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

2. 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)

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

5. 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)

6. 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

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

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

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

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

11. 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

13. 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)

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