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PubMed1997

Relating agonist-antagonist electromyograms to joint torque during isometric, quasi-isotonic, nonfatiguing contractions.

This paper describes an experimental study which relates simultaneous elbow flexor-extensor electromyogram (EMG) amplitude to joint torque. Investigation was limited to the case of isometric, quasi-isotonic (slowly force-varying), nonfatiguing contractions. For each of the flexor and extensor muscle groups, the model relationship between muscle group torque contribution and EMG amplitude was constrained to be a sum of basis functions which had a linear dependence on a set of fit parameters. With these constraints, the problem of identifying the EMG-to-torque relationship was reduced to a linear least squares problem. Surface EMG's from elbow flexors and extensors, and joint torque were simultaneously recorded for nonfatiguing, quasi-isotonic, isometric contractions spanning 0-50% maximum voluntary contraction. Single-/multiple-channel unwhitened/whitened/adaptively whitened EMG amplitude processors were used to identify an EMG-to-torque relation, and then estimate joint torque based on this relation. Each unwhitened multiple-channel EMG-to-torque estimator had a standard error (SE) approximately 70% of its respective single-channel estimator. The adaptively whitened multiple-channel joint torque estimator had an SE approximately 90% of the unwhitened multiple-channel estimator, providing an estimation error approximately 3% of the combined flexion/extension torque range. The experimental studies demonstrated that higher fidelity EMG amplitude processing led to improved joint torque estimation.

Adult↗
PubMed1985

Distension-dependent changes in noradrenaline sensitivity in small arteries from the rat.

The effects of distension on the sensitivity to noradrenaline and potassium were assessed in ring preparations of 200 micron branches from the superior mesenteric artery of the rat. In response to noradrenaline, maximal tension development occurred at smaller luminal diameters than with potassium activation. The sensitivity to noradrenaline increased with increasing distension whereas potassium sensitivity remained largely unchanged. A distension-dependent increase in noradrenaline sensitivity was observed also in vessels depolarized with high-potassium solution. The role of the distension-dependent sensitivity to noradrenaline for isotonic contractions was also studied. Upon activation with applied noradrenaline, submaximal isotonic responses were somewhat smaller than isometric responses at the same dose. This may be ascribed to a reduction in sensitivity during the isotonic contraction. Upon neurogenic activation, no difference between isotonic and isometric responses was found. The results show that distension increases noradrenaline sensitivity in resistance vessels in vitro, but that the consequences for their function as neuroeffectors may be small.

Animals↗
PubMed1996

Muscle chamber with strain gauge adapted for isotonic/isometric recording.

Simple devices are described that can facilitate student experiments on mechanical and electrical aspects of muscle contraction, for example, of isolated preparations such as frog gastrocnemius and sartorius muscles and gut. The system includes a general-purpose preparation bath that was previously described for electrophysiological observations with suction electrodes [D.M. Easton. Am.J. Physiol. 265 (Adv. Physiol.Educ. 10): S35-S46, 1993]. A tower installed in the bath allows the muscle to be oriented vertically but completely submerged in a limited volume. Recording of isotonic contraction, which is the more familiar behavior of skeletal muscle, is made convenient by an accessory to convert the commonly available Statham force transducer to isometric recording. Phenomena not conveniently accessible by isometric recording, such as muscle work, and also the length-tension relation, not ordinarily observed for isotonic contraction, can be demonstrated.

Animals↗
PubMed1985

Effects of physiological beating on the contractility of cat ventricular muscle.

Force and length changes corresponding to those undergone by muscle fibers in the ventricular wall of the heart during systole and diastole were generated in isolated cat ventricular muscles contracting at 30 degrees C. The effect on contractility of such cycles of force and length changes ("physiological" contractions) was assessed by measuring force production in isometric test beats. Contractility rose over the 1st min of physiological beating but then fell over the next 7-10 min. These changes in contractility were compared with those occurring during a series of isotonic contractions or modified isotonic contractions in which muscles were held at the short length during relaxation (reversed relaxation contractions). In all cases contractility increased to the same extent over the 1st min, but over the next 10 min it fell significantly more when the contractions were physiological than when they were isotonic (normal or reversed relaxation). This difference could be accounted for experimentally by the diastolic length changes that occur in physiological contractions. This study demonstrates 1) that contractility depends on length changes occurring in the muscle during both systole and diastole, and 2) that length-dependent changes in contractility contain both positive and negative components.

