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PubMed1990

The biology and medicine of sailing.

The physiological demands of sailing are highly specific, varying with wind conditions, type of craft, and crew position. In a light wind, the only physiological variable yet shown to influence performance is the resting blood sugar. Under high wind conditions, the skipper should be light (less than 60 kg), but crew members should be heavy (greater than 80 kg). Height does not seem a great advantage to crew, possibly because they then lack the muscular strength to exploit the added leverage. Muscle strength, endurance and a tolerance of anaerobic metabolism are all desirable attributes of crew, and competitive performance can be improved by a winter training programme that develops these aspects of muscle performance in the abdominal and thigh regions. The skipper must meet intense and prolonged cerebral demands in the face of periodic isometric work; performance may thus be helped by ingestion of carbohydrate over the course of a race. The ability to sustain isometric contractions in the 'hiking' position may also be improved if the muscles are preloaded with glycogen. The combination of a heavy body build, above average age for an athlete and sustained isometric contraction probably makes the yachting enthusiast vulnerable to ischaemic heart disease. Advisors to a sailing team must further take account of the risks presented by immersion in cold water, loss of sleep, circadian variations of performance over an event, and problems of motion sickness in rough weather.

Circadian Rhythm↗

PubMed2000

Modulation of triceps surae H-reflexes as a function of the reflex activation history during standing and stepping.

The facilitatory effectiveness of spindle afferent feedback is controlled by modulation of segmental reflex excitability such that the level of muscle activation is appropriate for the task. Phase-dependent modes of reflex modulation have been well-characterized. We hypothesized that segmental reflex excitability of the triceps surae was also modulated in a manner associated with the activation history of the spindle afferents and the segmental reflex pathway during isometric contractions, standing and stepping. In the first experiment. pairs of soleus (S) H-reflexes were evoked 80 ms apart with equal strength stimuli at rest and while subjects isometrically contracted their S against loads of 10%. 20%. and 50% of their maximum voluntary efforts. The percent depression of the second H-reflex relative to the first was used as a measure of the effect of reflex activation history. At rest, the second H-reflexes were depressed an average of 73% relative to the first. The degree of depression was progressively reduced as the plantarflexion torque increased. In the second experiment, paired H-reflexes were obtained from the S and medial (MG) and lateral gastrocnemii (LG) muscles while subjects were standing and during the stance phase of step initiation. The degree of depression of the second H-reflex during standing ( > 78%) was similar in magnitude to that produced at rest in Experiment I. At the end of the stance phase of stepping. depression of the second H-reflex of all three muscles was reduced to less than 25%. We conclude that the segmental reflex excitability is modulated as a function of the reflex activation history during these tasks.

Adult↗

PubMed1975

Creatine kinase equilibrium and lactate content compared with muscle pH in tissue samples obtained after isometric exercise.

Muscle biopsies taken from the musculus quadriceps femoris of man were analysed for pH, ATP, ADP, AMP, creatine phosphate, creatine, lactate and pyruvate. Biopsies were taken at rest, after circulatory occlusion and after isometric contraction. Muscle pH decreased from 7.09 at rest to 6.56 after isometric exercise to fatigue. Decrease in muscle pH was linearly related to accumulation of lactate plus pyruvate. An increase of 22mumol of lactate plus pyruvate per g of muscle resulted in a fall of 0.5pH unit. The apparent equilibrium constant of the creatine kinase reaction (apparent K(CK)) increased after isometric contraction and a linear relationship between log(apparent K(CK)) and muscle pH was obtained. The low content of creatine phosphate in muscle after contraction as analysed from needle-biopsy samples is believed to be a consequence of an altered equilibrium state of the creatine kinase reaction. This in turn is attributed mainly to a change in intracellular pH.

Adenosine Diphosphate↗

PubMed1998

Electromyogram activity and mean power frequency in exercise-damaged human muscle.

Eight volunteers performed two bouts of 50 voluntary maximal eccentric contractions of the knee extensors of one leg 3 weeks apart. During maximal voluntary isometric contractions performed at intervals after each bout, electromyogram (EMG) mean power frequency declined after bout one (P < 0.01 Duncan's test), whereas integrated EMG did not change after either bout. These results suggest that unaccustomed eccentric contractions produce a temporary reduction in mean muscle activation frequency during subsequent maximal isometric contractions.

Adult↗

PubMed1998

Metabolic fate of glucose in vascular smooth muscle during contraction induced by norepinephrine.

