Muscle metabolism relies on ATP as the direct source of energy for contraction. ATP stores are quickly depleted after 4-6 seconds of contraction and must be regenerated through creatine phosphate interaction, anaerobic glycolysis, and aerobic respiration. When muscle activity reaches 70% of maximum, oxygen delivery is impaired and lactic acid builds up, diffusing into the bloodstream. Muscle fatigue occurs when ATP production cannot keep up with demand, leading to relative ATP deficit, contractures, and lactic acid accumulation.

1. 9 Muscles and Muscle Tissue P A R T C

3. Muscle Metabolism: Energy for Contraction Figure 9.20

12. Force of Muscle Contraction Figure 9.21a–c

13. Length Tension Relationships Figure 9.22