Muscle biology is the study of muscle structure and function in living organisms. Muscles are made up of specialized cells called muscle fibers, which contract to produce movement. There are three main types of muscle: skeletal muscle, which is under voluntary control; smooth muscle, found in organs and not under voluntary control; and cardiac muscle, which makes up the heart. Muscle contraction occurs through a process called the sliding filament theory, where actin and myosin filaments slide past each other. This process is powered by adenosine triphosphate (ATP), the energy currency of cells. Understanding muscle biology is essential for fields like physiology, medicine, and sports science.