The metabolism of glucose in porcine carotid artery was tracked by isotopic methods during sustained isometric contraction induced by 100 microM norepinephrine (NE). In resting muscles, 74 and 18% of glucose taken up was accounted for by glycolysis and glycogen synthesis, respectively. Lactate production accounted for 69%, pyruvate production for 12%, and glucose oxidation accounted for 14% of glycolytic flux. The oxidation of glucose accounted for 57% of the consumption of O2 and thus constituted the primary oxidative substrate. During contraction by NE, glucose-uptake declined modestly below the resting basal rate. Glycolysis of external glucose and lactate production decreased and then increased with sustained contraction. Norepinephrine stimulated simultaneous glycogenolysis and glycogen synthesis, with net glycogen synthesis prevailing over 90 min of isometric contraction. Furthermore, NE modified the distribution of glucosyl units throughout the glycogen pool. The steady state rate of oxidation of glucose did not increase during NE contraction, even though O2-consumption increased. In contrast, increased glucose oxidation was demonstrable during contraction induced by 80 mm KCl. Furthermore, oxidation of exogenous fatty acid could be demonstrated during NE-induced contraction. Thus, NE exerts multiple effects on glucose and glycogen metabolism in smooth muscle, but it does not stimulate glucose oxidation.

Animals↗

PubMed1991

Acoustic myographic activity increases linearly up to maximal voluntary isometric force in the human quadriceps muscle.

Sounds produced during voluntary isometric contractions of the quadriceps muscle were recorded by acoustic myography (AMG) in seven healthy adults. With the subject seated, surface AMG and electromyography (EMG) were recorded over rectus femoris (RF) during isometric contractions, at different levels of maximum voluntary force. The AMG and EMG signals were amplified and integrated (IAMG and IEMG). The relationships between force and IAMG (r = 0.98 +/- 0.01, mean +/- 1 SD) and force and IEMG (r = 0.99 +/- 0) were linear in all subjects. The results for EMG confirm previous reports but those for AMG differ from the relationships reported for other muscles. Physiological and technical explanations are proposed for these differences and the necessity for further validation of the AMG technique is stressed.

Acoustics↗

PubMed2003

The conformation of myosin head domains in rigor muscle determined by X-ray interference.

In the absence of adenosine triphosphate, the head domains of myosin cross-bridges in muscle bind to actin filaments in a rigor conformation that is expected to mimic that following the working stroke during active contraction. We used x-ray interference between the two head arrays in opposite halves of each myosin filament to determine the rigor head conformation in single fibers from frog skeletal muscle. During isometric contraction (force T(0)), the interference effect splits the M3 x-ray reflection from the axial repeat of the heads into two peaks with relative intensity (higher angle/lower angle peak) 0.76. In demembranated fibers in rigor at low force (<0.05 T(0)), the relative intensity was 4.0, showing that the center of mass of the heads had moved 4.5 nm closer to the midpoint of the myosin filament. When rigor fibers were stretched, increasing the force to 0.55 T(0), the heads' center of mass moved back by 1.1-1.6 nm. These motions can be explained by tilting of the light chain domain of the head so that the mean angle between the Cys(707)-Lys(843) vector and the filament axis increases by approximately 36 degrees between isometric contraction and low-force rigor, and decreases by 7-10 degrees when the rigor fiber is stretched to 0.55 T(0).

Actins↗

PubMed1988

EMG computerized analysis of localized fatigue in Duchenne muscular dystrophy.

EMG power spectra obtained during a sustained isometric contraction were analyzed in a group of 10 children affected by Duchenne muscular dystrophy (DMD) and compared with those obtained in a control group of 5 normal children. In myopathic subjects the isometric contraction caused an increase of the total power, a progressive increase of power of the lower frequencies, a decrease of that of the higher frequencies, and a shift downward of the median frequency. In normal children an increase of the total power without a significant median frequency shift was noted. The modifications observed in DMD children were explained by a decrement of the firing rate of the more damaged fast twitch motor units. This decrease was probably induced by a relative predominance of activity of the slow twitch motor units, which are less damaged by the pathological process.

Child↗

PubMed2005

Modulation of passive force in single skeletal muscle fibres.

