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PubMedDate not supplied

Interaction of halothane and antibiotics on isometric contractions on rat-heart muscle.

Halothane (H), kanamycin (KM), streptomycin (SM), and chloramphenicol (CM) had direct negative inotropic effects on isometric contractions of isolated rat-heart muscles. Potassium penicillin-G did not show any significant changes in isometric contractions. The depression produced by these antibiotics was characterized by an abrupt onset, rapid progression, and rapid complete recovery, which suggests direct physicochemical rather than metabolic effects. On the other hand, the depression produced by H progressed slowly. When KM, SM, or CM were combined with H, there was a greater depression in isometric contractions than seen in the absence of H, suggesting that the innate characteristics of the antibiotics are augmented when any of them is administered together with halothane.

Animals↗
PubMed1998

Changes in reflex excitability following isometric contraction in humans.

Enhancement of muscle stretch following isometric contraction has been thought to occur as a result of inhibitory reflex mechanisms. Experiments with electrical stimulation (H-reflex) have demonstrated maximal H-reflex suppression during force relaxation followed by gradual recovery over the following 20 s. There has been considerable speculation as to whether electrical and mechanical stimulation elicit similar response behaviour. The present study examined postisometric reflex modulation following both stimulation modalities. In ten subjects dorsiflexion stimuli varying in speed and amplitude were applied after 30% and 60% maximal voluntary contraction (MVC). Modulation of the mechanically and electrically evoked responses following isometric plantarflexion was investigated. Reflex responses following both stimulation modalities were depressed during the course of force relaxation. A rather fast recovery was observed in mechanical stimulation. Postisometric response modulation was neither altered by the amount of isometric plantarflexion, nor by the amplitude of the applied stretch stimulus. With increasing velocity of the applied dorsiflexion, however, the shape of the reflex modulation persisted, but the magnitude of the responses was significantly enhanced. In electrical stimulation, however, recovery was delayed. It is suggested that postisometric reflex modulation is due to presynaptic inhibition. Moreover, possible peripheral mechanisms resulting from alpha-gamma-coactivation may also affect the stretch receptor itself because of inherent stiffness properties. The latter possibility particularly would explain the differences between mechanical and electrical stimulus modalities. With respect to practical implications, the very fast recovery (< 400 ms) of the stretch responses to control values strongly contradicts the interpretation that after isometric precontraction, suppression of reflex activity might be used for more efficient stretching of the tendomuscle system.

Adult↗
PubMed1999

Human muscle energetics during voluntary and electrically induced isometric contractions as measured by 31P NMR spectroscopy.

Electrical stimulation (ES) and voluntary contraction (VC) were compared in the quadriceps muscle of ten male volunteers. In both modes, a workload corresponding to 20% of maximal voluntary contraction was applied during 64 isometric contraction (5.5 s)-relaxation (5.5 s) cycles. The protocols were performed in a 1.5 T whole-body magnet. The Pi/PCr ratio and the intracellular pH (pHi) were monitored by 31P NMR spectroscopy during baseline, exercise and recovery periods, in a superficial region of the vastus medialis. During baseline, the Pi/PCr ratio (0.12 vs. 0.10) and the pHi (7.01 vs. 7.00) were comparable in both conditions. During exercise, the Pi/PCr ratio was higher (0.36 vs. 0.14) and the pHi was lower (6.85 vs. 7.07) during ES than during VC. For the same external work production, these results reflect a different metabolic solicitation in the ES quadriceps than in the VC ones.

Adult↗
PubMed1983

Physiological responses to sustained isometric contractions during one- and two-armed work. Role of muscle mass on cardiovascular responses to isometric work.

Cardiovascular and ventilatory responses were investigated during one- and two-armed isometric contractions at the same level of maximum voluntary contraction force (MVC). Significant differences were observed in heart rate, blood pressure, pulse arrival time, and oxygen intake in the contractions as compared with each other. These findings contradict the generally accepted view that cardiovascular responses at the same relative force in isometric muscle contractions, irrespective of the size of muscle mass engaged, are equal.

