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PubMed1978

Dynamic stiffness of rabbit mesotubarium smooth muscle: effect of isometric length.

The dynamic stiffness of mesotubarium smooth muscle from nonpregnant adult rabbits was measured continuously during isometric contraction by applying small (0.5 percent of the muscle length) sinusoidal length perturbations and measuring the amplitude and phase of the resulting tension perturbations. Stiffness during contraction was directly proportional to muscle tension; during relaxation stiffness at all tensions was significantly increased as compared to the values encountered during the rise of tension. Peak isometric tension and dynamic stiffness (determined at a common tension level) both decreased at shorter muscle lengths; the relative falloff in stiffness was significantly less than the tension decrease. Varying levels of muscle activation (obtained by changing stimulus strength and by applying quick releases to active muscle) had little effect on the measured elastic modulus. Comparisons of these results with published data on single-cell contractile properties imply a cellular locus for a portion of the measured stiffness.

Animals↗

PubMed1999

Dissociation between force and [Ca2+]i during extra systoles in guinea-pig ventricular muscle microinjected with fura-2.

Thin trabeculae were dissected from the right ventricle of guinea-pig heart and stimulated to contract isometrically at 0.5 Hz (26 degrees C). Rapid and transient changes of force were obtained by inducing three extra systoles (ES1-3) at 450-ms intervals. The two regular contractions (P1-2) following (ES1-3) were potentiated. Fura-2 salt was microinjected into the preparation to monitor intracellular calcium ([Ca2+]i). Three distinct phases of [Ca2+]i were seen: (1) a rapid rising phase to about 200 nmol L(-1), (2) a slower rising phase to a peak at 400 nmol L(-1), and (3) a slow decline to about 50 nmol L(-1). During ES1, there was a discrepancy between force, which decreased, and peak [Ca2+]i, which increased to 600 nmol L(-1). It is likely that the increased [Ca2+]i during the extra systoles reflects increased sarcolemmal calcium inflow, causing post-extra-systolic potentiation. Ryanodine (1-2 microM) was added to inhibit the intracellular calcium release and thus reduce the intracellular [Ca2+]i gradients following excitation. Ryanodine inhibited phase 1 of [Ca2+]i and abolished post-extra-systolic potentiation. There was a close relationship between dF/dt and [Ca2+]i with ryanodine during control and ES1-3. It is likely that fura-2 reports a spatially averaged [Ca2+]i and that phase 1 of the signal therefore apparently underestimates activator calcium in the close vicinity of the contractile elements.

Animals↗

PubMed1995

The dynamic response of Golgi tendon organs to tetanic contraction of in-series motor units.

Golgi tendon organs (TOs) display a dynamic response related to the rate of tension development by the motor units (MUs) which activate each TO. When several MUs are activated together the response increases non-linearly with respect to tension. The dynamic response has been re-examined by recording the responses of TOs, in the adult cat, to tetanic, isometric contractions of one or more MUs and has been found to comprise two components: for each MU-TO pair the instantaneous frequency between the first two impulses of the response was linearly related to the rate of rise of the tension, the slope of the relationship being greatest for the slow units. Similarly, when activating MUs were stimulated together, the relationships between frequency and the initial rate of tension rise were linear. When the relationships were expressed in terms of relative tension rate the organs were found to be most strongly activated by the IIb muscle fibres, the responsiveness at the level of the whole muscle being reduced by unloading effects. The later component of the response showed an exponential relationship between discharge frequency and rate of tension development, the slope of the relationship between frequency and the logarithm of tension rate being independent of MU size. Although absolute tension increases during a contraction, this did not appear to impinge on the response which was dominated by the dynamic parameters of the contraction.

Animals↗

PubMed1992

Muscle fatigue in frog semitendinosus: alterations in contractile function.

