises: FS is the fatigue carried out at short muscle length (S) (S = 40 degrees of knee flexion) and FL is the fatigue at long muscle length (L) (L = 100 degrees). Before and after each fatiguing exercise (i.e., three maximal isometric contractions maintained until 80, 60, and 40% of the initial maximal torque, respectively), activation level (AL, assessed by means of twitch interpolation technique), EMG activity (RMS), and peak doublet torque (Pd) were measured at the two lengths (S and L). RESULTS: First, AL was greater (P < 0.05) at L compared with S before and after both exercises. Second, despite a similar decrease in maximal voluntary torque (approximately 21% of the initial value) after the two exercises, AL and RMS were significantly reduced after FS (P < 0.05) but remained unchanged after FL, whereas the Pd decrease was more pronounced after FL than FS (P < 0.05). Nevertheless, after a given fatiguing exercise (i.e., FS or FL), AL, RMS, and Pd changes were similar at both postexercise test lengths (S and L). CONCLUSION: These results clearly demonstrate that i) the neural activation dependence on quadriceps muscle length is maintained with fatigue, and ii) neuromuscular fatigue after maximal isometric contractions is dependent on the muscle length at which the exercise is performed: short length preferentially induces neural activation impairment, whereas long length leads to higher contractile failure.

Adult↗
PubMed1992

The potentiating effect of prestretch on the contractile performance of rat gastrocnemius medialis muscle during subsequent shortening and isometric contractions.

The aim of the present study was to investigate the effect of an active stretch during the onset of a muscle contraction on subsequent active behaviour of the contractile machinery within an intact mammalian muscle-tendon complex. Muscle length and shortening velocity were studied because they may be important variables affecting this so-called prestretch effect. Seven gastrocnemius medialis (GM) muscles of the rat were examined. Tetanic, isovelocity shortening contractions from 3 mm above muscle optimum length (l0) to l0 - 2 mm, at velocities of 10-50 mm s-1 (dynamic experiments), were preceded by either an isometric contraction (PI) or an active stretch (PS). By imposing quick length decreases between the prephase and the concentric phase, all excess force generated in the prephase was instantaneously eliminated. This procedure only allowed small force changes during subsequent shortening (caused by the intrinsic properties of the contractile machinery). In this way, the influence of series elastic structures on subsequent muscle performance was minimized. Experiments were also performed at lengths ranging from l0 + 2.5 mm to l0 - 1.5 mm, keeping the length constant after the initial quick length changes (isometric experiments). For the dynamic experiments, enhancement of the performance of the contractile machinery (potentiation) was calculated as the ratio of the average force level over each millimetre of shortening during PS to that during PI conditions (PS/PI). For the isometric experiments, the PS/PI force ratio after 300 ms of stimulation was used. The main result of the present study confirmed results reported in the literature and experiments on isolated muscle fibres. For all conditions, a potentiation effect was found, ranging from about 2 to 16%. Muscle length appeared to have a large positive effect on the degree of potentiation. At the greatest lengths potentiation was largest, but at lengths below optimum a small effect was also found. A negative influence of shortening velocity was mainly present at increased muscle lengths (l0 + 2.5 mm and l0 + 1.5 mm). For the dynamic experiments, no interaction was found between the effects of muscle length and shortening velocity on potentiation. However, there was a clear difference between the isometric and dynamic responses: the dependence of potentiation on muscle length was significantly greater for the isometric contractions than for the dynamic ones. These isometric-dynamic differences indicate that the processes underlying prestretch effects operate differently under isometric and dynamic conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1999

Changes in conformation of myosin heads during the development of isometric contraction and rapid shortening in single frog muscle fibres.

1. Two-dimensional X-ray diffraction patterns were recorded at the European Synchrotron Radiation Facility from central segments of intact single muscle fibres of Rana temporaria with 5 ms time resolution during the development of isometric contraction. Shortening at ca 0.8 times the maximum velocity was also imposed at the isometric tetanus plateau. 2. The first myosin-based layer line (ML1) and the second myosin-based meridional reflection (M2), which are both strong in resting muscle, were completely abolished at the plateau of the isometric tetanus. The third myosin-based meridional reflection (M3), arising from the axial repeat of the myosin heads along the filaments, remained intense but its spacing changed from 14.34 to 14.56 nm. The intensity change of the M3 reflection, IM3, could be explained as the sum of two components, I14.34 and I14.56, arising from myosin head conformations characteristic of rest and isometric contraction, respectively. 3. The amplitudes (A) of the X-ray reflections, which are proportional to the fraction of myosin heads in each conformation, changed with half-times that were similar to that of isometric force development, which was 33.5 +/- 2. 0 ms (mean +/- s.d., 224 tetani from three fibres, 4 C), measured from the end of the latent period. We conclude that the myosin head conformation changes synchronously with force development, at least within the 5 ms time resolution of these measurements. 4. The changes in the X-ray reflections during rapid shortening have two temporal components. The rapid decrease in intensity of the 14.56 nm reflection at the start of shortening is likely to be due to tilting of myosin heads attached to actin. The slower changes in the other reflections were consistent with a return to the resting conformation of the myosin heads that was about 60 % complete after shortening of 70 nm per half-sarcomere.

