Muscle contraction dynamics

Muscle contraction dynamics include the mechanical properties of muscle tissues and tendons, which are expressed as force-length and force-velocity relations. The activation dynamics include the voluntary and nonvoluntary (reflex) excitation signal and motor unit recruitment level in the muscle. It is well known that regardless of fatigue, the generated torque in each joint is dependent on muscle activation levels (MALs) and joint angle when in a stationary position. This was first observed by Tnman et al. 

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-... 

Muscle contraction is a delicate dynamic balance of the attachment and detachment of myosin heads to F-actin, subject to fine regulation via the nervous system. 

Muscle spindles are composed of nuclear bag (dynamic) and chain (static) fibres known as intrafusal fibres and these are innervated by y motor neurones. Extrafusal fibres make up the muscle bulk and are innervated by a motor neurones. Stimulation of the muscle spindle leads to increased skeletal muscle contraction, which opposes the initial stretch and maintains the length of the fibre. This feedback loop oscillates at 10 Hz, which is the frequency of a physiological tremor. 

Microfilaments of F actin traverse the microvilli in ordered bundles. The microfila-ments are attached to each other by actin-as-sociated proteins, particularly fimbrin and vil-lin. Calmodulin and a myosin-like ATPase connect the microfilaments laterally to the plasma membrane. Fodrin, another microfila-ment-associated protein, anchors the actin fibers to each other at the base, as well as attaching them to the cytoplasmic membrane and to a network of intermediate filaments. In this example, the microfilaments have a mainly static function. In other cases, actin is also involved in dynamic processes. These include muscle contraction (see p. 332), cell movement, phagocytosis by immune cells, the formation of microspikes and lamellipo-dia (cellular extensions), and the acrosomal process during the fusion of sperm with the egg cell. 

Phospholamban is a homopentameric membrane protein involved in muscle contraction through regulation of the calcium pump in cardiac muscle cells. The stmcture of the unphospho-rylated protein solved in DPC micelles reveals a symmetric pentamer of phospholamban monomers (Fig. 2g) stabilized by leucine/isoleucine zipper motifs along the transmembrane domains (51). Notably, another stmcture was produced for phospholamban (Fig. 2h) that used a variant of the traditional simulated annealing and molecular dynamics protocol that reduced the chances of entrapment in local minima (52). 

Myosins, kinesins, and dyneins move by cycling between states with different affinities for the long, polymeric macromolecules that serve as their tracks. For myosin, the molecular track is a polymeric form of actin, a 42-kd protein that is one of the most abundant proteins in eukaryotic cells, typically accounting for as much as 10% of the total protein. Actin polymers are continually being assembled and disassembled in cells in a highly dynamic manner, accompanied by the hydrolysis of ATP. On the microscopic scale, actin filaments participate in the dynamic reshaping of the cytoskeleton and the cell itself and in other motility mechanisms that do not include myosin. In muscle, myosin and actin together are the key components responsible for muscle contraction. 

In the body, the energy derived from food is released as body heat and also used in the synthesis of ATP. The energy captured in ATP is then transformed into other forms, i.e., chemical (synthesis of new compounds), mechanical (muscle contraction), electrical (nerve activity), electrochemical (various ion pumps), thermal (maintenance of body temperature), and informational (base sequences in nucleic acids, amino acids in proteins). In general, the energy of food provides for the specific dynamic action of food, the maintenance of the body s basal metabolism, and the energy expenditure associated with various types of activity. 

Palladino, J.L. and Noordergraaf, A. 1998. Muscle contraction mechanics from ultrastructural dynamics. In Analysis and Assessment of Cardiovascular Function, G.M. Drzewiecki and J.K.-J. Li, Eds., Springer-Verlag, New York, chap. 3, pp. 53-57. 

Isoinertial — a static or dynamic muscle contraction where the external load is held constant [Kroemer, 1983]. 

Permits dynamic testing of most major body segments especially useful for stronger movements most devices provide good stabilization measures reciprocal muscle contractions widespread clinical acceptance also records angular data, work, power, and endurance-related measures provides a number of different reporting options also used as exercises devices... 

---