Fatigue of mouse diaphragm muscle in isometric and isotonic contractions

Fatiguabilities of mouse diaphragm muscle in vitro in isometric and isotonic contractions were compared in this study. Isolated mouse diaphragm muscle was stimulated repetitively to induce fatigue during both isometric and isotonic contractions. The supramaximal electrical stimulation used was a train of 100-Hz, 0.5-ms pulses delivered to the muscle every 2 s for 0.5 s. The percentage decrease in isometric tension from beginning to end of the fatiguing process was used as the index of fatigue. The experiments were carried out at different PO2 levels in both normal and zero-glucose Ringer solutions. It was found that fatigue developed more rapidly in isotonic contractions than in isometric ones. Also, the extracellular glucose level demonstrated little effect on the muscle's short-term fatiguability, whereas reductions in the extracellular PO2 exerted a profound effect, especially in the case of isotonic fatigue.     

Dynamic and steady state characteristics of the rabbit gastrocnemius muscle determined in series measurements

1. Within the range of the given conditions of measuring static and dynamic properties of the rabbit gastrocnemius muscle the following results were obtained: a) the dependence of the maxima of isotonic shortening upon the relative length of the muscle at constant load is linear; b) the parameters of the non-linear dependence of the passive elastic force of the muscle upon its relative length (measured in series) were identified using asymptotic regression; c) the time course of isotonic contractions (at an interval from 0 to 0.3 s after the beginning of stimulation) could be satisfactorily approximated by responses of a linear system to a step-function; d) the time course of isometric contractions (at an interval from 0 to 0.3 s after the beginning of stimulation) could be closely approximated by responses of a linear system to a step-function. 2. The time constants of isotonic and isometric contractions were determined as the parameters of the corresponding linear systems. 3. The maximum rates of the isometric and isotonic contractions were determined as maxima of the first derivatives of the responses of the corresponding models. 4. The experimental set-up made it possible to compare the values of the parameters concomitantly followed at various muscle lengths and at various loads.     

The effect of shortening history on isometric and dynamic muscle function

Despite numerous reports on isometric force depression, few reports have quantified force depression during active muscle shortening (dynamic force depression). The purpose of this investigation was to determine the influence of shortening history on isometric force following active shortening, force during isokinetic shortening, and velocity during isotonic shortening. The soleus muscles of four cats were subjected to a series of isokinetic contractions at three shortening velocities and isotonic contractions under three loads. Muscle excursions initiated from three different muscle lengths but terminated at a constant length. Isometric force produced subsequent to active shortening, and force or shortening velocity produced at a specific muscle length during shortening, were compared across all three conditions. Results indicated that shortening history altered isometric force by up to 5%, force during isokinetic shortening up to 30% and shortening velocity during isotonic contractions by up to 63%. Furthermore, there was a load by excursion interaction during isotonic contractions such that excursion had the most influence on shortening velocity when the loads were the greatest. There was not a velocity by excursion interaction during isokinetic contractions. Isokinetic and isotonic power–velocity relationships displayed a downward shift in power as excursions increased. Thus, to discuss force depression based on differences in isometric force subsequent to active shortening may underestimate its importance during dynamic contractions. The presence of dynamic force depression should be realized in sport performance, motor control modeling and when controlling paralyzed limbs through artificial stimulation.     

A phenomenological model for estimating metabolic energy consumption in muscle contraction

A phenomenological model for muscle energy consumption was developed and used in conjunction with a simple Hill-type model for muscle contraction. The model was used to address two questions. First, can an empirical model of muscle energetics accurately represent the total energetic behavior of frog muscle in isometric, isotonic, and isokinetic contractions? And second, how does such a model perform in a large-scale, multiple-muscle model of human walking? Four simulations were conducted with frog sartorius muscle under full excitation: an isometric contraction, a set of isotonic contractions with the muscle shortening a constant distance under various applied loads, a set of isotonic contractions with the muscle shortening over various distances under a constant load, and an isokinetic contraction in lengthening. The model calculations were evaluated against results of similar thermal in vitro experiments performed on frog sartorius muscle. The energetics model was then incorporated into a large-scale, multiple-muscle model of the human body for the purpose of predicting energy consumption during normal walking. The total energy estimated by the model accurately reflected the observed experimental behavior of frog muscle for an isometric contraction. The model also accurately reproduced the experimental behavior of frog muscle heat production under isotonic shortening and isokinetic lengthening conditions. The estimated rate of metabolic energy consumption for walking was 29% higher than the value typically obtained from gait measurements.     

Recruitment of single human low-threshold motor units with increasing loads at different muscle lengths.