Animals↗
PubMed2002

A rheological motor model for vertebrate skeletal muscle in due consideration of non-linearity.

A rheological motor model that satisfies the major mechanical properties of the skeletal muscle is proposed. The model consists of two Maxwell elements and a Voigt element connected in parallel with each other and has a force generator in it. The model well explains the mechanical behavior in quick and slow recovery phases in the isometric contraction of the muscle and achieves a sufficient isotonic shortening speed. The energy liberation of the motor in isotonic contraction is calculated and a mechanism of control is proposed, which operates so as to decrease the dissipated energy by altering the weights of the elastic and viscous constants in Maxwell elements. And thereby it becomes possible for the motor to possess non-linearity in energy liberation and load-velocity relation alike in muscle. The model would be a base model to be utilized for analyzing the kinetics of human macrosystems and/or for modeling the human neuromuscular system of motion control.

Animals↗
PubMed1980

Spontaneous mechanical alternans in papillary muscles from atherosclerotic rabbits.

Mechanical alternans developed spontaneously in 5 of 17 right ventricular papillary muscles from atherosclerotic rabbits. The rabbits had eaten an atherogenic diet of 5% lard, 5% peanut oil, 0.5% cholesterol, and 89.5% rabbit pellets for 116-184 days. In one muscle mechanical alternans developed slowly and persisted. When the condition had developed fully, in the weaker contraction the muscle shortened only 20% as far when contracting isotonically and developed only 38% as much tension when contracting isometrically. Added to the superfusate, calcium (5.0 mM) or norepinephrine (1.5 X 10(-5) M) abolished the alternans. In four muscles mechanical alternans was only temporary. Compared with the 12 muscles that did not develop alternans, these 4, when contracting isometrically 12 times/min but not in alternans, had longer latency and required more time to develop tension at the maximum rate and to develop peak tension. Contracting isotonically, they required more time to accelerate to maximum velocity of shortening and to shorten maximally. Raising the contraction frequency to 24/min decreased the differences in performance between the two groups. Norepinephrine (1.5 X 10(-5) M) made the differences smaller still. We think that both the mechanical alternans and the differences in performance between the muscles that developed alteranans and those that did not resulted from a defect in the cardiac cell's handling of calcium. Diet-induced changes in the lipid composition of the sarcoplasmic reticulum or sarcolemma or both seem probable causes.

Animals↗
PubMedDate not supplied

Skeletal muscle architecture: implications for muscle function and surgical tendon transfer.

Skeletal muscles have mechanical properties that are well-described by the length-tension relationship (for isometric contractions) and the force-velocity relationship (for isotonic contractions). These two intrinsic properties are scaled for a given muscle based on its architectural properties. Muscle active force-generating range is determined by muscle fiber length, while maximum muscle force is determined by physiologic cross-sectional area. These and other architectural properties should be matched between muscles when planning surgical tendon transfers in order to closely match donor and recipient muscles. Finally, the fiber length/moment arm ratio of a muscle-joint combination must be considered when describing strength because strength is a manifestation of both muscle and joint properties and not either alone. Unfortunately, detailed description of normal musculoskeletal design or optimal transfer strategy cannot be made until more basic science studies of the musculoskeletal system are conducted.

Finger Injuries↗
PubMed2001

Knee Extensor Electromyographic Activity-to-Work Ratio is Greater With Isotonic Than Isokinetic Contractions.