The purpose of this study was to characterize the contractile properties of the frog semitendinosus (ST) muscle before and during recovery from fatigue, to relate the observed functional changes to alterations in specific steps in the crossbridge model of muscle contraction, and to determine how fatigue affects the force-frequency relationship. The frog ST (22 degrees C) was fatigued by direct electrical stimulation with 100-ms 150-Hz trains at 1/s for 5 min. The fatigue protocol reduced peak twitch (Pt) and tetanic (Po) force to 32 and 8.5% of initial force, respectively. The decline in Pt was less than Po, in part due to a prolongation in the isometric contraction time (CT), which increased to 300% of the initial value. The isometric twitch duration was greatly prolonged as reflected by the lengthened CT and the 800% increase in the one-half relaxation time (1/2RT). Both Pt and Po showed a biphasic recovery, a rapid initial phase (2 min) followed by a slower (40 min) return to the prefatigue force. CT and 1/2RT also recovered in two phases, returning to 160 and 265% of control in the first 5 min. CT returned to the prefatigue value between 35 and 40 min, whereas even at 60 min 1/2RT was 133% of control. The maximal velocity of shortening, determined by the slack test, was significantly reduced [from 6.7 +/- 0.5 to 2.5 +/- 0.4 optimal muscle length/s] at fatigue. The force-frequency relationship was shifted to the left, so that optimal frequency for generating Po was reduced.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of Variance↗

PubMed2002

Changes in upper limb joint torque patterns and EMG signals with fatigue following a stroke.

PURPOSE: Little is known concerning changes in neuromuscular fatigue following a stroke. The purpose of this study was to evaluate the effect of a stroke on fatigue-related changes in upper limb torque patterns and electromyographic signals. METHOD: The paretic and non-paretic upper limb of 10 adults (51-79 years) who had a stroke (time since stroke: 3-75 months) were compared. Subjects had to perform a fatigue task consisting of a sustained maximal isometric contraction in elbow flexion until torque decreased to below 50% of initial. The main variables of interest assessed before, during and after fatigue were: (1) the torque in elbow flexion, as well as associated forces/torques at the shoulder and forearm; (2) the level of voluntary activation; (3) the amplitude (RMS); and (4) frequency content (median frequency) of electromyographic signals. RESULTS: Compared to the non-paretic side, the paretic side showed a lower level of voluntary activation and higher relative torque levels at the forearm and shoulder which could both be exaggerated with fatigue, and a lesser fatigue-related decrease in median frequency. CONCLUSIONS: Thus, greater fatigue-related changes in features of the central command (ability to maximally activate a muscle and ability to isolate effort to a muscle group) were observed for the paretic compared with the non-paretic side. This could be a confounding factor when assessing changes in peripheral measures of fatigue following a stroke using voluntary contractions.

Aged↗

PubMed1989

The effect of ethanol on rat vas deferens.

The effect of ethanol (3.9-390 mM) on resting tension of vas deferens, isometric contractions produced by field stimulation of the vas deferens and on the contractions produced by neurotransmitters, acetylcholine (ACh 0.55-55 mM), noradrenaline (NA 0.058-580 microM) and 5-hydroxytryptamine(5-HT 0.25-250 microM) was studied in rat vas deferens to see if ethanol modified neurotransmission and muscle contraction in this preparation. The results showed that ethanol had no significant effect on resting tension of the muscle, but reduced the contractions produced by field stimulation and by neurotransmitters. It was concluded that ethanol modifies neurotransmission in the rat vas deferens preparation, by reducing the contractions produced by field stimulation and by depolarizing agents.

Acetylcholine↗

PubMed1992

Reactive oxygen in skeletal muscle. I. Intracellular oxidant kinetics and fatigue in vitro.

We hypothesized that muscle fiber bundles produce reactive oxygen intermediates and that reactive oxidant species contribute to muscular fatigue in vitro. Fiber bundles from rat diaphragm were mounted in chambers containing Krebs-Ringer solution. In studies of intracellular oxidant kinetics, bundles were loaded with 2',7'-dichlorofluorescin, a fluorochrome that emits at 520 nm when oxidized; emissions were quantified using a fluorescence microscope. Emissions from unstimulated muscles increased over time (P < 0.001). Accumulation of fluorescence was slowed by addition of catalase (P < 0.001) or superoxide dismutase (P < 0.001) and was accelerated by repetitive muscular contraction (P < 0.05). To determine effects of reactive oxygen intermediates on fatigue, curarized bundles were stimulated to contract isometrically; force was measured. Catalase, superoxide dismutase, and dimethyl sulfoxide were screened for effects on low- and high-frequency fatigue. Antioxidants inhibited low-frequency fatigue [after 5 min of repetitive contractions, force at 30 Hz was 20% greater than control (P < 0.015)] and increased the variability of fatigue at 30 Hz (P < 0.03). Antioxidants did not alter high-frequency (200-Hz) fatigue. We conclude that 1) diaphragm fiber bundles produce reactive oxygen intermediates, including O2-. and H2O2; 2) muscular contraction increases intracellular oxidant levels; and 3) reactive oxygen intermediates promote low-frequency fatigue in this preparation.