We investigated the recruitment behaviour of low threshold motor units in flexor digitorum superficialis by altering two biomechanical constraints: the load against which the muscle worked and the initial muscle length. The load was increased using isotonic (low load), loaded dynamic (intermediate load) and isometric (high load) contractions in two studies. The initial muscle position reflected resting muscle length in series A, and a longer length with digit III fully extended in series B. Intramuscular EMG was recorded from 48 single motor units in 10 experiments on five healthy subjects, 21 units in series A and 27 in series B, while subjects performed ramp up, hold and ramp down contractions. Increasing the load on the muscle decreased the force, displacement and firing rate of single motor units at recruitment at shorter muscle lengths (P<0.001, dependent t-test). At longer muscle lengths this recruitment pattern was observed between loaded dynamic and isotonic contractions, but not between isometric and loaded dynamic contractions. Thus, the recruitment properties of single motor units in human flexor digitorum superficialis are sensitive to changes in both imposed external loads and the initial length of the muscle.     

Differences between VO2 maxima of twitch and tetanic contractions are related to blood flow

The purpose of this investigation was to compare oxygen uptake (VO2) and fatigue characteristics of isotonic tetanic contractions with those observed during isotonic twitches in dog gastrocnemius-plantaris muscle. Tetanic contractions (1/s, 200-ms trains of 50 impulses/s) elicited a peak VO2 of 9.01 +/- 0.42 mumol.g-1.min-1, which declined 29% in 30 min. The peak was significantly lower during 4/s twitches (6.23 +/- 0.36 mumol.g-1.min-1), but the rate of decline was similar. Peak blood flow (Q) was 37% higher and decreased more slowly during tetanic than twitch contractions. VO2/Q and VO2/venous PO2 were similar in both groups at peak VO2 and later declined or remained constant over time. Power was significantly greater with tetanic contractions with the relative decline between 3 and 30 min similar in both groups (32 and 37%). In conclusion, tetanic contractions result in significantly higher VO2 and power than do twitch contractions. This was derived primarily from increased Q because the arteriovenous O2 difference was similar. A significant determinant of the difference in Q between twitch and tetanic contractions is mechanical hindrance of Q. There is relatively more time for unhindered flow in the tetanic contractions. In electrically stimulated muscles, maximal VO2 is related to Q and reflects mainly Q through the muscle rather than the VO2 capacity of the muscle.     

Energy Production in Cardiac Isotonic Contractions

The energy output of rabbit papillary muscle is examined and it is shown that there is more energy liberated in an afterloaded isotonic contraction than in an "equivalent" isometric contraction. This statement holds true regardless of whether equivalence is based on the proposition that tension or the time integral of tension is the best index of muscle energy expenditure. Besides the external work performed there is additional heat production in isotonic contractions and this heat increases as the afterload is decreased. The additional heat is more evident when tension rather than the time integral of tension is made the determinant of energy expenditure. It is shown in single contractions that the rate of isotonic heat production, regardless of afterload size, never exceeds the heat rate recorded in an isometric contraction at the same initial length. Experiments reveal no simple linear correlation between isotonic energy output and contractile element work. Problems associated with the compartmentalization of the energy output of a contraction are discussed.     

Calculation of muscle maximal shortening velocity by extrapolation of the force-velocity relationship: afterloaded versus isotonic release contractions

The maximal shortening velocity of a muscle (V(max)) provides a link between its macroscopic properties and the underlying biochemical reactions and is altered in some diseases. Two methods that are widely used for determining V(max) are afterloaded and isotonic release contractions. To determine whether these two methods give equivalent results, we calculated V(max) in 9 intact single fibres from the lumbrical muscles of the frog Xenopus laevis (9.5-15.5 °C, stimulation frequency 35-70 Hz). The data were modelled using a 3-state cross-bridge model in which the states were inactive, detached, and attached. Afterloaded contractions gave lower predictions of Vmax than did isotonic release contractions in all 9 fibres (3.20 ± 0.84 versus 4.11 ± 1.08 lengths per second, respectively; means ± SD, p = 0.001) and underestimated unloaded shortening velocity measured with the slack test by an average of 29% (p = 0.001, n = 6). Excellent model predictions could be obtained by assuming that activation is inhibited by shortening. We conclude that under the experimental conditions used in this study, afterloaded and isotonic release contractions do not give equivalent results. When a change in the V(max) measured with afterloaded contractions is observed in diseased muscle, it is important to consider that this may reflect differences in either activation kinetics or cross-bridge cycling rates.     

The effect of SO24-, NO-3, Ca2+ and temperature upon sodium-free contracture, isotonic and isometric contraction of the isolated rat right ventricle.