In this study, we investigated the effects of activation and stretch on the passive force-sarcomere length relationship in skeletal muscle. Single fibres from the lumbrical muscle of frogs were placed at varying sarcomere lengths on the descending limb of the force-sarcomere length relationship, and tetanic contractions, active stretches and passive stretches (amplitudes of ca 10% of fibre length at a speed of 40% fibre length/s) were performed. The passive forces following stretch of an activated fibre were higher than the forces measured after isometric contractions or after stretches of a passive fibre at the corresponding sarcomere length. This effect was more pronounced at increased sarcomere lengths, and the passive force-sarcomere length relationship following active stretch was shifted upwards on the force axis compared with the corresponding relationship obtained following isometric contractions or passive stretches. These results provide strong evidence for an increase in passive force that is mediated by a length-dependent combination of stretch and activation, while activation or stretch alone does not produce this effect. Based on these results and recently published findings of the effects of Ca2+ on titin stiffness, we propose that the observed increase in passive force is caused by the molecular spring titin.

Animals↗

PubMed1984

Factors affecting the equatorial X-ray diffraction pattern from contracting frog skeletal muscle.

Changes in the equatorial X-ray diffraction pattern from tetanized frog sartorius muscles (Rana catesbiana ) were studied by use of time-resolved data collection technique (time resolution, 0.5 sec) to give information about the dynamic properties of the cross-bridges. No significant changes in the intensity ratio of two equatorial reflections (I1,0/I1,1) were observed when isometrically contracting muscles were slowly stretched by 5-6%, in spite of marked force changes. The intensity ratio also showed no significant changes when the load on isometrically contracting muscles was suddenly increased from Po to 1.2-1.5 Po to produce isotonic muscle lengthening. Closer examination of the data indicated that a small decrease in the value of I1,1 was caused by both slow stretch and isotonic lengthening. Because of the scatter of experimental plots in I1,0, the effect of small change in I1,1 on the intensity ratio fell within the range of accuracy of measurement. It is suggested that no marked changes in myosin head orientation or in the number of the cross-bridges in the vicinity of the thin filaments take place in response to slow stretches or isotonic lengthening, and that the decreased regularity of the filament lattice may produce the change in I1,1.

Animals↗

PubMed2000

Motor unit synchronization in young and elderly adults.

In an effort to determine whether aging might alter the manner in which pairs of motor units are coactivated, the extent of motor unit synchronization was assessed in the first dorsal interosseous (FDI) muscle in seven young (mean 28 years) and eight older adults (mean 75 years). During constant-force isometric contractions at either 50% or 100% of maximal effort, motor units were recorded using a four-wire needle electrode and a multi-channel recording technique. Customized software was utilized to identify the occurrences of motor unit action potentials from the resultant three-channel signals. The magnitude of motor unit synchronization was determined using six variables that have been described previously in the literature. The extent of motor unit synchronization was similar in both young and older adults, occurring in nearly every motor unit pair observed. During the 50% maximum voluntary contractions, the CIS synchronization measure (the number of synchronized discharges per unit time) averaged 2.5 in the young subjects and 2.4 in the older individuals. The intensity of motor unit synchronization was somewhat greater during maximal force isometric contractions. We conclude that motor unit synchronization in the FDI muscle is a ubiquitous phenomenon that is not affected by the aging process.

Action Potentials↗

PubMed1997

Substance P released in the rostral brainstem of cats interacts with NK-1 receptors during muscle pressor response.

The release of immunoreactive substance P-like substances (irSP) was measured from sites in the rostral brainstem (at a level 1.3 mm anterior to the obex) of anesthetized cats in response to fatiguing isometric contractions using SP antibody-coated glass microprobes. The contractions caused a pressor and tachycardic response. irSP were released from sites in the medial subnucleus of the nucleus tractus solitarius (mNTS), the solitary tract and lateral tegmental field at this level of the brainstem. Injections of the specific NK-1 receptor antagonist, GR 82334, bilaterally into the mNTS significantly reduced the muscle pressor response, while bilateral injections of the SP NK-1 agonist, GR 73632, into the mNTS significantly increased the pressor and tachycardic responses during the contractions. Neither the antagonist nor the agonist, at the doses tested, affected the resting arterial pressure or heart rate. These data indicate that irSP are released from sites in the mNTS during the reflex pathways activated by isometric contractions and that they interact with NK-1 receptors in the area of the mNTS to affect the cardiovascular responses during the muscle pressor reflex.

Animals↗

PubMed2002

SEMG power spectrum changes during a sustained 50% Maximum Voluntary Isometric Torque do not depend upon the prior knowledge of the exercise duration.

