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ANPS 019 Beneyto-Santonja 10/12/12 Muscle Metabolism: Energy Use and Muscle Contraction Learning Objectives Describe how muscle fibers obtain energy for contraction Distinguish between aerobic and anaerobic contraction, muscle fiber types, and muscle performance. Muscle Contraction requires large amount of energy Creatine phosphate releases stored energy to convert ADP to ATP Aerobic metabolism provides most ATP needed for contraction At peak activity, anaerobic glycolysis needed to generate ATP Creatine phosphate; ATP and Glycogen; 15 billion thick/fiber; each one 2500 molecules of ATP/second; thousands of muscle fibers per muscle Cell 3.75 10(16) power ATP molecules required. Muscle Metabolism Resting muscle: fatty acids are catabolized; the ATP produced is used to build energy reserves of ATP, CP, and glycogen Moderate activity: Glucose and fatty acids are catabolized; the ATP produced is used to power contraction Peak activity: Most ATP is produced through glycolysis, with lactic acid as a by-product. Mitochondria activity (not shown) now provides only about one-third of the ATP consumed. Energy use and level of muscular activity Energy production and use patterns mirror muscle activity Fatigued muscle no contracts o Build up of lactic acid o Exhaustion of energy resources Recovery Period Begins immediately after activity ends Oxygen debt (excess post-exercise oxygen consumption) o Amount of oxygen required during resting period to restore muscle to normal conditions Muscle Performance Types of skeletal muscle fibers o Fast Fibers larger diameter, paler color; easily fatigued o Slow Fibers smaller diameter, darker color due to myoglobin; fatigue resistance o Intermediate fibers Similar to fast fibers; greater resistance to fatigue Fast Fibers Large in diameter Contain densely packed myofibrils Large glycogen reserves Relatively few mitochondria Produce rapid, powerful contractions of short duration Slow Fibers Half the diameter of fast fibers Take three times as long to contract after stimulation Abundant mitochondria Extensive capillary supply High concentrations of myoglobin Can contract for long periods of time Muscle performance and the distribution of muscle fibers Pale muscles dominated by fast fibers are called white muscles Dark muscles dominated by slow fibers and myoglobin are called red muscles Training can lead to hypertrophy of stimulated Physical conditioning Anaerobic endurance time over which muscular contractions are sustained by glycolysis and ATP/CP reserves Aerobic endurance time over which muscle can continue to contract while supported by mitochondrial activities Physiological profiles of motor units: all fibers in a motor unit are of the same fiber type Slow motor units contain slow fibers: o Myosin with long cycle time and therefore uses ATP at a slow rate o Many mitochondria, so large capacity to replenish ATP o Economical maintenance of force during isometric contractions and efficient performance of repetitive slow isotonic contractions Fast motor units contain fast fibers: o Myosin with rapid cycling rates o For higher power or when isometric force produced by slow motor units is insufficient. o Type 2A fibers are fast and adapted for producing sustained power Muscle is adaptable! Muscle “adapts” to meet the habitual level of demand placed on it, i.e. level of physical activity Level of physical activity determined by the frequency of recruitment and the load Increase muscle use o Endurance training o Strength training (cannot be optimally trained for both strength and endurance) Decrease muscle use o Prolonged bed rest o Limb casting o Denervation o Space flight Endurance training Little hypertrophy but major biochemical adaptations within muscle fibers Increased numbers of mitochondria; concentration and activities of oxidative enzymes (e.g. succinate dehydrogenase, see below) Increased use: Strength training Early gains in strength appear to be predominantly due to neural factors… optimizing recruitment patterns Long term gains almost solely the result of hypertrophy i.e. increased size Disuse causes atrophy – USE IT OR LOSE IT! Individual fiber atrophy (loss of myofibrils) with no loss in fibers. Effect more pronounced in Type II fibers (Fast, white fibers) “Completely reversible” (in young healthy individuals) ATPase activity: o Type I fibers light o Type II fibers dark Motor unit remodeling with aging Fewer motor units; more fibers/motor unit Muscle injury may play a role in the development of atrophy with aging Muscles in old animals are more susceptible to contraction-induced injury than those in young or adult animals Muscles in old animals show delayed and impaired recovery following contractioninduced injury Following severe injury, muscles in old animals display prolonged, possibly irreversible, structural and functional deficits. You should now be comfortable with the following concepts! The organization of muscle and the unique characteristics of skeletal muscle cells The structural components of the sarcomere The events at the neuromuscular junction The key concepts involved in skeletal muscle contraction and tension production How muscle fibers obtain energy for contraction Aerobic and anaerobic contraction, muscle fiber types, and muscle performance.