Adult↗
PubMed1980

Frequency analysis of the surface electromyogram during sustained isometric contractions.

Four male and four female volunteers served as subjects in these experiments to assess the frequency components of the surface EMG during and following brief (3 s) and sustained isometric contractions of the handgrip muscles. Two types of fatiguing contractions were performed. Contractions were either maintained to fatigue at a constant tension of up to 100% of their strength or were maintained as a sustained maximal effort in the unfatigues or previously fatigued muscle. The frequency components of the surface EMG were assessed by calculating the power spectra of 1.5 s samples of the EMG from a fundamental frequency of 4 Hz through the first 128 harmonics by Fourier analysis; the centre frequencies of the resultant power spectra were then used as an index of the mean frequency of the EMG. The results of these experiments showed that the centre frequency was independent of the tension exerted by the muscle during brief isometric contractions but decreased linearly with time throughout the duration of fatiguing isometric contractions at tensions between 25 and 100% MVC. During sustained maximal effort, the frequency initially decreased linearly with time. However, once the target tension could no longer be maintained, the centre frequency remained constant throughout the remainder of the contraction. The frequency was found to recover within 1 min following exercise at all tensions examined.

Adult↗
PubMed2006

Discharge rate during low-force isometric contractions influences motor unit coherence below 15 Hz but not motor unit synchronization.

The purpose of the study was to determine whether pairs of motor units that discharge action potentials at different rates during isometric contractions exhibit different levels of motor unit synchronization or coherence. Twelve subjects (28.6 +/- 6.1 years) performed isometric contractions at target forces slightly above the recruitment threshold (1.02-20.9%) of an isolated motor unit. Based on audio feedback, subjects maintained a relatively constant discharge rate of the isolated unit for about 80 s. Intramuscular electrodes were used to record the discharge of 47 pairs of motor units at rates that ranged from 8.07 to 13.6 pps. Correlated discharge between pairs of motor units was quantified with the common input strength (CIS) index, k' index, and coherence spectrum. Greater discharge rates across pairs of motor units were predicted (R2 = 0.36, P < 0.001) by higher coherence from 8 to 13 Hz (r = -0.52) and lower coherence from 0 to 4 Hz (r = 0.37). Indexes of motor unit synchronization (CIS and k') were strongly associated with motor unit coherence from 16 to 32 Hz (CIS: R2 = 0.63; k': R2 = 0.4; P = 0.001). The CIS index of motor unit synchronization and the motor unit coherence from 16 to 32 Hz did not vary with discharge rate. In contrast, the k' index of motor unit synchronization declined with discharge rate (r2 = 0.20, P = 0.001). Furthermore, greater discharge rates across pairs of motor units were accompanied by higher motor unit coherence in the 8-13 Hz band and lower motor unit coherence in the 0-4 Hz band. These results demonstrate that differences in discharge rate between pairs of motor units in first dorsal interosseus during low-force, isometric contractions were associated with modulation of the correlation in the discharge times of the two motor units at frequencies less than 15 Hz.

Acoustic Stimulation↗
PubMedDate not supplied

Electromyography tension and frequency spectrum analysis of some masticatory muscles at rest, isotonic and isometric contractions.

On a population of 52 subjects surface electromyographic recordings of temporals and masseters, simultaneously with mandible dynamic of closure and clenching, were performed, in order to study tension and frequency behaviour in three postural conditions: rest, isotonic and isometric contractions. Frequency was studied using the median resulting from FFT calculation, and a new computing method, which presents the proportion of frequencies making up the whole EMG signal, by steps of 50 Hz. Tension was calculated as well. The results permit us to draw the following conclusions: 1. a period of EMG silence was present in 51 of 52 subjects at mandible closure (SPA--Silent Period Area); 2. SPA onset was before teeth contact (22.5 msec., during the motion of the mandible), while its end was after closure (10.2 msec., during motionless phase of clenching). This allowed to use the SPA as a tool to clearly distinguish isotonic from isometric contraction; 3. the comparison of tension and frequency, expressed as median, showed that at rest a muscle presents low frequency and low tension. In active contraction both increase their values. Nevertheless, in active contraction, while no differences were found in frequency behaviour, tension showed a difference: although higher than at rest, isotonic contraction presented lower values than during isometric contraction; 4. the study performed by the new program showed that the low frequency at rest was due to the high proportion (30-40%) of frequencies of less than 50 Hz, while the increase at function was due to the parallel increase of frequencies comprised between 100 and 250 Hz. Because it is known that muscles are composed of fibers at low frequency and at high frequency of discharge, which play different functional roles, the last finding suggests that the mathematical analysis of the spectrum of frequencies, could provide a functional-histological image of the muscle.