Endurance time (Tlim) is a relevant indicator of muscular resistance to fatigue. It has been recently shown that SEMG changes computed during shorter periods (sub-maximal durations) than the whole test duration could serve to predict Tlim. The aim of the present study was to test whether the prior knowledge of the sub-maximal duration had any influence on Tlim prediction throughout SEMG changes. For this purpose, we compared myoelectric changes estimated over a 30-s isometric contraction whose duration was known by the subjects, to those changes measured during the first 30 s of a test prolonged until exhaustion. The effort intensity was set at 50% of the maximal voluntary isometric torque (MVIT). The myoelectric manifestations of muscle fatigue appeared to change in a similar way during both sessions for temporal and spectral analyses. In conclusion, the prior knowledge of the duration of sustained isometric contraction did not influence the motor unit recruitment strategy throughout surface EMG evaluation. These results confirmed that the use of SEMG changes computed over shorter periods than expected endurance time may be applied to investigate the capability of the initial rate of SEMG changes to predict muscle endurance capacity at 50%MVIT. This may be of particular interest for patient evaluation in the clinical field.

Adult↗

PubMed2005

Structure-function relation of the myosin motor in striated muscle.

Force and shortening in striated muscle are driven by a structural working stroke in the globular portion of the myosin molecules-the myosin head-that cross-links the myosin-containing filaments and the actin-containing filaments. We use time-resolved X-ray diffraction in single fibers from frog skeletal muscle to link the conformational changes in the myosin head determined at atomic resolution in crystallographic studies with the kinetic and mechanical features of the molecular motor in the preserved sarcomeric structure. Our approach exploits the improved brightness and collimation of the X-ray beams of the third generation synchrotrons by using X-ray interference between the two arrays of myosin heads in each bipolar myosin filament to measure with A sensitivity the axial motions of myosin heads in situ during the synchronous execution of the working stroke elicited by rapid decreases in length or load imposed during an active isometric contraction. Changes in the intensity and interference-fine structure of the axial X-ray reflections following the mechanical perturbation allowed to establish the average conformation of the myosin heads during the active isometric contraction and the extent of tilt during the elastic response and during the subsequent working stroke. The myosin working stroke is 12 nm at low loads, which is consistent with crystallographic studies, while it is smaller and slower at higher loads. The load dependence of the size and speed of the myosin working stroke is the molecular determinant of the macroscopic performance and efficiency of muscle.

Animals↗

PubMed1984

The relative isometric strength of type I and type II muscle fibres in the human quadriceps.

In isometric contractions, the type II fibres of animal muscle may have a greater specific strength than the type I fibres. This paper reports two retrospective analyses of the influence of fibre-type composition on the ratio of the voluntary isometric strength of the quadriceps to its cross-sectional area at mid-thigh. In 15 normal quadriceps, the ratio of the muscle's strength to its cross-sectional area was weakly correlated with the percentage contribution of type II fibres to a biopsy from the lateral mass of the muscle (r = 0.55, P less than 0.05). Linear regression suggested that type II fibres might have about three times the specific strength of type I fibres. Measurements of the atrophied quadriceps of 11 patients with unilateral knee injury/immobilization were standardized by comparison with the contralateral muscles. Strength/cross-sectional area was again correlated with the percentage area as type II fibres (r = 0.62, P less than 0.05). The data suggested that type II fibres might have about twice the specific strength of type I fibres. Despite the wide confidence limits of each of the analyses, they agree that, in isometric contractions, the type II fibres of the human quadriceps seem stronger, for their cross-sectional area, than the type I fibers.

Adolescent↗

PubMed2002

Mibefradil improves beta-adrenergic responsiveness and intracellular Ca(2+) handling in hypertrophied rat myocardium.

The present study investigated the effects of mibefradil, a novel T-type channel blocker, on ventricular function and intracellular Ca(2+) handling in normal and hypertrophied rat myocardium. Ca(2+) transient was measured with the bioluminescent protein, aequorin. Mibefradil (2 microM) produced nonsignificant changes in isometric contraction and peak systolic intracellular Ca(2+) concentration ([Ca(2+)](i)) in normal rat myocardium. Hypertrophied papillary muscles isolated from aortic-banded rats 10 weeks after operation demonstrated a prolonged duration of isometric contraction, as well as decreased amplitudes of developed tension and peak Ca(2+) transient compared with the sham-operated group. Additionally, diastolic [Ca(2+)](i) increased in hypertrophied rat myocardium. The positive inotropic effect of isoproterenol stimulation was blunted in hypertrophied muscles despite a large increase in Ca(2+) transient amplitude. Afterglimmers and corresponding aftercontractions were provoked with isoproterenol (10(-5) and 10(-4) M) stimulation in 4 out of 16 hypertrophied muscles, but were eliminated in the presence of mibefradil (2 microM). In addition, hypertrophied muscles in the presence of mibefradil had a significant improvement of contractile response to isoproterenol stimulation and a reduced diastolic [Ca(2+)](I), although a mild decrease of peak Ca(2+)-transient was also shown. However, verapamil (2 microM) did not restore the inotropic and Ca(2+) modulating effects of isoproterenol in hypertrophied myocardium. Mibefradil partly restores the positive inotropic response to beta-adrenergic stimulation in hypertrophied myocardium from aortic-banded rats, an effect that might be useful in hypertrophied myocardium with impaired [Ca(2+)](i) homeostasis.