Animals↗

PubMed1984

Mechano-mathematical model of excitation-contraction coupling in muscle tissue.

A system of equations, describing the excitation-contraction coupling in cardiac muscle is obtained on the basis of a physiological model and of a thermodynamical one of a system with external control, the latter model being proposed by the authors. A single isometric contraction is modelled for latent, activation and relaxation periods. The obtained theoretical results coincide qualitatively with the experimental data.

Animals↗

PubMed2004

Muscle activation and blood flow do not explain the muscle length-dependent variation in quadriceps isometric endurance.

We investigated the role of central activation in muscle length-dependent endurance. Central activation ratio (CAR) and rectified surface electromyogram (EMG) were studied during fatigue of isometric contractions of the knee extensors at 30 and 90 degrees knee angles (full extension = 0 degree). Subjects (n = 8) were tested on a custom-built ergometer. Maximal voluntary isometric knee extension with supramaximal superimposed burst stimulation (three 100-mus pulses; 300 Hz) was performed to assess CAR and maximal torque capacity (MTC). Surface EMG signals were obtained from vastus lateralis and rectus femoris muscles. At each angle, intermittent (15 s on 6 s off) isometric exercise at 50% MTC with superimposed stimulation was performed to exhaustion. During the fatigue task, a sphygmomanometer cuff around the upper thigh ensured full occlusion (400 mmHg) of the blood supply to the knee extensors. At least 2 days separated fatigue tests. MTC was not different between knee angles (30 degrees : 229.6 +/- 39.3 N.m vs. 90 degrees: 215.7 +/- 13.2 N.m). Endurance times, however, were significantly longer (P < 0.05) at 30 vs. 90 degrees (87.8 +/- 18.7 vs. 54.9 +/- 12.1 s, respectively) despite the CAR not differing between angles at torque failure (30 degrees: 0.95 +/- 0.05 vs. 90 degrees: 0.96 +/- 0.03) and full occlusion of blood supply to the knee extensors. Furthermore, rectified surface EMG values of the vastus lateralis (normalized to prefatigue maximum) were also similar at torque failure (30 degrees : 56.5 +/- 12.5% vs. 90 degrees : 58.3 +/- 15.2%), whereas rectus femoris EMG activity was lower at 30 degrees (44.3 +/- 12.4%) vs. 90 degrees (69.5 +/- 25.3%). We conclude that differences in endurance at different knee angles do not find their origin in differences in central activation and blood flow but may be a consequence of muscle length-related differences in metabolic cost.

Adult↗

PubMed2001

Tibialis anterior muscles in mdx mice are highly susceptible to contraction-induced injury.

Skeletal muscles of patients with Duchenne muscular dystrophy (DMD) and mdx mice lack dystrophin and are more susceptible to contraction-induced injury than control muscles. Our purpose was to develop an assay based on the high susceptibility to injury of limb muscles in mdx mice for use in evaluating therapeutic interventions. The assay involved two stretches of maximally activated tibialis anterior (TA) muscles in situ. Stretches of 40% strain relative to muscle fiber length were initiated from the plateau of isometric contractions. The magnitude of damage was assessed one minute later by the deficit in isometric force. At all ages (2-19 months), force deficits were four- to seven-fold higher for muscles in mdx compared with control mice. For control muscles, force deficits were unrelated to age, whereas force deficits increased dramatically for muscles in mdx mice after 8 months of age. The increase in susceptibility to injury of muscles from older mdx mice did not parallel similar adverse effects on muscle mass or force production. The in situ stretch protocol of TA muscles provides a valuable assay for investigations of the mechanisms of injury in dystrophic muscle and to test therapeutic interventions for reversing DMD.

Age Factors↗

PubMedDate not supplied

Dynamic and isometric protocols of knee extension: effect of fatigue on the EMG signal.