Sodium-free contracture was directly dependent on the presence of Ca2+ in Tyrode sulphate solution. The first phase of contracture increased in Tyrode nitrate solution more quickly than in sulphate solution, while the second phase was the same. Higher amplitude of isotonic contractions was observed on substituting nitrates for chlorides in the Tyrode solution than in sulphate substitution. In Tyrode sulphate solution, spontaneous contractions appeared in 50% of the experiments. The relationship between the duration and tension of isotonic contraction, given the same pre-loading, showed greater diminution of the contractions, at the lower temperature (22 degrees C) than at the higher temperature (35 degrees C), whereas passive stretching of the muscle was not influenced by the temperature. Isometric contrations lasted longer at the lower temperature. The amphlitude of isometric contractions was directly correlated to the frequency of electrical stimulation at a constant temperature (22 degrees C). The results showed that shifts of the calcium participating in contraction of the rat ventricle are influenced both by the anion and cation content of the medium and by its temperature.     

Complex myograph allows the examination of complex muscle contractions for the assessment of muscle force, shortening, velocity, and work in vivo

Background

The devices used for in vivo examination of muscle contractions assess only pure force contractions and the so-called isokinetic contractions. In isokinetic experiments, the extremity and its muscle are artificially moved with constant velocity by the measuring device, while a tetanic contraction is induced in the muscle, either by electrical stimulation or by maximal voluntary activation. With these systems, experiments cannot be performed at pre-defined, constant muscle length, single contractions cannot be evaluated individually and the separate examination of the isometric and the isotonic components of single contractions is not possible.

Methods

The myograph presented in our study has two newly developed technical units, i.e. a). a counterforce unit which can load the muscle with an adjustable, but constant force and b). a length-adjusting unit which allows for both the stretching and the contraction length to be infinitely adjustable independently of one another. The two units support the examination of complex types of contraction and store the counterforce and length-adjusting settings, so that these conditions may be accurately reapplied in later sessions.

Results

The measurement examples presented show that the muscle can be brought to every possible pre-stretching length and that single isotonic or complex isometric-isotonic contractions may be performed at every length. The applied forces act during different phases of contraction, resulting into different pre- and after-loads that can be kept constant - uninfluenced by the contraction. Maximal values for force, shortening, velocity and work may be obtained for individual muscles. This offers the possibility to obtain information on the muscle status and to monitor its changes under non-invasive measurement conditions.

Conclusion

With the Complex Myograph, the whole spectrum of a muscle's mechanical characteristics may be assessed.     

The synthesis and biological activity of alkyloxy prostaglandin analogues.

Prostaglandin analogues in which the alkyl chain attached to C-15 in the natural compounds is replaced by an alkyoxyalkyl group have been synthesised. Compounds of the 17-oxa series are particularly potent luteolytic agents and are selective in the sense that they are less effective than PGE-2alpha in causing isotonic contractions of isolated uterus muscle.     

Studies of arterial smooth muscle relaxation in younger (16-18 week) and older (28-31 week) spontaneously hypertensive rats

Both isometric and isotonic relaxation rates have previously been reported to be decreased in caudal arterial and mesenteric resistance arterial smooth muscle from 16- to 21-week-old spontaneously hypertensive rats (SHR) compared with muscle from age-matched normotensive Wistar-Kyoto rats (WKY). An increased maximum velocity of shortening (Vmax) and an increased shortening ability (delta Lmax) have also been reported for arterial smooth muscle from 16- to 21-week-old SHR. It has been suggested that both increased narrowing and prolonged narrowing of arteries contribute to the development of hypertension. However, SHR Vmax is not different from WKY Vmax when studying arterial muscle from older (28- to 31-week-old) rats. Thus increased arterial narrowing ability cannot be a contributing factor to the maintenance of hypertension. In this study the role of relaxation rate in the maintenance of hypertension was examined by comparing the relaxation rates of isometric and isotonic contractions of caudal arterial strips from 16- to 21-week-old SHR (n = 9) and WKY (n = 8) and from 28- to 31-week-old SHR (n = 7) and WKY (n = 5). While relaxation rates were lower for 16- to 21-week-old SHR compared with age-matched WKY preparations for both isometric and isotonic contractions, only isometric relaxation rates were found to be different in 28- to 31-week-old SHR compared with 28- to 31-week-old caudal arterial muscle (p less than 0.05). Vmax tended to normalize from a once-elevated velocity, while isometric relaxation rate remained decreased in SHR with ageing and (or) with progression of the hypertensive condition.(ABSTRACT TRUNCATED AT 250 WORDS)     

Force-velocity shifts with repetitive isometric and isotonic contractions of canine gastrocnemius in situ.