Adrenergic beta-Agonists↗

PubMed2004

Role of mitochondrial superoxide dismutase in contraction-induced generation of reactive oxygen species in skeletal muscle extracellular space.

Contractions of skeletal muscles produce increases in concentrations of superoxide anions and activity of hydroxyl radicals in the extracellular space. The sources of these reactive oxygen species are not clear. We tested the hypothesis that, after a demanding isometric contraction protocol, the major source of superoxide and hydroxyl radical activity in the extracellular space of muscles is mitochondrial generation of superoxide anions and that, with a reduction in MnSOD activity, concentration of superoxide anions in the extracellular space is unchanged but concentration of hydroxyl radicals is decreased. For gastrocnemius muscles from adult (6-8 mo old) wild-type (Sod2(+/+)) mice and knockout mice heterozygous for the MnSOD gene (Sod2(+/-)), concentrations of superoxide anions and hydroxyl radical activity were measured in the extracellular space by microdialysis. A 15-min protocol of 180 isometric contractions induced a rapid, equivalent increase in reduction of cytochrome c as an index of superoxide anion concentrations in the extracellular space of Sod2(+/+) and Sod2(+/-) mice, whereas hydroxyl radical activity measured by formation of 2,3-dihydroxybenzoate from salicylate increased only in the extracellular space of muscles of Sod2(+/+) mice. The lack of a difference in increase in superoxide anion concentration in the extracellular space of Sod2(+/+) and Sod2(+/-) mice after the contraction protocol supported the hypothesis that superoxide anions were not directly derived from mitochondria. In contrast, the data obtained suggest that the increase in hydroxyl radical concentration in the extracellular space of muscles from wild-type mice after the contraction protocol most likely results from degradation of hydrogen peroxide generated by MnSOD activity.

Animals↗

PubMed2004

Effects of gentamicin sulfate on the contractility of myometrium isolated from non-pregnant cows.

Gentamicin sulfate, an aminoglycoside antibiotic known to cause depression of neuromuscular function, is a drug of choice in intrauterine antibiotic treatment of bovine chronical or subclinical uterine infections but its effects on the contractility of the cow uterus have not been studied. The aim of this study was to characterize, in vitro, the effect of gentamicin sulfate on spontaneous as well as prostaglandin F2alpha (PGF2alpha) and oxytocin-induced contractility of the non-pregnant cow uterus. Myometrial strips were isolated from non-pregnant cows in follicular phase and suspended in a jacketed organ bath filled with Krebs solution at 37 degrees C (pH 7.4) continuously bubbled with 95% oxygen and 5% carbon dioxide and isometric contractions were recorded using isometric force displacement transducer. After manifestation of the spontaneous contractions during equilibration period the test substances PGF2alpha (1 microM), oxytocin (2.5 mIU/ml bath fluid) and gentamicin sulfate (150-600 microm) were added to the bath. The effects of gentamicin sulfate on amplitude (g) and frequency of spontaneous and the agonist-induced contractions were evaluated by 20 min intervals. Data were statistically analyzed using the Student's t-test and Wilcoxon signed-rank test where appropriate. P <0.05 was considered to be significant. Gentamicin sulfate inhibited spontaneous, as well as oxytocin or PGF2alpha-induced contractions in a dose-dependent manner. Although both the frequency and amplitude of contractions were significantly inhibited by gentamicin sulfate, the effect on the frequency of the spontaneous and agonist-induced contractions were more prominent than on the amplitude. The result from this in vitro study indicated that gentamicin sulfate inhibits spontaneous as well as oxytocin and PGF2alpha-induced contractions of myometrium isolated from non-pregnant cows. This may be of importance considering the potentially negative effect of gentamicin sulfate on uterine involution in cows with puerperal endometritis, resulting in impairment of fertility performance.

Animals↗