The objective of this study was to analyze the electromyographic (EMG) signal behavior of rectus femoris (RF), vastus medialis (VM), vastus lateralis (VL) and biceps femoris (caput longum) (BFCL) from nine women during fatiguing dynamic and isometric knee extensions tests and to determine their EMGFT (Electromyographic Fatigue Threshold). Surface electrodes, biological signal acquisition module, analogical-digital converter board and specific software were used. The RMS (Root Mean Square) values obtained from concentric phase (80 to 30 degrees) of the dynamic knee extension and from isometric contraction were correlated with time on each load by linear regression analysis. The respective slopes were correlated with the correspondent load to determine the EMGFT. Force (Kgf) and median frequency--MF (Hz) obtained during MIVC (Maximal Isometric Voluntary Contraction) performed before and after the fatiguing tests were calculated in Matlab environment. The results demonstrated that the endurance time decreases with higher loads, the EMG amplitude increase with time and was greater at higher loads, between muscles in dynamic exercise the RF and VL showed higher slopes, and in isometric exercise the VL showed the same behavior. The EMGFT values were similar in both exercises; the force values predominantly decreased after fatiguing tests; however the MF only decreased after some loads. The protocols proposed allowed standardizing protocols at least to induce the fatigue process and to determine the EMGFT as an endurance indicative, which may be used to evaluate the effectiveness of rehabilitative or training interventions indicated to reduce muscle weakness and fatigue.

Adolescent↗

PubMed2003

Intracellular sodium in mammalian muscle fibers after eccentric contractions.

The effect of eccentric contractions on intracellular Na(+) concentration ([Na(+)](i)) and its distribution were examined in isolated rat and mouse muscle fiber bundles. [Na(+)](i) was measured with either Na(+)-binding benzofuran isophthalate or sodium green. Ten isometric contractions had no significant effect on force (measured after 5 min of recovery) and caused no significant change in the resting [Na(+)](i) (7.2 +/- 0.5 mM). In contrast 10 eccentric contractions (40% stretch at 4 muscle lengths/s) reduced developed force at 100 Hz to 45 +/- 3% of control and increased [Na(+)](i) to 16.3 +/- 1.6 mM (n = 6; P < 0.001). The rise of [Na(+)](i) occurred over 1-2 min and showed only minimal recovery after 30 min. Confocal images of the distribution of [Na(+)](i) showed a spatially uniform distribution both at rest and after eccentric contractions. Gd(3+) (20 microM) had no effect on resting [Na(+)](i) or control tetanic force but prevented the rise of [Na(+)](i) and reduced the force deficit after eccentric damage. These data suggest that Na(+) entry after eccentric contractions may occur principally through stretch-sensitive channels.

Animals↗

PubMed1996

ATPase activity and force production in skinned rat cardiac muscle under isometric and dynamic conditions.

ATPase activity and force were determined in maximally activated skinned cardiac trabeculae from rat at 20 degrees C, 2.2 microns sarcomere length and pH 7.0. Measurements were performed during isometric contraction, and when square-wave shaped length changes with an amplitude of 0.5-5% of the resting length (Lo) and a repetition frequency of 2-167 Hz were imposed. Isometric ATPase activity amounted to 0.48 +/- 0.04 mM/s, and isometric force was 53 +/- 3 kN/m2. During length changes, an increase in ATP consumption was observed, reminiscent of the Fenn effect, to a maximal value of 1.7 times the isometric value, while the average force decreased. The frequency dependent increase in ATPase activity during length changes is compatible with a mono exponential recovery process, with a rate constant of 32 +/- 7/s. In a two-state scheme of crossbridge cycling, an apparent attachment rate of 28.6/s and an apparent detachment rate of 3.4/s were determined. To relate these observations with crossbridge kinetics during isotonic shortening, ramp-shaped length changes (2.5% Lo, 23 Hz) were imposed. ATPase activity and average force during ramp shortenings were similar to the values obtained during square-wave shaped length changes with the same amplitude and frequency. During lengthening ramps, the increase in ATPase activity and the decrease in average force were significantly less than during the square-wave protocol. These results indicate that the energetic effect of stretches is relatively small.

Adenosine Diphosphate↗

PubMed1985

[A study of tracking behavior in adults with cerebral palsy: on muscular contraction mechanism and motor perception].

Two tracking experiments were carried out to investigate the motor control system in adults with cerebral palsy. The first experiment analysed the frequency response characteristics and the surface electromyogram from biceps brachii and triceps brachii, when the cerebral palsy subjects made isometric contractions during the pursuit tracking task with sine wave targets. Patterns of reciprocal innervation of the antagonists explained the frequency response characteristics of tracking behavior of the cerebral palsy subjects. The second experiment analysed perceived response velocity during the compensatory tracking task by means of magnitude estimation method. Fitting power function to the relation between the input frequency and the perceived response velocity, coefficients of determination for the cerebral palsy subjects were found to be lower than those for the normal subjects. It suggests that the cerebral palsy subjects can discriminate their response velocity less exactly than the normal subjects in manual tracking system.

