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PubMed1976

Reflex activity in the masseter muscle of young individuals. I. Experimental procedure - results.

The monosynaptic myotatic (jaw jerk) potential (MSP) evoked in the relaxed masseter by a tap on the chin and recorded electromyographically in this muscle (latency 6-3 ms, duration 5-9 ms) was similar in subjects with, or without TMJ clicking. The latency of the myotatic potential was significantly shorter (5.8 ms) when evoked during isometric contraction. The duration was 5-2 ms. A potential with similar characteristics as the MSP was evoked by TMJ clicking, as we believe by excitation of muscle spindles from tissue transmitted vibrations (latency 5-5 ms, duration 5-6 ms). The TMJ click evoked MSP could in the contracting massester be followed by silent periods (SPs) both of the early (latency about 10 ms) and the late type (latency about 50 ms). A tap on the chin during isometric contraction of the jaw elevators, was followed by an SP of the early type (latency 12-2 ms, duration 17-5 ms) and sometimes by an SP of the late type (latency 58 ms, duration 18 ms). The late SP occurred significantly more often in patients with TMJ-dysfunction. The period of lower activity following an MSP can be divided into two phases: a first phase with a complete electrical silence, denoted SP, and a second phase during which the activity is slowly increasing to reach the control level. This latter phase will be called "depressed activity" (DA) and is introduced as a new parameter. The SP with tooth tapping (latency 10-5 ms, duration 9-4 ms) was similar in groups with or without TMJ clicking. The superimposed MSP (SMSP) preceding the SP at tooth tapping is supposed to be a manifestation of the "load-compensation reflex".

Adult↗

PubMed2005

Attenuation of the effect of remote muscle contraction on the soleus H-reflex during plantar flexion.

OBJECTIVE: We investigated to what extent the facilitation of the soleus (Sol) Hoffmann (H-) reflex during a phasic voluntary wrist flexion (Jendrássik maneuver, JM) can be modulated by graded plantar flexion force and conditioning wrist flexion force. METHODS: The subjects were asked to perform phasic wrist flexion under a reaction time condition. Sol H-reflex was evoked by stimulating the right tibial nerve at various time intervals (50-400ms) after the 'Go' signal for initiating JM while the ankle was at rest and while plantarflexing. The level of tonic plantar flexion force (isometric contraction of 10, 20 and 30% of maximal EMG) and conditioning wrist flexion (isometric contraction of 30, 50 and 80% of maximum voluntary contraction) during JM was graded systematically. RESULTS: Although JM facilitation could be seen 80-120ms after the flexor carpi radialis (FCR) EMG onset even while plantarflexing, the magnitude of JM facilitation under plantar flexion was significantly decreased compared to that at rest. The degree of decrease in JM facilitation did not depend on the level of plantar flexion force. In contrast, the degree of JM facilitation was proportional to the level of wrist flexion force while the ankle was at rest and while plantarflexing, though the amount of JM facilitation significantly decreased while plantarflexing. CONCLUSIONS: JM facilitation of Sol H-reflex is decreased while performing tonic voluntary contraction of the homonymous muscle. The degree of decrease in JM facilitation is independent of the level of homonymous muscle contraction, but depends on the level of remote FCR contraction. In clinical application, when we intend to elicit a maximum stretch reflex by JM, full relaxation of homonymous muscle should be carefully confirmed. SIGNIFICANCE: Our results provide evidence for better understanding of the features of JM and insight into its clinical application.

Acoustic Stimulation↗

PubMed1974

Effect of stretching on the elastic characteristics and the contractile component of frog striated muscle.

