Final answer:
Energy is released in muscles through the breakdown of ATP during contraction, with fuel sources including muscle glycogen, blood glucose, and fat reserves. For high-intensity activities like sprinting, muscles rely on a different, faster process. ATP provides the necessary energy for muscle contraction, with energy stored in myosin released during the power stroke.
Step-by-step explanation:
Energy is released during muscular activities through the breakdown of adenosine triphosphate (ATP). Initially, muscle glycogen is converted to glucose, which is used in glycolysis and further in the Krebs cycle, where oxygen helps produce carbon dioxide and water, releasing energy for muscle contractions. When muscle glycogen stores are depleted, the liver releases glucose into the bloodstream and fat metabolism increases to fuel the necessary aerobic pathways. For quick, high-intensity activities like sprinting, muscles use a process other than aerobic cellular respiration due to its speed and intensity. ATP is essential for muscle contraction, as it provides the required energy for cross-bridge formation and filament sliding within the muscle fibers.
Moreover, exercise enhances muscular strength and endurance, which are the muscle's ability to exert force and continue contracting over time respectively. The free energy of ATP hydrolysis is stored in a high-energy myosin conformation and is released during the power stroke of muscle contractions, facilitating the work done by muscles.