OBJECTIVE: To determine whether isotonic or isokinetic contractions produced greater electromyographic (EMG) activity per unit of work during isotonic and isokinetic knee-extension exercise. DESIGN AND SETTING: Subjects performed three 3-second maximal voluntary isometric contractions of the dominant knee extensors for EMG normalization. Exercise testing performed on the Biodex System 3 Dynamometer involved 10 isokinetic contractions at 180 degrees.s(-1) and 10 isotonic contractions with the resistance set at 50% of the previously recorded maximal voluntary isometric contraction. SUBJECTS: Recreationally active college students (10 men and 11 women). MEASUREMENTS: Surface EMG signals were collected from the vastus medialis and lateralis muscles and then integrated (IEMG) over the concentric phase of each repetition for both exercises. The IEMG was divided by the total work performed during the concentric phase for each exercise (IEMG/W). RESULTS: We analyzed the IEMG/W data using a 1-between (sex), 2-within (exercise and muscle) repeated-measures analysis of variance. There was a significant main effect for exercise, with the isotonic IEMG/W value being significantly greater than the isokinetic IEMG/W value. Additionally, the IEMG/W relationship did not appear to be affected by sex or individual muscle tested. CONCLUSIONS: Per unit of work performed, the isotonic contractions resulted in greater motor unit recruitment or an increased rate of firing, or both. This finding may have implications for the early phase of rehabilitation, when goals include complete motor unit recruitment of injured or atrophied muscles.

Journal Article↗
PubMed1982

Contractile properties of isolated frog skeletal muscles under the influence of Na-octanoate.

Na-octanoate (2-10 mM) altered the characteristics of the isometric twitch of isolated frog skeletal muscles. The peak tension, the maximum rate of tension rose, the time to peak tension, and the half relaxation time decreased. The tetanus tension was reduced by the same amount in percent as a single isometric twitch so that no changes occurred in the twitch/tetanus relationship. The fusion frequency of tetanic contractions increased. The isotonic contractions had a reduced shortening period. The shortening velocity was not affected. The K+ induced contractures were diminished. The S-shaped curve which relates peak tension to potassium concentration was shifted to more positive potentials. The maximum contractile strength was not attained even by complete depolarization. 1-2 mM caffeine removed the octanoate effects described above. The contracture induced by 7.5 mM caffeine produced the same maximum tension as that obtained in Ringer's solution, but the contracture began later and the rise of tension was retarded. The tension development of glycerol-extracted muscles was not affected by octanoate. The results are consistent with the hypothesis that the octanoate-induced changes of muscle contraction are due to a reduced Ca++ release from cellular stores.

Animals↗
PubMed1990

Actions of acetylcholine and GABA on spontaneous contractions of the filariid, Dipetalonema viteae.

1. Isotonic contractions were recorded from the filarial nematode, Dipetalonema viteae (Acanthocheilonema viteae), in an isolated tissue chamber. 2. Nicotine (10(-6) M) and pilocarpine (10(-5) M) increased the spontaneous contractions in the intact filariid, but acetylcholine (ACh, 10(-4) M) and muscarine (10(-5) M) were inactive. 3. When ACh was applied to an opened D. viteae, it was 10,000 times more potent. This indicates that the cuticle is an effective barrier to the penetration of ACh to the muscle cells. 4. The effects of ACh on the opened D. viteae were not affected by hexamethonium (10(-3) M) or atropine (10(-5) M) and were only partially reduced by (+)-tubocurarine (10(-4) M). 5. gamma-Aminobutyric acid (GABA, 10(-3) M) reduced the spontaneous activity of the intact D. viteae; however, the effect of GABA had a slow onset and recovery. Muscimol (10(-5) M) was more potent than GABA and had a more rapid onset and recovery. 6. GABA was 1,000 times more potent on the opened D. viteae than on the intact D. viteae. Baclofen (10(-3) M) was inactive on both preparations. 7. The effect of GABA was not antagonized by bicuculline (10(-4) M), picrotoxin (10(-5) M or penicillin G (10(-3) M). 8. It is concluded that the filariid cuticle acts like a lipid structure and blocks the penetration of polar substances, such as ACh and GABA. Also, due to the lack of efficacy of the ACh and GABA antagonists, it was concluded that the nematode receptors are somewhat different from the mammalian ACh and GABA receptors.