1. The force-velocity relationship and the stress-strain curve of the so-called series elastic component (s.e.c.) of frog sartorius, semitendinosus and gastrocnemius have been determined during shortening against a given force (isotonic quick-release) and at high speed (controlled release): (a) from a state of isometric contraction and (b) after stretching of the contracted muscle. In both cases the muscle was released from the same length: this was usually slightly greater than the muscle's resting length.2. The muscle released immediately after being stretched is able to shorten against a constant force, P, equal to or even greater than the isometric force, P(0), at the same length. When the force P applied to the muscle is reduced below P(0) the velocity of shortening is greater after stretching, and the force-velocity curve is therefore shifted along the velocity axis: the shift is maximal when P is near to P(0) and it decreases rapidly with decreasing P.3. The extent of shortening of the s.e.c. required to make the force fall from P(0) to zero is 50-100% greater when the muscle is released immediately after stretching than when it is released from a state of isometric contraction. This difference is found by using either the controlled release method or the isotonic quick-release method.4. If a time interval is left between the end of stretching and the onset of shortening of the contracted muscle (controlled release method), the length change of the s.e.c., for a given fall of the force, is reduced and approaches that taking place when the muscle is released from a state of isometric contraction.5. Curare does not affect the results described above, indicating that these do not depend on modification of the neuromuscular transmission.6. It is concluded that stretching a contracted muscle modifies temporarily: (a) its elastic characteristics, as shown by the greater amount of mechanical energy released for a given fall of the force at the muscle's extremities, and (b) its contractile machinery, as it is suggested by the change of the force-velocity relationship.

Animals↗

PubMed1993

Current X-ray diffraction experiments using a synchrotron radiation source.

A Fuji imaging plate and synchrotron radiation are the most distinct innovations of the last twenty years in the X-ray diffraction experiments on biological materials. Here we present results of recent experiments on skeletal muscles made at Photon Factory, Tsukuba. It is now possible to record a two-dimensional X-ray diffraction pattern from a rabbit or frog single skinned fiber with a 30-sec exposure. Although weaker compared with those from whole muscles, it shows layer-lines up to 5.1 nm. When the fiber is activated by Ca2+, the pattern changes in a way similar to that observed when a live muscle is electrically stimulated. Use of single fibers makes various types of structural experiments much easier than using whole muscles or fiber bundles. Not only suitable for physiological experiments, better diffusion makes it also suitable for biochemical experiments using various kinds of labels. Time-resolved experiments with imaging plates are possible by using an imaging-plate exchanger devised by Dr. Y. Amemiya. By combining this and a fast-acting mechanical shutter, it is possible to record a two-dimensional diffraction pattern from a frog whole muscle shortening at the maximum speed. The pattern thus obtained shows weakening of the 5.1 and 5.9-nm actin layer-lines and the third (14.3 nm) and the sixth (7.2 nm) myosin meridional reflections, compared with the pattern from isometrically contracting muscles. On the other hand, the second meridional reflection from the thick filament is intensified. These results suggest very different arrangement of myosin heads during active shortening from that during isometric contraction.

Animals↗

PubMed1994

NEUROMUSCULAR CONTROL AND KINEMATICS OF INTERMITTENT FLIGHT IN BUDGERIGARS (MELOPSITTACUS UNDULATUS)

Kinematic and electromyographic data were collected from budgerigars (parakeets), Melopsittacus undulatus, flying at different speeds in a variable-speed wind tunnel. Birds exhibited flap-gliding at low speeds and flap-bounding at high speeds. The percentage of time spent flapping generally decreased at intermediate speeds. These behavior patterns are consistent with minimizing energy expenditure according to aerodynamic theory. During intermittent glides, the pectoralis exhibited an isometric contraction while the supracoracoideus was inactive. During bounds, both muscles were inactive. Contrary to earlier work, our studies indicate that budgerigars do not exhibit simultaneous twitch contractions of the pectoralis during each wingbeat, but rather generate typical multiple-spike electromyographic bursts that represent motor unit action potential trains or asynchronous twitch contractions from different motor units. The relative intensity of electromyographic bursts from the primary flight muscles increased with flight speed. This may indicate an increase in force production. Our observations of isometric contractions during glides, along with patterns of variation in muscle activity and wingbeat frequency, do not support the hypothesis that small birds such as the budgerigar use flap-bounding as their only means of reducing power output during flight.

Journal Article↗

PubMed2004

Incubation of rat aortic rings produces a specific reduction in agonist-evoked contraction: effect of age of donor.

