Muscles are the effectors of movement, and their physiology is critical to understanding exercise. There are three types of muscle fibers: slow-twitch (Type I), fast-twitch (Type IIa), and fast-twitch (Type IIb). Slow-twitch fibers are used for low-intensity, long-duration activities, while fast-twitch fibers are used for high-intensity, short-duration activities. During exercise, muscles use energy from ATP, phosphocreatine, and glucose to fuel contractions. The 7th edition of "Physiology of Sport and Exercise" discusses the importance of muscular adaptations to exercise, including hypertrophy (increased muscle size) and changes in muscle fiber type.