Acetylcholine↗
PubMed1984

A kinetic model of muscle contraction and its oscillatory characteristics.

A set of five differential equations has been found which gives a satisfactory account of the isotonic and isometric properties of striated muscle. Four of these differential equations give an equally satisfactory account of the results of length-drive experiments with sinusoidal variation of length. In this case, the fifth equation (of motion) is redundant. These sets of equations predict a number of results not yet measured relating to the superposition of oscillatory length changes on isotonic contraction. The equations predict correctly the variation of tension with time when the amplitude of the driven oscillation increases beyond the region where it can be treated as a perturbation, and the deviation of the mean tension per cycle from the steady-state tension for isotonic contraction with superimposed oscillations in length or velocity. The equations can be derived rigorously from a more complex set of eight equations which themselves are formulated from the basic principles of chemical physics, the theory of molecular force fields and radiationless transitions. The reduced model may be consistent with many other molecular theories and its predictive success does not prove the correctness or otherwise of the level 1 assumptions of the seven-state theory. By the same token, macroscopic mechanical experiments of the type presently carried out cannot give information on level 1 questions such as the existence or otherwise of 'binding' of crossbridges to the thin filament. The experimental kinetic results can be described with or without this assumption. The theory needs considerable development in so far as it does not consider elastic elements at all at present, nor have detailed conclusions yet been extracted from the equations for the case of stretching, except for isotonic steady states where agreement is encouraging.

Adenosine Triphosphate↗
PubMedDate not supplied

Age-related differences in excitation-contraction coupling in rat papillary muscle.

To investigate the possible role of an alteration in excitation-contraction coupling during development, aging and senescence we compared simultaneously recorded mechanical and electrical activity of left ventricular papillary muscles from 3, 6, 12, and 24-month-old male rats. In addition, the effects of calcium and verapamil on excitation-contraction coupling were evaluated. We recorded transmembrane action potentials during both isometric and isotonic contractions. At al external bath calcium = 2.4 mM, action potential duration at 75% complete repolarization (APD75) was significantly prolonged as a function of age (3 mo = 28.2 +/- 2.7; 6 mo = 29.5 +/- 2.6; 12 mo = 49.5 +/- 5.6; 24 mo = 121 +/- 8.5 msec) while peak developed tension (DT) was not significantly altered (3 mo = 5.13 +/- 0.53; 6 mo = 4.75 +/- 0.53; 12 mo = 7.26 +/- 0.51; 24 mo = 6.01 +/- 0.67 g/mm2). The correlation coefficient (r value) for APD75 and DT was strong for 3-month-old animals (4 = 0.99) but weakened as a function of age (6 mo = 0.93; 12 mo = 0.81; 24 mo = 0.57). Similar results were observed when APD75 was correlated with time-to-peak tension (TPT) (3 mo = 0.95; 6 mo = 0.98; 12 mo = 0.85; 24 mo = 0.68), time-to-one-half relaxation (T1/2R) 3 mo = 0.91; 6 mo = 0.97; 12 mo = 0.85; 24 mo = 0.81) and time to peak shortening (TPS) (3 mo = 0.89; 6 mo = 0.81; 12 mo = 0.82; 24 mo = 0.51). Correlations between action potential duration and contractile parameters became weak in all age groups upon the addition of verapamil (V). The correlation between APD75 and DT for 3-month-old animals decreased by 34% upon the addition of V while a 70% decrease was seen in 24-month-old animals. Similar results were seen when APD75 was correlated with TPT, T1/2R and TPS when V was added to the perfusate. Our results indicate that excitation-contraction coupling, as evidence by alterations in not only the contractile apparatus but also in the surface membrane, may be altered in ventricular muscle as in function of age.

Action Potentials↗
PubMed1995

Motor unit recruitment pattern during low-level static and dynamic contractions.