The force-velocity (F-V) relationships of canine gastrocnemius-plantaris muscles at optimal muscle length in situ were studied before and after 10 min of repetitive isometric or isotonic tetanic contractions induced by electrical stimulation of the sciatic nerve (200-ms trains, 50 impulses/s, 1 contraction/s). F-V relationships and maximal velocity of shortening (Vmax) were determined by curve fitting with the Hill equation. Mean Vmax before fatigue was 3.8 +/- 0.2 (SE) average fiber lengths/s; mean maximal isometric tension (Po) was 508 +/- 15 g/g. With a significant decrease of force development during isometric contractions (-27 +/- 4%, P < 0.01, n = 5), Vmax was unchanged. However, with repetitive isotonic contractions at a low load (P/Po = 0.25, n = 5), a significant decrease in Vmax was observed (-21 +/- 2%, P < 0.01), whereas Po was unchanged. Isotonic contractions at an intermediate load (P/Po = 0.5, n = 4) resulted in significant decreases in both Vmax (-26 +/- 6%, P < 0.05) and Po (-12 +/- 2%, P < 0.01). These results show that repeated contractions of canine skeletal muscle produce specific changes in the F-V relationship that are dependent on the type of contractions being performed and indicate that decreases in other contractile properties, such as velocity development and shortening, can occur independently of changes in isometric tension.     

Intramuscular pressure, force and blood flow in rabbit tibialis anterior muscles during single and repetitive contractions

The elevated intramuscular pressure (IMP) associated with sustained muscle contraction can affect blood flow, and could influence the long-term viability of functional skeletal muscle grafts. We therefore examined the relationship between force, peak IMP and blood flow in the tibialis anterior muscle of the anaesthetized rabbit. During isometric contractions, IMP was related linearly to force, and only the slope of the relationship varied between animals. During isotonic contractions, however, the highest values of IMP were found at the lowest force levels, and IMP appeared to be related to the amount and speed of shortening. During repeated isometric contractions, the ratio of IMP to force varied with time, stimulation pattern and subject. Mean blood flow did not differ appreciably between␣repetitive isometric contractions at duty cycles of 10–40%, and was unrelated to integrated pressure, integrated force, or depth from the surface. We conclude: (1) that IMP is unlikely to affect mean blood flow during cyclic activity that has a duty cycle less than 40%; and (2) that the clinical use of IMP as a predictor of muscle force appears to be justified only for single isometric contractions, and needs to be interpreted cautiously when contractions involve shortening or fatigue. Accepted: 17 November 1997     

Attenuated fatigue in slow twitch skeletal muscle during isotonic exercise in rats with chronic heart failure

During isometric contractions, slow twitch soleus muscles (SOL) from rats with chronic heart failure (chf) are more fatigable than those of sham animals. However, a muscle normally shortens during activity and fatigue development is highly task dependent. Therefore, we examined the development of skeletal muscle fatigue during shortening (isotonic) contractions in chf and sham-operated rats. Six weeks following coronary artery ligation, infarcted animals were classified as failing (chf) if left ventricle end diastolic pressure was >15 mmHg. During isoflurane anaesthesia, SOL with intact blood supply was stimulated (1s on 1s off) at 30 Hz for 15 min and allowed to shorten isotonically against a constant afterload. Muscle temperature was maintained at 37°C. In resting muscle, maximum isometric force (F(max)) and the concentrations of ATP and CrP were not different in the two groups. During stimulation, F(max) and the concentrations declined in parallel sham and chf. Fatigue, which was evident as reduced shortening during stimulation, was also not different in the two groups. The isometric force decline was fitted to a bi-exponential decay equation. Both time constants increased transiently and returned to initial values after approximately 200 s of the fatigue protocol. This resulted in a transient rise in baseline tension between stimulations, although this effect which was less prominent in chf than sham. Myosin light chain 2s phosphorylation declined in both groups after 100 s of isotonic contractions, and remained at this level throughout 15 min of stimulation. In spite of higher energy demand during isotonic than isometric contractions, both shortening capacity and rate of isometric force decline were as well or better preserved in fatigued SOL from chf rats than in sham. This observation is in striking contrast to previous reports which have employed isometric contractions to induce fatigue.     

The synthesis and biological activity of alkyloxy prostaglandin analogues

Prostaglandin analogues in which the alkyl chain attached to C-15 in the natural compounds is replaced by an alkyloxyalkyl group have been synthesised. Compounds of the 17-oxa series are particularly potent luteolytic agents and are selective in the sense that they are less effective than PGF_2α in causing isotonic contractions of isolated uterus muscle.     