We studied the effect of age on the response of aortic rings to injury produced by three days' incubation, and the mechanism of this response. Five-mm rings of the thoracic aorta isolated from Wistar rats were incubated or not in culture medium. Isometric contraction evoked by agonists (norepinephrine or serotonin) or high [K(+)](e) was determined in the presence and absence of endothelium. Experiments were repeated in the presence of propranolol (0.3 microM), polymixin B (36 microM), pyrrolidine dithiocarbamate (50 microM) or glutathione (3 mM). Inductible NO-synthase and cyclo-oxygenase-2 mRNA were determined by real-time PCR, and glutathione-related enzymes and catalase activity by spectrophotometry. Incubation reduced the isometric contraction evoked by agonists but not by high [K(+)](e). The reduction in agonist-evoked contraction was greater in rings from adult (norepinephrine Emax-80%) than in young (-40%) rats. The removal of the endothelium had no effect. The reduction in norepinephrine-evoked contraction was not due to endotoxin contamination, beta-adrenoceptor-mediated dilation or any change in ring structure (no fibrosis or edema). Inductible NO-synthase (but not cyclo-oxygenase-2) mRNA increased on incubation. N(G)-nitro-L-arginine methyl ester partially restored contractility in rings from adult animals, further addition of an anti-oxidant restored norepinephrine-evoked contraction. Catalase fell with age and glutathione reductase increased upon incubation in rings from young donors only. In conclusion, incubation of the aorta produces a specific reduction in agonist-evoked contraction that involves induction of smooth muscle cell oxidative stress and iNOS. The reaction is greater in rings from older animals.

Age Factors↗

PubMed1989

[In vitro effect of calcitonin on guinea pig gallbladder].

Calcitonin (CT) is a 32 amino acidic polypeptide hormone which has been found in almost all species and whose effects are mainly concerned with calcium and phosphorous homeostasis. Three preparations are employed for therapeutic uses: salmon (sCT), porcine (pCT) and human CT (hCT). The sCT is the most powerful one and in human volunteers a strong relaxing effect has been shown on gallbladder (GB) basal volume and emptying in response to a meal, intraduodenal instillation of a liquid meal and i.v. cholecystokinin (CCK) infusion. Our study was aimed at investigating if a direct sCT effect could be demonstrated on smooth muscle strips from guinea pig GBs "in vitro" (organ bath). Isometric contractions were measured in response to maximal doses of acetylcholine (ACh: 10(-4) M), KCl (80 mM) and cholecystokinin octapeptide (CCK-OP: 10(-6) M), in absence and in presence of four doses of sCT (1 x 10(-9), 1 x 10(-8), 1 x 10(-7) and 1 x 10(-6) M). sCT did not affect the initial strip basal tone. ACh, CCK-OP and KCl caused, as expected, a powerful contraction of the strips, but no effect was shown when each of the sCT doses was administered before ACh (1.28+ 0.69 SEM without sCT vs 1.28g+ 0.69 with sCT; n = 6) and CCK-OP (1.46g+ 0.19 without sCT vs 1.46g+ 0.19 with sCT; n = 8) or 5 min after the induced KCl contraction. On the basis of these preliminary results, we conclude that no evidence of a direct sCT effect was found on guinea pig GBs when considering either basal smooth muscle tone or isometric contraction in response to ACh, KCl and CCK-OP. Further studies are therefore required to clarify the influence of CT on GB dynamics in vivo and to elucidate its the physiological significance.

Animals↗

PubMed2006

Sex differences in the rate of fatigue development and recovery.

BACKGROUND: Many musculoskeletal injuries in the workplace have been attributed to the repetitive loading of muscle and soft tissues. It is not disputed that muscular fatigue is a risk factor for musculoskeletal injury, however the disparity between gender with respect to muscular fatigability and rate of recovery is not well understood. Current health and safety guidelines do not account for sex differences in fatiguability and may be predisposing one gender to greater risk. The purpose of this study was to quantify the sex differences in fatigue development and recovery rate of lower and upper body musculature after repeated bouts of sustained isometric contractions. METHODS: Twenty-seven healthy males (n = 12) and females (n = 15) underwent bilateral localized fatigue of either the knee extensors (male: n = 8; female: n = 8), elbow flexors (male: n = 8; female: n = 10), or both muscle groups. The fatigue protocol consisted of ten 30-second sub-maximal isometric contractions. The changes in maximum voluntary contraction (MVC), electrically evoked twitches, and motor unit activation (MUA) were assessed along with the ability to control the sustained contractions (SLP) during the fatigue protocol using a mixed four-factor repeated measures ANOVA (gender x side x muscle x time) design with significance set at p < 0.05. RESULTS: There was a significant loss of MVC, MUA, and evoked twitch amplitude from pre- to post-fatigue in both the arms and legs. Males had greater relative loss of isometric force, a higher rate of fatigue development, and were less capable of maintaining the fatiguing contractions in the legs when compared to the females. CONCLUSION: The nature of the induced fatigue was a combination of central and peripheral fatigue that did not fully recover over a 45-minute period. The results appear to reflect sex differences that are peripheral, and partially support the muscle mass hypothesis for explaining differences in muscular fatigue.