Adolescent↗
PubMed1994

Inhibition in the release of immunoreactive beta-endorphin from the periaqueductal grey during isometric contractions of cat hind-limb muscles: the effects of clonidine.

Glass microelectrodes, coated with antibodies specific for beta-endorphin, were inserted into the right periaqueductal grey (PAG) (at PO.5-1.0 mm, LR 2.0 mm and 6.0 mm below the dorsal surface of the colliculi) of cats anesthetized with alpha-chloralose to determine whether immunoreactive beta-endorphins (ir-END) were released in response to fatiguing isometric contractions of the hind-limb muscles. Probes were inserted into the PAG prior to, during and following muscle contraction in the absence or presence of clonidine. ir-END was released from the PAG up to 3 h after surgery was completed while cats remained at rest. In the absence of clonidine, mean arterial pressure increased by 65 +/- 12 mmHg during contractions and the release of ir-END was inhibited during the contraction periods compared to resting periods. Clonidine abolished the pressor response to muscular contraction when injected into the PAG, but did not cause the release of ir-END during fatiguing isometric contractions. These data suggest that isometric contractions of skeletal muscle do not induce the release of ir-END-like substances from the PAG and clonidine does not attenuate the muscle pressor response by causing the release of ir-END from this level in the PAG.

Animals↗
PubMed2000

Effects of 1- or -adrenoceptor stimulation on work-loop and isometric contractions of isolated rat cardiac trabeculae.

1. We studied the effects of alpha1- or beta-adrenoceptor stimulation on the contractility of isolated rat ventricular trabeculae at 24 degrees C using the work-loop technique, which simulates the cyclical changes in length and force that occur during the cardiac cycle. Some muscles were injected with fura-2 to monitor the intracellular Ca2+ transient. 2. Comparison of twitch records revealed that peak force was greater and was reached earlier in work-loop contractions than in corresponding isometric contractions. This was attributed to the changes in muscle length and velocity during work-loop contractions, since the Ca2+ transients were largely unaffected by the length changes. 3. Stimulation of alpha1-adrenoceptors (with 100 microM phenylephrine) increased net work, power production, the frequency for maximum work, and the frequency for maximum power production (fopt). The increase in net work was due to the positive inotropic effect of phenylephrine, which was similar at all frequencies investigated (0. 33-4.5 Hz). The increase in fopt was attributed to an abbreviation of twitch duration induced by alpha1-stimulation at higher frequencies (> 1 Hz), even though the twitch became longer at 0.33 Hz. 4. beta-Adrenoceptor stimulation (with 5 microM isoprenaline) produced marked increases in net work, power output, the frequency for net work, and fopt. These effects were attributed both to the positive inotropic effect of beta-stimulation, which was greater at higher frequencies, and to the reduction in twitch duration. beta-stimulation also abolished the frequency-dependent acceleration of twitch duration. 5. The increase in power output and fopt with alpha1- as well as beta-adrenoceptor stimulation suggested that both receptor types may contribute to the effects of catecholamines, released during stress or exercise, although the greater effects of beta-stimulation are likely to predominate.

Animals↗
PubMed1995

Fatiguing isometric contraction of hind-limb muscles results in the release of immunoreactive neurokinins from sites in the rostral medulla in the anesthetized cat.