Mechanical control of the rising phase of contraction of frog skeletal and cardiac muscle

The effect of shortening on contractile activity was studied in experiments in which shortening during the rising phase of an isotonic contraction was suddenly stopped. At the same muscle length and the same time after stimulation the rise in tension was much faster, if preceded by shortening, than during an isometric contraction, demonstrating an increase in contractile activity. In this experiment the rate of tension rise determined in various phases of contraction was proportional to the rate of isotonic shortening at the same time after stimulation. Therefore, the time course of the isotonic rising phase could be derived from the tension rise after shortening. The rate of isotonic shortening was found to be unrelated to the tension generated at various lengths and to correspond closely to the activation process induced by shortening. The length response explains differences between isotonic and isometric contractions with regard to energy release (Fenn effect) and time relations. These results extend previous work which showed that shortening during later phases of a twitch prolongs, while lengthening abbreviates contraction. Thus the length responses, which have been called shortening activation and lengthening deactivation, control activity throughout an isotonic twitch.     

The effects of isotonic contractions on the rate of fatigue development and the resting membrane potential in the sartorius muscle of the frog, Rana pipiens.

The goal of this study was to characterize how isotonic contractions affect the rate of fatigue development. Muscle bundles dissected from frog sartorius muscles were stimulated with 100-ms long train of pulses (0.5 ms, 6 V, 140 Hz). To measure the effect of the isotonic contractions, isometric tetanus were elicited at regular time intervals during the stimulation to fatigue. In general, isotonic contractions caused a faster decrease in tetanic force than isometric contractions. The difference in tetanic force between an isotonic and isometric fatigue increased gradually over a 20-min period to 7.9 and 13.5% at 0.04 and 0.1 trains/s (TPS), respectively. At 0.2, 0.5, and 1.0 TPS, the decrease in tetanic force was also faster during an isotonic fatigue, which resulted in an initial difference in tetanic force between the two types of fatigue. The difference did not exceed 18.5% and did not persist throughout the stimulation period; i.e., the difference disappeared before the end of the fatigue stimulation. The half-relaxation time was prolonged during fatigue development, and the prolongation was greater during an isotonic fatigue, except at 0.04 TPS. The increases in the half-relaxation time at 0.2, 0.5, and 1.0 TPS were followed by a decrease, and the decreases were especially pronounced during an isotonic fatigue at 0.5 and 1.0 TPS. The results showed for the first time that isotonic contractions cause a faster rate of fatigue development in frog sartorius muscles, and this effect depends on the frequency of stimulation.     

Catchlike-inducing train activation of human muscle during isotonic contractions: burst modulation.

Stimulation trains that exploit the catchlike property [catchlike-inducing trains (CITs)] produce greater forces and rates of rise of force than do constant-frequency trains (CFTs) during isometric contractions and isovelocity movements. This study examined the effect of CITs during isotonic contractions in healthy subjects. Knee extension was electrically elicited against a load of 10% of maximum voluntary isometric contraction. The stimulation intensity was set to produce 20% of maximum voluntary isometric contraction. The muscle was tested before and after fatigue with a 6-pulse CFT and 6-pulse CITs that contained an initial doublet, triplet, or quadruplet. For prefatigue responses, the greatest isotonic performance was produced by CITs with initial doublets. When the muscles were fatigued, triplet CITs were best. CITs produce greater excursion, work, peak power, and average power than do CFTs, because CITs produced more rapid rates of rise of force. Faster rates of rise of force enabled the preload on the muscle to be exceeded earlier during the stimulation train.     

Surface EMG-force modelling for the biceps brachii and its experimental evaluation during isometric isotonic contractions

The aim of this study was to evaluate a surface electromyography (sEMG) signal and force model for the biceps brachii muscle during isotonic isometric contractions for an experimental set-up as well as for a simulation. The proposed model includes a new rate coding scheme and a new analytical formulation of the muscle force generation. The proposed rate coding scheme supposes varying minimum and peak firing frequencies according to motor unit (MU) type (I or II). Practically, the proposed analytical mechanogram allows us to tune the force contribution of each active MU according to its type and instantaneous firing rate. A subsequent sensitivity analysis using a Monte Carlo simulation allows deducing optimised input parameter ranges that guarantee a realistic behaviour of the proposed model according to two existing criteria and an additional one. In fact, this proposed new criterion evaluates the force generation efficiency according to neural intent. Experiments and simulations, at varying force levels and using the optimised parameter ranges, were performed to evaluate the proposed model. As a result, our study showed that the proposed sEMG–force modelling can emulate the biceps brachii behaviour during isotonic isometric contractions.