Isotonic contractions, muscle contraction

Figure 4. When a muscle contracts isotonically or a constant resisting force is imposed on it during a contraction, the velocity at which it shortens quickly comes to a constant. The force-velocity curve shows the relationship between the force applied to a muscle and the steady-state velocity of shortening. As in all other muscles, the force-velocity curve of smooth muscle is a rectangular hyperbola for all positive shortening velocities. In order to compare the behavior of muscles of different lengths and diameters, it is common to normalize force and velocity by dividing each by its maximum value and expressing the result as a percentage, nd...

In terms of muscle function, muscle is very adaptable. Depending on the type of stimulation, muscle can either twitch or contract tetanically for a variable length of time. If the ends are held fixed, then it contracts isometrically and the force produced is maximal. If one or both ends of the muscle are not held fixed then the muscle is able to shorten. The muscle can shorten at a fixed load (isotonic contraction) where the velocity of shortening is also constant. Power output (force X velocity) is maximum where the velocity of shortening is about one third of the maximal rate. Finally, the muscle can shorten at maximum velocity (unloaded shortening). However, the molecular basis of the interaction of myosin with actin to produce force, or shortening, is the same in each case. 

Isotonic contraction occurs when the muscle shortens under a constant load. For example, when an object is lifted, the muscle contracts and becomes shorter although the weight of the object remains constant. In addition to moving external objects, isotonic contractions are performed for movements of the body, such as moving the legs when walking. 

Many activities require both types of muscle contraction. An example is running when one of the legs hits the ground, isometric contraction of the muscles within this limb keep it stiff and help to maintain body support. At the same time, isotonic contractions in the opposite leg move it forward to take the next stride. 

Isotonic muscle contraction was used to measure the effects of selected nematode FaRPs on the body-wall muscle of H. contortus. AF2 was found to have inhibitory effects on muscle activity and inhibited acetylcholine (ACh) -induced contractions in the worm whereas AF8 had excitatory effects on the muscle and enhanced ACh-induced contractions (Marks et al., 1999a). There were obvious differences in the methodologies used to evaluate the effects of these peptides on Haemonchus muscle compared with those used to examine these peptide effects on Ascaris. How comparable the results are has yet to be determined. 

Figure 8. Original tracings showing the effects of (R)a-methylhistamine (MHA, pM), thioperamide (TH, pM), the selective a2 adrenoceptor agonist UK-14304 (UK, nM) and the a2-adrenoceptor antagonist idazoxan (ID, pM) on different in vitro assays from the guinea pig ileum A) Electrically-evoked longitudinal contractions of the whole ileum B) Peristaltic waves of the perfused ileum, C) Reflex-evoked circular muscle contractions Vertical calibrations represent (A and C) centimeters of isotonic contractions or (B) changes in perfusion pressure. Horizontal calibration is the chart speed, w = washing of the preparation...

Martin-Femandez, M. L., Bordas, J., Diakun, G., Harries, J., Lowy, J., Mant, G. R., Svennson, A., and Towns-Andrews, E. (1994). Time-resolved X-ray diffraction studies of myosin head movements in live frog sartorius muscle during isometric and isotonic contractions./. Mus. Res. CellMotil. 15, 319-348. 

Description of muscle contraction has essentially evolved into two separate approaches — lumped whole muscle models and specialized crossbridge models of the sarcomere. The former seek to interpret muscle s complex mechanical properties with a single set of model elements. Muscle experiments measure muscle force and length subjected to isometric (fixed length) conditions, isotonic (fixed load) conditions, and transient analysis where either length or load is rapidly changed. 

Figure 8.17 shows computed isotonic contractions for different fixed loads. Although not shown, this muscle model exhibits an inverse relation between isotonic load and amount of shortening, and a direct relation between initial length and shortening (Starhng s law). Results are also consistent with experiments,... 

FIGURE 8.17 Isotonic contractions for muscle strip computed from Equation 8.15 for three intermediate and one isometric load. Top shows muscle length (right y-axis). 

The idea for this distributed muscle fiber model arose in 1990 [44]. At that time, muscle fibers were assumed to be functionally similar to muscle strips. Recently, experiments on isolated muscle libers show this to be the case. Predictions from the model have recently been borne out, for example, the magnitude of computed peak isometric force compared to that measured on isolated guinea pig myocytes [34]. Peak isometric stress measured on isolated rabbit myocytes (5.4 mN/mm ) is very close to peak stress from rabbit papillary muscle strips (6.4 mN/mm ) [45]. The distributed model generates peak isometric stress of 2.5 mN/mm. Other muscle phenomena measured on the isolated fiber include a quadratic force-length relation [45], inotropic changes in contractile state [37], quick release and stretch [37,39], and isotonic contractions [46], all in agreement with model predictions. 

Brutsaert, D.L., Claes, V. A., and SonnenbHck, E.H. 1971. Effects of abrupt load alterations on force-velocity-length and time relations during isotonic contractions of heart muscle load clamping. /. Physiol. 216 319. 

M. Amin, Theory of Muscle Contraction II Isotonic Contraction, J. Biol. Phys. 11, 123-126 (1984). 

Stimull/s 20 Stimuli/s 50 Stimuli/s Frog muscle, 20°C, approx. Isotonic contraction... 

An isotonic type of contraction (reduction in length under constant stress) is exemplified by an isolated muscle excited to lift a weight hanging on one end - see Fig. 9.8. (The cross-sectional area experiencing loading is assumed to remain approximately constant.) By fixing both ends and applying an... 

Some of the applicable muscle models include the Maxwell, Voigt, Hill and Carlson models (Figure 1). In particular, the Carlson (1957) equation is used in much of this work to describe the stress-velocity relationship of cardiac muscle over the entire cardiac cycle. Min et al. (1978) found very little difference in analyzing ventricular dynamics when he alternately used Carlson s equation only during isotonic contraction and Hill s equation during isovolumic contraction. 

During muscle contraction, the force is transmitted to the skeleton by the tendon or aponeurosis and may or may not result in joint motion. There are three different types of muscle contraction isometric, when the muscle contracts but there is no change in its length (Fig. 3.3a,d) isotonic, when the muscle contracts and simultaneously shortens (Fig. 3.3b,e) and eccentric, when the muscle contracts and, at the same time, lengthens (Fig. 3.3c,0. 

The patient should be examined in a comfortable position during complete relaxation, isometric and isotonic contraction. Before starting the examination, some notes on the patient s clinical history should be collected by the examiner with special reference to previous sport trauma (date and mechanism of the injury). Inspection of the affected body area is also needed to rule out local swelling and ecchymosis then, palpation of the muscle may reveal local tenderness and a mass effect. Especially... 

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