Anatomy & Physiology

Anatomy & Physiology Study Guide Sarcomere shortening and muscle fiber stimulation produce tension: • The amount of tension produced by a muscle fiber depends on the number of cross-bridges formed. • Skeletal muscle fibers can contract most forcefully when stimulated over a narrow range of resting lengths. • A twitch is a cycle of contraction and relaxation produced by a single stimulus. • Repeated stimulation at a slow rate produces treppe, a progressive increase in twitch tension. • Repeated stimulation before the relaxation phase ends may produce a summation of twitches (wave summation), in which one twitch is added to another. Incomplete tetanus is when tension peaks and falls at intermediate stimulus rates, or complete tetanus, in which the relaxation phase is eliminated by very rapid stimuli. • The number and size of a muscle’s motor units determine how precisely controlled its movements are. • Resting muscle tone stabilizes bones and joints. • The return to resting length after a contraction may involve elastic forces, the contraction of opposing muscle groups, and gravity. • Load and speed of contraction are inversely related. • Normal activities generally include both isotonic contractions (in which the tension in a muscle rises and the length of the muscle changes) and isometric contractions (in which tension rises, but the length of the muscle remains constant). ATP is the energy source for muscle contraction: • Muscle contractions require large amounts of energy. • Creatine phosphate (CP) can release stored energy to convert ADP to ATP. • At rest or moderate levels of activity, aerobic metabolism can provide most of the ATP needed to support muscle contractions. • At peak levels of activity, the cell relies heavily on the anaerobic process of glycolysis to generate ATP because the mitochondria cannot obtain enough oxygen to meet the existing ATP demands. • As muscular activity changes, the pattern of energy production and use changes. • A fatigued muscle can no longer contract because of a drop in pH due to the buildup of lactic acid, the exhaustion of energy resources, or other factors. • Circulating hormones may alter metabolic activities in skeletal muscle fibers. • The recovery period begins immediately after a period of muscle activity and continues until conditions inside the muscle have returned to pre-exertion levels. The oxygen debt, or excess post-exercise oxygen consumption (EPOC), created during exercise is the amount of oxygen required during the recovery period to restore the muscle to its normal condition.

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