Journal Article↗

PubMed1989

The fatigability of two agonistic muscles in human isometric voluntary submaximal contraction: an EMG study. II. Motor unit firing rate and recruitment.

The recruitment and firing rate of biceps brachii (BB) and brachioradialis (BR) motor units (MUs) were studied in the course of fatiguing isometric contractions at 20%-30% of maximal voluntary contraction (MVC). MU recruitment generally occurred throughout the maintained contraction and was similar for BB and BR muscles. Newly recruited MUs started to discharge in the form of bursts, the duration of which increased until a continuous rhythmical firing was achieved. Within each burst, the first interval between two consecutive discharges was usually the shortest. MU threshold was lowered just after the limit time of the maintained contraction. The MU's firing rate either increased or remained stable as a function of the elapsed time. It is concluded that (1) in fatiguing isometric contractions at 20%-30% MVC contractile failure is mainly compensated for by MU recruitment and a lowered MU threshold and (2) differences between in surface changes in the electromyogram of BB and BR muscles cannot easily be explained by related differences in MU firing rate and recruitment.

Adult↗

PubMed1991

[The static exercise-induced arterial hypertension test].

The exercise tests used in the detection and follow-up of hypertensive patients fall into two categories: isotonic (dynamic) and isometric (static). The isometric exercise-induced arterial hypertension test consists of an isometric contraction--usually a grip by the dominant hand--sustained for 3 minutes, with a strength that is 30 percent of maximum voluntary strength. This results in a continuous and parallel increase of systolic and diastolic blood pressures during the hand grip. The heart rate also continuously rises, though not to the level obtained by the isotonic exercise test. The increase in cardiac output observed, despite a fall in stroke volume, and the absence of decrease in peripheral vascular resistance contribute to a sometimes considerable rise in blood pressure, particularly diastolic pressure. These cardiovascular responses do not depend on the muscular mass involved but on the relative strength developed during isometric contraction. Owing to the lack of strict standardization and longitudinal studies, the isometric exercise test is of little value in the early detection of arterial hypertension. However, it is useful in the follow-up of hypertensive patients, as it provides precise information on the point of impact and mode of action of several antihypertensive drugs.

Blood Pressure↗

PubMed1989

Cross-bridge movement in rat cardiac muscle as a function of calcium concentration.

1. By applying the X-ray diffraction method to chemically skinned papillary muscles of the rat, the transfer of myosin heads from the thick to the thin filaments was studied as a function of Ca2+ concentration. 2. No significant transfer of the heads occurred when the Ca2+ concentration was below the threshold of contraction (pCa 6.2). 3. During the maximum isometric contraction at pCa 4.4, 80% of the myosin heads were transferred to the thin filament. 4. When the muscle was activated isometrically at low Ca2+ concentrations (pCa 6.2-5.8), where the average tension was less than 20% of the maximum, a disproportionately large number of myosin heads were transferred to the thin filament. 5. It was concluded that a significant fraction of the heads transferred at the low Ca2+ concentrations does not produce tension.

Animals↗

PubMed1984

A comparison between the physiological and histochemical characterisation of urethral striated muscle in the guinea pig.

In this in vitro study comparison has been made between the actomyosin ATPase activity and the contractile properties of the external urethral sphincter in the guinea pig. Histochemical analysis showed the external urethral sphincter to contain a mixture of alkali and acid stable actomyosin ATPase positive fibres in the ratio of 3:1. External urethral sphincter isometric contraction measurements were undertaken using specimens mounted transversely or longitudinally with respect to the urethral lumen. These contraction parameters have shown that the majority of fibres which constitute the external urethral sphincter correspond to the fast twitch type. In order to determine whether the relatively small fibre diameter of urethral striated muscle influenced the contraction results, when compared with controls, corrections for different volume ratios of slow and fast fibres were applied to the results. These modified values revealed close correlation between muscle fibre actomyosin ATPase content and isometric contraction responses.