Antibody-coated microprobes were used to determine whether immunoreactive neurokinins (irNK) were released from sites in the brainstem during fatiguing isometric contractions of the triceps surae muscles in cats anesthetized with alpha-chloralose. Contractions were generated by stimulating the tibial nerve using a microprocessor-controlled stimulator. Microprobes were inserted into the periaqueductal grey (P 0.5-1.0 mm) or the medullary brainstem (either 3.0 or 3.5 mm rostral to the obex) prior to, during and following fatiguing contractions. No release of irNK was detected from the periaqueductal grey as a result of fatiguing isometric contractions. When probes were inserted 3.0 mm rostral to the obex, a basal release of irNK was detected from the medulla but this was inhibited during isometric contractions from sites corresponding to the lateral tegmental field. When probes were inserted into the more rostral site in the medulla (3.5 mm rostral to the obex), irNK were released in response to contractions from sites corresponding to lateral reticular nucleus, ventral regions of the nucleus tractus solitarius and the medial vestibular nucleus. No irNK were released from this site (3.5 mm rostral to obex) either during passive leg flexing, during nerve stimulation following gallamine injection and muscle paralysis or during stimulation of the central end of the cut tibial nerve. These results demonstrate that neurokinins are released from discrete sites in the medulla in response to fatiguing muscle contractions and suggest that tachykinin neurons may be a component of the pathways regulating blood pressure during ergoreceptor activation.

Anesthesia↗
PubMed1999

Hemoglobin near-infrared spectroscopy and surface EMG study in muscle ischaemia and fatiguing isometric contraction.

BACKGROUND: To assess how muscle ischaemia and isometric fatiguing contraction influence oxygen content in striated muscle. METHODS: We simultaneously measured changes in hemoglobin near-infrared (NIR) spectroscopy and in surface EMG before, during, and after muscle ischaemia and ischaemia plus muscle isometric fatiguing contraction. Seventeen healthy male subjects (age range: 19-40 yrs) were examined in our Clinical Neurophysiology Unit. Test I (9 subjects): hemoglobin NIR spectroscopy and stimulated surface EMG were measured for 2 minutes at rest, for 4 minutes during complete ischaemia of tibialis anterior muscle, and for twelve minutes during recovery. Test II (all subjects): hemoglobin NIR spectroscopy and surface EMG were measured for 2 minutes with the subjects performing brief non-fatiguing contractions, for 4 minutes with the subject performing maximal isometric contraction in complete ischaemia, and for twelve minutes during recovery. EMG parameters measured: median density frequency (MDF); muscle fiber conduction velocity (MFCV). NIR spectroscopy parameters measured: percentage of amplitude decrement (% AD) and nadir time (NT) during ischaemia and ischaemic effort; half-recovery time (1/2 RT) from ischaemia effort. RESULTS: At EMG, we observed a significant shift towards lower values of both MFCV and MDF during fatiguing isometric contraction. MDF recovery was faster then MFCV recovery. At NIR spectroscopy, the 1/2 RT slowed a fast pattern in twelve subjects and a slow pattern in five. A significant relationship was found between AD% and 1/2 RT values of test I and AD% and 1/2 values of test II. We found a positive relationship between NT and 1/2 RT in test II. CONCLUSIONS: Surface EMG and hemoglobin NIR spectroscopy can be applied simultaneously to evaluate both fatigue intensity and blood flow changes in striated muscle.

Adult↗
PubMed1987

Effects of carotid artery occlusion on the pressor response induced by sustained isometric contraction in the cat.

The effects of clonidine, a central alpha 2 agonist, on changes in blood pressure caused by muscle afferent nerve (ergoreceptor) activation and baroreceptor manipulation were studied in cats. Prolonged isometric contractions (ergoreceptor activation) of the gastrocnemius and plantaris muscles increased mean arterial pressure by 53 mmHg. This pressor response was not altered by naloxone (0.5 mumol.litre-1) but was eliminated by clonidine (0.5-2.0 micrograms) when injected into the cerebral aqueduct. Brief occlusion of the carotid artery (15-30 s) caused mean arterial pressure to increase by 32-42 mmHg at rest. Neither naloxone nor clonidine altered the magnitude of the reflex pressor response to carotid occlusion. Similar increases in pressure were measured when occlusion was applied during fatiguing isometric contractions; thus baroreceptor induced increases in pressure were superimposed on the ergoreceptor induced blood pressure changes. Naloxone did not affect the changes in pressure caused by either reflex response. Clonidine continued to eliminate the pressor response to muscular contraction but did not affect the pressure increase when the carotid occlusion was applied during contractions. Electrical stimulation of the carotid sinus nerve caused blood pressure to decrease by 36 mmHg during rest and by 41 mmHg during fatiguing isometric contractions. Clonidine did not alter the depressor response to carotid sinus nerve stimulation. These data may indicate that separate pathways centrally mediate the changes in blood pressure caused by ergoreceptor and baroreceptor afferent activation. The integration of the ergoreceptor pathway may involve a catecholaminergic-opioidergic system but the present results do not suggest a similar interaction for the baroreceptor integration.