Motor unit (MU) recruitment patterns were studied during dynamic and static contractions at workloads corresponding to 10% of maximal voluntary contraction force. The dynamic contraction consisted of a 20 degrees flexion and extension of the elbow performed with a velocity of 10 degrees/s. Motor unit potential trains were recorded from the brachial biceps muscle of 6 healthy females using a quadripolar needle electrode and a computerized decomposition program. Properties of the identified MUs were derived from concentric needle EMG. A total of 119 MUs were identified during dynamic contractions, 107 MUs during static anisotonic contractions, and 96 MUs during static isotonic contractions. The main result was that MUs recruited during different contractions showed similar properties and may belong to the same part of the motoneuron pool. This indicates that MU recruitment patterns during dynamic contractions may be almost as stereotypical as during static contractions and may even activate the same MUs.

Action Potentials↗
PubMed1980

Direct effects of ethanol on myocardial performance and coronary resistance.

Ethanol has been infused into an isolated heart-lung type preparation in which the "isotonically" contracting left ventricle pumps blood and does work at a lower than normal value while afterload is maintained constant. The isotonic contraction permits observation of the influence of a drug or other agent on the coronary vasculature independent of the compressive effects of the musculature. Infusions of ethanol to concentrations of approximately 210 mg/100 ml over a 30- to 60-min period resulted in significant increases in coronary blood flow, decreases in coronary vascular resistance and depression of myocardial performance. Cardiac work increased concomitantly with the increase in coronary blood flow and cardiac output without significant alterations in tension-time index and oxygen consumption. A second group of animals received a smaller amount of ethanol over a 5-min period, accompanied by injection of 86Rb and 125I for nutritional flow measurements and of radioactive microspheres for flow distribution. Extraction increased slightly after alcohol but flow distribution was not significantly altered. Small concentrations of ethanol are concluded to have a direct vasodilating effect on the coronary vasculature.

Animals↗
PubMed1989

Comparison of the effects of inotropic interventions on isometric tension and shortening in isolated ferret ventricular muscle.

In this study, we have compared the effects of a range of positive and negative inotropic interventions on isometric tension and shortening at low load in isolated ferret papillary muscles. It was found that the majority of interventions produced broadly similar effects on tension and shortening. Positive inotropic interventions which behaved in this way included increasing extracellular calcium and increasing frequency at moderate stimulation rates. Acidosis, the addition of cyanide, and metabolic blockade produced by the addition of iodoacetic acid and cyanide, all produced negative inotropic effects which showed a similar pattern. For all these interventions, it was noted that the fractional effect on tension was 1.5-2 times larger than the fractional effect on shortening. This was attributed to the shape of the tension-length relation for cardiac muscle in different inotropic states. When frequency was increased to a high rate, tension rose to an initial peak, but then showed a marked decline, while shortening rose to a new level and was then well maintained. When protocols consisting of varying amounts of isometric and isotonic contraction were applied, the amount of the secondary decline for both tension and shortening was related to the period spent doing isotonic contractions, and hence to the energy consumption of the muscle. Thus the difference in the behaviour of tension and shortening under these conditions can be accounted for by the lower energy requirements of shortening, rather than by other factors. Over the range of inotropic interventions studied, peak shortening velocity was a more sensitive index of contractility than shortening, being roughly comparable to isometric tension.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1999

Normalisation of EMG amplitude: an evaluation and comparison of old and new methods.

