Muscle Physiology II Dr Waleed R Ezzat MECHANICS

Muscle Physiology II Dr. Waleed R. Ezzat

MECHANICS OF SKELETAL MUSCLE CONTRACTION § A single muscle action potential leads to a single brief contraction of the muscle called a simple muscle twitch. § The duration of the action potential in a skeletal muscle fiber, however, is shorter (about 5 ms) than the duration of a twitch (tens or hundreds of milliseconds).

MECHANICS OF SKELETAL MUSCLE CONTRACTION § If many stimuli are given repeatedly and rapidly to the muscle fiber, the result is an amplifi ed, sustained contraction called a tetanus. § The repetition rate at which tetanization occurs is typically 20 to 60 stimuli per second, with the higher rates found in muscles that contract and relax rapidly.

MECHANICS OF SKELETAL MUSCLE CONTRACTION The Motor Unit: § A group of muscle fibers innervated by branches of the same alpha motoneuron is called a motor unit. § The motor unit is normally the smallest functional unit of a muscle. § Motor units are of different sizes. In muscles adapted for fine and precise control, only a few muscle fibers are associated with a given motor axon. In muscles in which high force is more important, a single motor axon controls many thousands of muscle fibers.

MECHANICS OF SKELETAL MUSCLE CONTRACTION The Force of Contraction: The degree of force of contraction in skeletal muscle is modulated by two factors; 1. Temporal summation = the degree of tetanization of each individual motor unit. 2. Motor unit summation or spatial summation (i. e. recruitment technique) = a more motor units are brought into play at any one time. Note: if small force is needed the CNS stimulates to tetany only the number of motor units needed for that particular task; other motor units within the muscle groups are not activated.

Isometric Versus Isotonic Contraction Isometric contraction: (Def) is the type of contraction when the muscle expresses its contractile activity by developing force without shortening (Isometric means = same length). Examples; pushing against a wall. The muscles contract, gets tense, and “harden” in such a situation, although no muscle or object is actually moved. Therefore, the external work is zero. The muscle, however, still consumes energy to fuel the processes that generate and maintain force.

Isometric Versus Isotonic Contraction Isotonic contraction: (Def) is the type of contraction when the muscle can generate a force larger than the load to which it is attached. And the muscle shortens with the development of force enough to equal the afterload to which it is attached (Isotonic means = same force). Note: in daily life, every isotonic contraction is preceded by isometric contraction development. The duration of the early isometric portion of the contraction varies, depending on the afterload. At low afterloads, the muscle requires little time to develop sufficient force to begin to shorten, and conditions will be isotonic for a longer time, and vice versa.

MECHANICS OF SKELETAL MUSCLE CONTRACTION The mechanics of isometric and isotonic contraction of muscle can be described in terms of two important relationships: 1. The length-tension curve, which represents the effect of changing the initial resting length to which muscle is stretched on the force generated by an isometric contraction. This passive precontraction or resting length is often called the preload. 2. The force-velocity curve, which represents the effect of afterload on the initial velocity of shortening of an isotonic contraction of muscle. The relationship between these two variables is not unchanging; it is altered by muscle preload.