Animals↗
PubMed2006

Effect of 1-acyl-5,6-dialkoxy-2-alkylthiobenzo[d] imidazoles on the action potential duration and isometric contraction in guinea pig atrium activated by carbachol and in guinea pig heart papillary muscles.

BACKGROUND: Studies have shown that some benzo[d]imidazole derivatives (1-(5,6-dimethoxy-2-methylthio-1H-benzo[d]imidazol-1-yl)-1-ethanone (1), 1-(6-ethylthio-5H[1,31dioxolo[4',5':4,5] benzo[d]imidazol-5-yl)-1-propanone (2), 1-(2-ethylthio-6,7-dihydro-1H[1,4]dioxino [2:3':4,5]benzo[d]imidazol-1-yl)-1-pro-panone (3) and 2,3,9,10-tetrahydro-8H [1,4]dioxino[2' 3":4',5']benzo[4,5]imid-azo[2,1-b][1,3]thiazin-10-one (4)) possess strong cardiotonic activity. The goal of this study was to investigate the effect of compounds 1-4 on the action potential (AP) duration and contractile force in guinea pig atrium activated by carbachol and in guinea pig heart papillary muscles. METHODS: The experiments were carried out on the guinea pig papillary muscles and atrium. Isometric contraction and transmembrane potential were recorded using a force transducer and standard microelectrode technique. RESULTS: Compounds 1-4 exerted a positive inotropic effect in a dose-dependent manner on the electrically driven left atrium and papillary muscles, more pronounced in atrium. In response to 1 micromol/L carbachol the AP duration at a 90 % repolarization in atrium shortened more than 70 %, the isometric contraction decreased to the similar level as well. Compounds 1 and 4 significantly antagonized the shortening of the AP duration induced by carbachol and increased it. Compound 1 abolished the reduction of isometric contraction as well. Derivative 3 significantly lengthened (31 ms) the AP duration at a 90 % repolarization in papillary muscles, while 1 and 4 failed to affect this index. The selective blockade of the rapid component of the delayed rectifier potassium current (Ikr) by dl-sotalol (1 micromol/L) did not show the substantial influence on benzimidazole effects. CONCLUSION: These findings support the hypothesis that the tested benzo[d] imidazole derivatives abolish the influence of carbachol on AP and the isometric contraction by inhibition of acetylcholine-activated potassium current (KACh) in guinea pig atrial myocytes and therefore may be beneficial for the prognosis of patients with advanced heart failure and atrial fibrillation.

Action Potentials↗
PubMed1996

The influence of location of electrode on muscle fiber conduction velocity and EMG power spectrum during voluntary isometric contraction measured with surface array electrodes.

A surface array electrode was used to investigate muscle fiber conduction velocity (MFCV) and EMG power spectrum during voluntary isometric contraction of m. biceps brachii. The mean power frequency (MPF) and the total power (TP) at various locations of the muscle were obtained from the power spectrum. MFCVs at various locations of the muscle were measured directly using the averaging method. The values of MPF, TP and MFCV were identified with respect to the electrode locations on the m. biceps brachii. MPF was shown as high near the end-plate and low near the tendon of the muscle during contraction of 40% of isometric maximum voluntary contraction (MVC). TP showed low value near the end-plate and the tendon of the muscle fiber and different values at different location setting electrode during voluntary isometric contraction. TP at each location on the muscle surface increased when the contraction levels increased in the contractions range of 20 to 60% MVC. MFCVs showed a high value near the end-plate and the tendon of the muscle during the contraction of 40% MVC. These results indicated that MPF, TP and MFCV at different locations on the muscle were different along the length of the muscle fiber.

