Final answer:
After intense exercise, muscles recover with increased oxygen intake and enhanced blood circulation, which help clear lactic acid, replenish energy stores, and reduce muscle soreness.
Step-by-step explanation:
During intense exercise, muscles build up an oxygen debt which must be repaid after the activity has ceased. Post-exercise, muscles recover as increased oxygen intake facilitates the conversion of lactic acid back into pyruvic acid and eventually glucose or glycogen in the liver. This process, along with enhanced blood circulation, helps clear the accumulated lactic acid from the muscles, further aiding in recovery. Additionally, more oxygen in the system can replenish ATP and creatine phosphate levels, essential for muscle contraction and energy. As the heart and lungs work to increase oxygen supply, muscle soreness and fatigue diminish, enabling muscles to function at their original efficiency or even improve due to long-term adaptations in muscle cells such as increased myoglobin content.