The purpose of this study was to evaluate and compare four different methods of normalising the amplitude of electromyograms (EMGs), from the biceps brachii. Five males performed isotonic contractions of the elbow flexors with an external force of 50 N, 100 N, 150 N and 200 N. These were followed by a single isometric maximal voluntary contraction (MVC) and ten isokinetic MVCs at 0.35 rad s(-1) intervals between 0.35 rad s(-1) and 3.50 rad s(-1). The processed EMGs recorded from the isotonic contractions were normalised by expressing them as a percentage of: (i) the mean (Dynamic Mean Method) and (ii) the peak EMG from the same contraction (Dynamic Peak Method), (iii) the EMG from the isometric MVC (Isometric MVC Method), and (iv) the EMG from an isokinetic MVC at the same elbow angle and angular velocity (Isokinetic MVC Method). The root mean square difference (RMSD) between the outputs of the Isokinetic MVC and Dynamic Mean methods was significantly greater (P<0.05) than between the Isokinetic MVC method and the Dynamic Peak and the Isometric MVC methods. The small (10%) difference between the Isokinetic MVC and the Isometric MVC Methods was a consequence, firstly, of the lack of difference in EMG recorded from the isometric and isokinetic MVCs and, secondly, the consistency in EMG over the range of motion and at different angular velocities of isokinetic MVC. We conclude that only the Isometric and Isokinetic MVC methods should be used to normalise the amplitude of EMGs from the biceps brachii.

Adult↗
PubMed1976

[Mechanism of catecholamine release from the sympathetic nerve endings, particularly in its relation tovacetylcholine an experiment on the isolated vas deferens of rats].

UNLABELLED: The mechanism of catecholamine (CA) release induced by transmural stimulation (TS) and the participation of acetylcholine (ACh) in this mechanism were studied using isotonic contraction of excised vas defernse of rats. Furthermore, the inhibitory effect of bretylium (Br) on sympathetic activity was observed. RESULTS: 1) The effects of various kinds of drugs on isotonic contractions induced by TS, exogenous ACh and exogenous noradrenaline (NA) were observed with the results shown in Table 1. The following has been concluded: (1) TS-induced contraction is due to stimulation of the endings of hypogastric nerve (sympathetic nerve), resulting in CA release from adrenergic fiber (AF) and ACh release from cholinergic fiber (CF) in this nerve. (2) The participation of ACh is not indispensable in the CA release from AF induced by TS. (3) Endogenous ACh release from CF by TS brings about CA release under eserine application. (4) CA release by exogenous ACh is not inhibited by ganglion blockade, but is inhibited by atropine, indicating the muscarinic receptor to be in AF endings. 2) Br exerted preferentially the irreversible inhibition on CA release from AF rather than that on ACh release from CF, while it caused a mild reversible inhibition on CA release by exogenous ACh. 3) On the TS contraction that had been abolished irreversibly by Br, NA incubation showed a mild lasting recovery, while methamphetamine (MAP) or calcium (Ca) incubation showed a strong lasting recovery. Furthermore, in this recovery of TS contraction, the incubation with NA or MAP exerted only the recovery of CA release while the Ca incubation exerted the recovery of both CA release and ACH release. It would appear that Br blocks both AF and CF by inhibiting the transmitter-releasing action of Ca.

Acetylcholine↗
PubMed1993

Variable cross-bridge cycling-ATP coupling accounts for cardiac mechanoenergetics.

Cardiac twitch contractions were simulated by Huxley's sliding filament cross-bridge muscle model coupled with parallel and series elastic components. The energetics of the contraction were based on the ATP hydrolysis for the cross-bridge cycling. Force-length area (FLA), as a measure of the total mechanical energy, was computed for both isometric and isotonic contractions in a manner similar to the pressure-volume area (PVA) (Suga, H. Physiol. Rev. 70: 247-277, 1990). PVA correlates linearly with cardiac oxygen consumption, and since FLA is analogous to PVA, FLA should correlate with the ATP expended. Simulations comparing FLA with the cross-bridge cycling ATP usage showed that at lower muscle fiber activation levels (shorter initial fiber lengths and lower preload levels) FLA decreased more rapidly than the number of muscle fiber cross-bridge cycles in both isometric and isotonic contraction cases. This suggests that one ATP can cause more than one cross-bridge cycle at lower activation levels as was proposed by Yanagida, Arata, and Oosawa (Nature 316: 366-369, 1985). If the number of cross-bridge cycles to ATP ratio is allowed to increase at lower activation levels as suggested by Yanagida et al., Huxley's model is compatible with the experimental findings on FLA and PVA.

Adenosine Triphosphate↗