Adenosine Triphosphatases↗

PubMed1991

Diastolic dysfunction in hypertrophic cardiomyopathy. Effect on active force generation during systole.

We tested the hypothesis that intracellular Ca++ [( Ca++]i) overload underlies the diastolic dysfunction of patients with hypertrophic cardiomyopathy. Myocardial tissue was obtained at the time of surgery or transplantation from patients with hypertrophic cardiomyopathy and was compared with control myocardium obtained from patients without heart disease. The isometric contractions and electrophysiologic properties of all myocardial specimens were recorded by standard techniques and [Ca++]i was measured with the bioluminescent calcium indicator aequorin. In contrast to the controls, action potentials, Ca++ transients, and isometric contraction and relaxation were markedly prolonged in the hypertrophic myocardium, and the Ca++ transients consisted of two distinct components. At 38 degrees C and 1 Hz pacing frequency, a state of relative Ca++ overload appeared develop, which produced a rise in end-diastolic [Ca++]i, incomplete relaxation, and fusion of twitches with a resultant decrease in active tension development. We also found that drugs with increase [Ca++]i, such as digitalis, exacerbated these abnormalities, whereas drugs that lower [Ca++]i, such as verapamil, or agents that increase cyclic AMP, such as forskolin, prevented them. These results may explain why patients with hypertrophic cardiomyopathy tolerate tachycardia poorly, and may have important implications with regard to the pharmacologic treatment of patients with hypertrophic cardiomyopathy.

Adult↗

PubMed1979

The time-course of energy balance in an isometric tetanus.

Unpoisoned sartorius muscles of Rana temporaria were stimulated tetanically in isometric contractions lasting up to 20 s at 0 degrees C. The observed enthalpy (heat + work) production and the chemical changes in these contractions were measured, and a comparison was made between the observed enthalpy and the enthalpy that could be explained by the chemical changes. Like earlier workers, we found that the only net known reaction of energetic significance that occurred was dephosphorylation of n-phosphoryl creatine (PC), and we found a significant evolution of unexplained enthalpy (UE), a portion of the observed enthalpy which could not be explained by the extent of PC dephosphorylation. We measured the total quantity and the rate of production of the UE, and we found that its rate of evolution, which was most rapid during the first 750 ms of contraction, fell progressively to zero by the 8th s of contraction: i.e., after 8 s of contraction, all the observed enthalpy is adequately explained by PC dephosphorylation. The time-course of evolution of the UE was slower than that of the labile enthalpy (a component of the enthalpy evolved in isometric contraction whose rate of production declines exponentially at approximately 1 s-1). We conclude that, although the magnitudes of these enthalpy quantities may be similar, they are not derived from the same chemical reaction in muscle.

Animals↗

PubMed1999

Evidence for a non-orthostatic origin of orthostatic tremor.

OBJECTIVES: Orthostatic tremor was first described by Heilman in 1984. It usually occurs in the legs during stance and decreases markedly during sitting or walking. The aim of this study was to determine if orthostatic tremor is invariably associated with the orthostatic and weight bearing conditions in the arms and legs, and to investigate the features of orthostatic tremor under different levels of peripheral loading. METHODS: Multichannel surface EMG recordings were obtained under different conditions (body posture and peripheral loading) from the proximal arm and leg muscles of seven patients fulfilling the clinical and electrophysiological criteria of orthostatic tremor. RESULTS: In weight bearing positions (stance; weight bearing on the hands on all fours), all patients showed 13 Hz-16 Hz tremor activity, predominantly in the active limb. No tremor activity could be found in a supine position with muscles at rest. Isometric contraction of the limbs in the supine position led to synchronous 13 Hz-16 Hz rhythmic activity in five patients. No tremor was seen when the subjects were suspended in a harness with relaxed legs. Isometric contraction of the legs in this position produced tremor in two patients. A stepwise reduction of the body weight by a harness reduced the tremor activity. Additional loading (10 kg-20 kg) during stance led to an increase in tremor amplitude, but tremor frequency remained unchanged. CONCLUSIONS: Orthostatic tremor is invariably present during stance or other weight bearing positions. It is not, however, always associated with orthostasis. In at least some patients it can be classified as an orthostasis independent action tremor. The failure of peripheral loading to modify tremor frequency indicates that orthostatic tremor may have a central, rather than a peripheral, origin.