Animals↗
PubMedDate not supplied

The effect of free isotonic and maximal isometric contraction exercises of the hip adduction on vastus medialis oblique muscle: an electromyographic study.

The purpose of this study was to analyse the activity of the vastus medialis oblique (VMO) muscle electromyographically during free isotonic and maximal isometric contraction of the hip adduction with knee extension performing exercises at sitting and in decubitus lateralis positions. Using a 8 channels Nicolet Electromyograph and Beckman surface mini electrodes, the activity of the vastus medialis oblique muscle was recorded among 15 healthy subjects, aged 19 to 33 years, (mean = 24.4, SD = 4.1) without prior knee and hip joints pathology. The sign was recorded in root mean square (RMS), expressed in microvolts. The data were statistically calculated employing an analysis of variance (ANOVA) and the Tukey test, at 5% of significance. The results showed that the electromyographic (EMG) activity of the vastus medialis oblique muscle was significantly greater in both exercises of maximal contraction when compared with free isotonic at decubitus lateralis. On the other hand, there was no significant difference of the electromyographic activity of the vastus medialis oblique muscle between the two exercises of maximal isometric contraction. These findings, within the experimental conditions used, suggest that prior stages of patellofemoral dysfunction, the exercises of hip adduction with knee extension could be performed isometrically or isotonically, at sitting and in decubitus lateralis positions, depending on the biological conditions of each patient.

Adult↗
PubMed1975

Myocardial oxygen consumption in isometric contraction of various strengths due to paired stimulation and quick-release.

In all of the experiments described, the oxygen consumption during isometric contraction was dependent solely upon the magnitude of active tension developed, and not on the rate of tension development or on the duration of contraction. There is a constant linear relationship between tension development and oxygen uptake. Any sudden interruption of active tension in the rising phase of the contraction terminates the energy utilization by the contractile system. These findings provide evidence to suggest that in isometric contraction of heart muscle, a rigid and constant coupling exists in the transformation of chemical energy to the mechanical tension.

Animals↗
PubMed1991

The relationship between isometric contraction durations and improvement in shoulder joint range of motion.

Proprioceptive Neuromuscular Facilitation (PNF) flexibility techniques are now being used in health and sports related activities, yet it is unclear as to the relationship between various isometric contraction time increments and joint range of motion. The purpose of this study, therefore, was to determine the relationship between a three-second, six-second, and ten-second maximum voluntary isometric contraction (MVIC). A modified PNF procedure referred to as the slow-reversal-hold-relax (SRHR) flexibility technique was employed in the investigation. It was hypothesized that longer MVIC time increments used with the SRHR flexibility technique would provide greater range of motion (ROM). Specifically, the ten-second MVIC was believed to be superior to the six-second and three-second MVIC. Furthermore, it was hypothesized that the six-second MVIC was superior to the three-second MVIC. Sixty subjects, ages 14-57 were randomly assigned to one of three treatment groups. Using a Leighton Flexometer, acute internal rotation of the shoulder joint was measured in degrees for six trials. Three passive stretch trials served as the baseline measurement for each subject (trials 1-3). The SRHR flexibility technique was used as the treatment for trials 4-6. A sixty-second rest interval common to clinical settings was integrated between each trial. The hypothesis was not accepted that a positive correlation existed between increased MVIC time and greater ROM.

Adolescent↗
PubMed2004

The effects of a sensorimotor training and a strength training on postural stabilisation, maximum isometric contraction and jump performance.

