Isometric Muscle 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. 

Although in in vivo circumstances an intracellular free calcium increase apparently acts as the primary modulator of contraction, it can be bypassed in highly permeabilized smooth muscle preparations where the active subunit of MLCK can be introduced to phosphorylate myosin and induce contraction. The MLCK catalyzed phosphorylation of serine-19 is seen as the necessary event in the activation of smooth muscle myosin to form crossbridges. Thus, the rising phase of force during an isometric smooth muscle contraction follows an increase in the degree of phosphorylation of myosin, and that in turn follows the transient rise of (a) cytosolic free Ca, (b) Ca-calmodulin complexes, and (c) the active form of MLCK. The regulation of the intracellular calcium is discussed below. The dynam-... 

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. 

Physical restraints (may be associated with isometric muscle contractions and worsening of hyperthermia)... 

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. 

Adhikari BB, Wang K. 2001. S100A1 modulates skeletal muscle contraction by desensitizing calcium activation of isometric tension, stiffness and ATPase. FEBS Lett 497(2, 3) 95-98. 

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. 

The modeling and control of movements in this chapter relates to external control of muscles via so-called functional electrical stimulation. Macroscopic viscoelastic models started from the observation that the process of electrical stimulation transforms the viscoelastic material from a compliant, fluent state into the stiff, viscous state. Levin and Wyman [35] proposed a three-element model— damped and undamped elastic element in series. Hill s work [36] demonstrated that the heat transfer depends upon the type of contraction (isometric, slow contracting, etc). The model includes the force generator, damping and elastic elements. Winters [37] generalized Hill s model in a simple enhancement of the original, which... 

Sukop J. and Nelson R.C. 1974. Effects of isometric training in the force-time characteristics of muscle contractions. In R.C. Nelson, and C.A. Morehouse (Eds.), Biomechanics IV, pp. 440—447. Baltimore,... 

Shiffman CA, Aaron R, Rutkove SB. 2003. Electrical impedance of muscle during isometric contraction. Physiol Meas 24, 213-234. 

M. Amin, Theory of Muscle Contraction I Isometric Contraction, J. Biol. Phys. 11 91-97 (1984). 

Isometric. A static exercise in which the muscle contracts with Uttle or no shorterring (static contraction). 

In active techniques, the patient assists the physician by actively contracting certain muscles under the guidance of the physician. There are two forms of active myofascial techniques. Active direct techniques are those in which the patient is asked to contract the involved muscle. These techniques use the Golgi tendon organ reflex to result in relaxation of the involved muscle(s). In these techniques, the physician applies isometric resistance to the contraction. (Isometric resistance The physician apphes resistance to the patient s contraction such that little shortening of the muscle is permitted but a great increase in muscle tension results.)... 

Henriksson J, Katz A, Sahlin K. (1986) Redox state changes in human skeletal muscle after isometric contraction. J Physiol 380(1) 441-451... 

---