Adult↗

PubMed2003

Quantitative assessment of co-contraction in cervical musculature.

The co-contraction of cervical musculature is quantitatively assessed. For the modeling procedure, electromyographic (EMG) signal and anatomical data of the neck musculature were used. EMG signals were collected from eight sites of the neck bilaterally using Ag-AgCl surface electrodes from ten adult male subjects. The subjects performed voluntary isometric contractions gradually developing to maximum efforts in flexion, extension, left lateral bending and right lateral bending. The EMG-assisted optimization modeling procedure was used to estimate muscle forces. To quantify co-contraction, muscle forces were decomposed into task subset and co-contraction subset of muscle forces. To show the degree of co-contraction, 'co-contraction ratio' was defined as the proportion of co-contraction muscle forces to total muscle forces. The ranges of co-contraction ratio are 0.08-0.16 during extension, 0.30-0.41 during flexion, 0.27-0.32 during left lateral bending, and 0.30-0.36 during right lateral bending. In all cases, co-contraction ratios increase as external moments increase. The ratios of compressive spinal loads from the co-contraction subset of muscle forces to those from total muscle forces are similar to co-contraction ratios during the whole ramp period. This study provides data demonstrating quantitative measures of the contribution of muscle co-contraction to cervical spinal loads.

Adult↗

PubMed2003

Finger flexors fatigue in trained rock climbers and untrained sedentary subjects.

The present series of experiments were conducted to access the surface EMG frequency parameters during repeated fingertip isometric contractions to determine if they can be used as a fatigue index under specific grip used in rock climbing. Electromyograms of the finger flexors and extensors were characterised in ten elite climbers and ten non-climbers. The exercise consisted in reaching 80 % of maximal isometric finger force as quickly as possible intermittently with a 5-s contraction followed by 5-s of rest until exhaustion (i. e. when the subject was unable to maintain 80 - 70 % MVC force range for the 5 s). The results clearly indicate that expert climbers performed significantly greater fingertip force than sedentary subjects (420 +/- 46 N vs. 342 +/- 56 N). This force was maintained during twelve repetitions (12.88 +/- 4.96) in sedentary subjects, whereas the climbers maintained the force during nineteen repetitions (19.33 +/- 4.84). The median frequency of both the flexor and extensor EMG power spectra decreased during fatiguing isometric contractions, but at different rates in climbers and non-climbers. In non-climbers, the results replicated previous findings, whereas in climbers the results were novel.

Adult↗

PubMedDate not supplied

Effect of muscular ultrasound stimulation on power spectrum electromyography during a strengthening training.

The purpose of this study is to describe the electromyographic (EMG) power spectral evolution during a muscle strengthening training based on ultrasound stimulation applied to the right rectus femoris muscle simultaneously with its maximum voluntary isometric contraction (MVIC) in ten healthy subjects. From the surface EMG of rectus femoris the Zero Crossing (ZC) and integrated EMG (IEMG) were calculated. The Mean Power Frequency (MPF) and Median Frequency (MF) from the EMG power spectrum were obtained using a maximum entropy method. An Analysis of Variance (ANOVA) for repeated measures showed significant changes for IEMG and ZC throughout sessions and for MPF, MF and ZC between the pretest and posttest. Immediate increases in MPF, MF and ZC were observed (paired t-test, p < 0.05) after training and an increment of IEMG during sessions (one-way ANOVA, p = 0.01). The ultrasound pulsed stimulation applied simultaneously with a MVIC produced the development of the muscle force. The mechanism of action of this application could be the mechanical effect, compression and rarefaction, produced by the ultrasound stimulation during the muscle contraction process.

Action Potentials↗

PubMed1993

Relation between work and power calculated from force-velocity curves to that done during oscillatory work.

The force-velocity relation during oscillatory work was compared with that measured in the traditional way with quick release and force clamps using toad sartorius, frog sartorius, and mouse soleus muscles. Plotting the force and corresponding velocity data in this way produces a 'power-loop'. The 'power-loop' has less intuitive value than the frequently reported 'work-loops' but it is useful because it permits comparison with the force-velocity curve produced using traditional methods. The force/velocity combinations for oscillatory work during a contraction often exceed those that would be predicted from the force-velocity curve. Although it has been known for many years that more force is developed by stimulated muscle when it is being stretched than can be developed during an isometric contraction, my results show that the increase in force is of importance at stretch velocities that probably occur in vivo during locomotion.

Animals↗