Previous studies revealed that adaptations following sensorimotor training, performed to improve functional joint or postural stability, were characterized by improvements in the rate of force development during maximum voluntary isometric contraction. In classical strength training studies using intense loads it has been shown that improvements in rate of force development is mainly due to adaptations in the intramuscular coordination. The purpose of the present study was to compare possible neuromuscular adaptations in two training groups following either sensorimotor or classical strength training over a period of four weeks. Additionally a control group was investigated to contrast the adaptations seen after training. Postural stability, maximum voluntary isometric contraction and performance in squat-jump and in drop-jump were measured before and after training. The results confirmed the positive effects of both training regimen on rate of force development and on maximum strength during maximum voluntary contraction as well as on jump performance, while only the improvements after the strength training were significant. Strength training reduced iMEG, while it was enhanced after sensorimotor training in most testing situations. Strength training had positive effects also on concentric contractions like squat-jump. The sensorimotor training improved performance in reactive drop-jump by enhanced neuromuscular activity immediately after ground contact. It is concluded that classical strength training with high loads basically improves the mechanical efficiency of the efferent drive on the motoneurons, whereas sensorimotor training alters the afferent input on the central nervous system. Both adaptations yield to specific effects during force development.

Adult↗
PubMed1993

Influence of force on muscle and skin sympathetic nerve activity during sustained isometric contractions in humans.

1. Our purpose was to test the hypothesis that efferent sympathetic nerve activity to non-active skeletal muscle (MSNA) and skin (SSNA) is independent of the level of force during sustained submaximal isometric contractions in humans. 2. In twelve healthy subjects, arterial blood pressure, heart rate, and MSNA (n = 6) or SSNA (n = 6) (peroneal microneurography) were recorded before and during isometric handgrip contractions sustained to exhaustion at 20, 40 and 60% of maximal force. Responses were examined at similar percentages of endurance time at each level of force. 3. Contraction duration decreased progressively with increasing force (495 +/- 54, 140 +/- 13, 73 +/- 8 s, respectively), but peak ratings of perceived effort were similar for the three force levels. 4. The peak increases in systolic pressure were not different among the three levels of force. The increases in diastolic and mean pressure were similar at 40 and 60% of maximal force, but were smaller at the end of 20% of maximal force. The contraction-induced rise in heart rate was directly related to the level of force. 5. The contraction-evoked stimulation of both MSNA and SSNA was similar during handgrip at 40 and 60% of maximal force, but was much less during handgrip at 20% of maximal force. The increases in SSNA were associated with increases in both skin blood flow and skin electrical conductance suggesting primarily sudomotor fibre activation. 6. These findings indicate that there is a minimum force necessary to elicit peak levels of MSNA and SSNA during sustained isometric contractions in humans. When normalized to endurance time, however, the regulation of these sympathetic outflows appears to be independent of force above this minimum level. The results also indicate that during this type of muscle activity the relationship between force and heart rate is different to that between force and peripheral sympathetic discharge.

Adult↗
PubMed1996

Relationships of the vibromyogram to the surface electromyogram of the human rectus femoris muscle during voluntary isometric contraction.

The relationship between vibromyographic (VMG) and electromyographic (EMG) signals during isometric contraction of the human rectus femoris muscles was studied. The method of least squares was used to obtain the best-fitting linear regression model to the root mean squared (RMS) values of the VMG and the EMG. It is shown that for the rectus femoris of four subjects, a linear VMG versus EMG relationship exists during 20-80% of the maximum voluntary contraction (MVC) at 30 degrees, 60 degrees, and 90 degrees of knee joint flexion angles. The relation between the VMG and the EMG may be explained by the order recruitment of motor units and by the "onion-skin" phenomenon of the firing rates of recruited motor units in the regulation of muscle force production as reported in electro-neurophysiologic studies.

Adult↗
PubMed2000

Fatigue responses of human triceps surae muscles during repetitive maximal isometric contractions.

Nine healthy men (22-45 yr) completed 100 repetitive maximal isometric contractions of the ankle plantar flexor muscles in two knee positions of full extension (K0) and flexion at 90 degrees (K90), positions that varied the contribution of the gastrocnemii. Electromyographic activity was recorded from the medial and lateral gastrocnemii and soleus muscles by using surface electrodes. Plantar flexion torque in K0 was greater and decreased more rapidly than in K90. The electromyographic amplitude decreased over time, and there were no significant differences between muscles and knee joint positions. The level of voluntary effort, assessed by a supramaximal electrical stimulation during every 10th contraction, decreased from 96 to 70% (P < 0.05) with no difference between K0 and K90. It was suggested that a decrease in plantar flexion torque was attributable to both central and peripheral fatigue and that greater fatigability in K0 than in K90 would result from a greater contribution and hence more pronounced fatigue of the gastrocnemius muscle. Further support for this possibility was provided from changes in twitch torque.

Adult↗