Adult↗
PubMed2006

Effects of isometric contraction on intramuscular level of neuropeptide Y and local pain perception.

OBJECTIVE: The release of neuropeptide Y (NPY) is reported to increase in ischemic conditions and may thus be involved in chronic myalgia. The purpose of this study was to investigate the effect of isometric contraction on intramuscular levels of NPY in relation to local pain development. MATERIAL AND METHODS: Intramuscular microdialysis was performed in the masseter and trapezius muscles to determine NPY levels before, during, and after isometric contraction in 16 healthy females. Pain intensity was assessed simultaneously with VAS. Repeated measures ANOVA, t-test, and Pearson correlation analysis were used for statistical analyses. RESULTS: The level of NPY in the trapezius muscle was increased during and after contraction, while there was no change in the masseter muscle. The level of NPY before contraction was higher in the masseter muscle than in the trapezius muscle, and the levels in the two muscles were correlated before and during contraction. Low-level pain in both muscles after probe insertion increased significantly during contraction, but the pain was not correlated to the NPY level. CONCLUSIONS: Pain is developed in the trapezius and masseter muscles during repeated isometric contraction. The NPY level is increased in the trapezius muscle but is not associated with the pain development.

Bite Force↗
PubMed1993

Cardiovascular changes during an isometric contraction combined to a cold pressor test.

Fifteen normotensive male subjects (age, 27.9 +/- 1.3 years) performed isometric contractions (handgrip) at 15, 30 and 50% maximal voluntary contraction respectively for 7, 3 and 1 min. The contractions were sustained with or without a cold pressor test of the same duration (immersion of one foot in water of 5.2 +/- 0.1 degrees C). At rest, under the influence of a cold pressor test for 3 min, the heart rate increased from 74.3 +/- 2.2 to 83.3 +/- 2.1 beats min-1 (P < 0.001) in less than 10 s, and returned to the control value in 3 min; on the contrary, at the end of 3 min of cold pressor test, the systolic blood pressure was still above the control value (135.5 +/- 4.2 and 121.0 +/- 3.2 mmHg, P < 0.001). During the contractions, there was no significant difference between heart rate and systolic blood pressure values obtained with or without cold pressor test. Similarly, there was no significant change in the concentrations of plasma catecholamines. This absence of cardiovascular effect of cold pressor test associated with isometric contraction (for three relative force levels) is probably due to the increasing importance of sympathetic cardiac activation produced by the contraction with respect to that produced by the cold pressor test, the absence of supplementary changes in total peripheral resistance and a partial reciprocal inhibition of pain produced by the simultaneous performance of these two tests.

Adult↗
PubMed1983

Gravitational effects on human cardiovascular responses to isometric muscle contractions.

Isometric exercise induces profound cardiovascular adaptations increasing mean arterial pressure and heart rate. We investigated effects of simulated +Gz and -Gz respectively on the central and peripheral cardiovascular system. Sustained handgrip exercise was performed at 40% of maximum for 2 minutes in five subjects. This maneuver increased mean arterial pressure by 40-45 mm Hg both during head out water immersion which simulates weightlessness, as well as bedrest during -25, 0, and +25 degrees tilt from the horizontal. Lower body negative pressure (-60 mm Hg for 10 min) attenuated the response to handgrip exercise to 30 mm Hg. It also increased the heart rate minimally by about 20 beats per minute while the water immersion, as well as head up, head down and horizontal bedrest showed increments of about 50 beats per min. It was concluded that the response to isometric contraction is mediated through the high pressure baroreceptors, because similar responses were seen during stresses producing a wide variation in central venous pressure. During lower body negative pressure the increased sympathetic nervous activity itself increased resting heart rate and mean arterial pressure. The responses to static exercise were, therefore, weaker.

Adaptation, Physiological↗