Energy for skeletal muscle contraction ATP sources The many causes of muscle fatigue Classification of skeletal muscle fiber types

ATP and muscle contraction Need ATP for Cross-bridge formation, power stroke (myosin ATPase) Ca++ transport to SR (Ca++ ATPase) Na+/K+ transport across sarcolemma (Na+/K+ ATPase)

Sources of ATP ATP pool Phosphocreatine. At rest, ATP phosphorylates creatine. During exercise, creatine kinase (creatine phosphokinase) moves phosphate from phosphocreatine to ATP

Sources of ATP Glucose (glycolysis) to pyruvate citric acid cycle  oxidative phosphorylation (about 30 ATP per glucose molecules) Anaerobic glycolysis: glucose  lactic acid (2 ATP per glucose molecule)

Sources of ATP Beta oxidation of fatty acids. Fatty acids are converted to acetyl CoA citric acid cycle in the mitochondria, need oxygen Slow During light exercise

Sources of ATP glucose catabolism during heavy exercise carbo loading builds up glycogen stores Protein catabolism during starvation

Fatigue Muscle is no longer able to generate sustained expected power output A variety of contributing factors depends on the degree of muscle activity

Fatigue: contributing factors Intensity of muscle activity Duration of muscle activity Aerobic/anaerobic metabolism Muscle composition Fitness level Ions Nutrients Neurotransmitter

Fatigue during extended submaximal exertion Not ATP shortage Glycogen depletion may affect Ca++ release from SR

Fatigue during short duration maximal exertion Lots of inorganic phosphate from ATP hydrolysis  may slow P release from myosin:ADP:Pi  slows power stroke Acidosis may inhibit some enzymes

More factors for muscle fatigue K+ : intracellular K+ lowered during repetitive action potentials affects Ca++ release channels on SR membrane Acetyl choline depletion at the myoneural junction low end-plate potential (disease)

More factors for muscle fatigue CNS: Subjective feelings preceding physiological fatigue Acidosis may influence perception of fatigue