Aged↗

PubMed1988

Modulation by the thyroid state of intracellular calcium and contractility in ferret ventricular muscle.

The purpose of this study was to determine whether the cardiac contractile abnormalities induced by hyperthyroidism or hypothyroidism are caused by changes in intracellular Ca2+ handling or by alterations in the distribution of isoenzymes of ventricular myosin. Right ventricular papillary muscles obtained from euthyroid ferrets and ferrets treated with L-thyroxine (hyperthyroid) or methimazole (hypothyroid) were loaded with the calcium indicator aequorin for recording intracellular Ca2+ levels during isometric contraction. In muscles from the hypothyroid ferrets, peak tension was reduced and the duration of contraction prolonged compared to the controls; these changes were associated with a Ca2+ transient of decreased amplitude and prolonged duration. Hyperthyroidism produced opposite changes in the time course of the Ca2+ transient and the associated isometric contraction. Native polyacrylamide gel electrophoresis was performed on myosin extracted from the right ventricular free wall of control and treated ferrets. The hyperthyroid state was associated with new myosin formation as indicated by the appearance of three myosin bands on the pyrophosphate gel. Gels of myosin from hypothyroid and euthyroid ferrets showed a single band that migrated with the slowest of the three bands from the hyperthyroid ferrets. These results suggest that changes in both Ca2+ handling and myosin isoenzymes may contribute to the contractile abnormalities observed in hyperthyroidism. Alterations in intracellular Ca2+ handling alone may account for the contractile changes induced by hypothyroidism.

Animals↗

PubMed1996

Nongenomic effects of aldosterone on phosphocreatine levels in human calf muscle during recovery from exercise.

Nongenomic in vitro effects of aldosterone on the sodium-proton antiport and intracellular second messengers have been described in human mononuclear leukocytes, vascular smooth muscle cells, and endothelial cells. To test the potential physiological relevance of these effects, an in vivo 31P magnetic resonance spectroscopy study on the human calf at rest and during exercise was performed in 10 healthy volunteers receiving either 1 mg aldosterone or placebo iv in a double blind, randomized, cross-over trial. Spectra were analyzed for phosphocreatine, ATP, phosphomonoesters, inorganic intracellular phosphate, and intracellular pH. Resting values remained unchanged by aldosterone. After isometric contraction of the calf (50% body weight for 3 min), phosphocreatine recovered to significantly higher levels after application of aldosterone compared with placebo. Other parameters were not significantly changed by aldosterone. Effects appeared immediately after isometric contraction and, thus, occurred within 8 min of aldosterone administration. They are, therefore, likely to represent the first contemporary evidence of nongenomic in vivo effects of aldosterone in man. These findings also point to an involvement of aldosteron in the acute stress adaptation of cellular oxidative metabolism in human muscle physiology.

Adult↗

PubMed1988

Mechanical behavior of rest contractions in cardiac muscle.

The effects of increasing rest periods on rest contractions and rest potentiation were analysed on isometric contractions of guinea pig papillary muscles at room temperature. Rest contraction amplitude is dependent on pause duration and potentiated for short pauses, which decreases as the rest period increases. their inotropic state is also dependent on the inotropic state of steady state control contractions. This statement is validated by the interdependence between control contractions and rest contractions amplitude and also because positive (5.0 mM CaCl2) and negative (0.5 mM NiCl2)inotropic interventions do not change the time course of dF/dt pause duration curves. They also show a slow activation characterized by a prolonged TTP that depends on pause duration being observed even in contractions occurring after short pauses. Rest contractions can be elicited under depolarized conditions produced by TKBa (Tyrode with high potassium and barium) and TKAdr (Tyrode with high potassium and adrenaline), showing that they are dependent on calcium influx occurring during the slow response of the cardiac action potential. The present study, carried out under depolarized conditions and in low extracellular Na+, shows that Na+ entry during the Na-dependent fast component of the action potential is necessary for the occurrence of rest potentiation. These findings suggest that the intracellular Na+, that can be exchanged by external calcium with the Na/Ca exchange mechanism, is necessary for the occurrence of the rest potentiation and to avoid the accelerated depression of the rest contractions with increasing pause duration.

Animals↗