Final answer:
Muscles are made up of cells called muscle fibers, which can contract and shorten. They do this through the sliding of proteins called actin and myosin that form long filaments within the muscle fibers.
Step-by-step explanation:
Muscles are made up of cells called muscle fibers, which can contract and shorten. They do this through the sliding of proteins called actin and myosin that form long filaments within the muscle fibers. When muscles contract, the actin and myosin filaments slide past each other, causing the muscle to shorten. This contraction is the basis for nearly all movement in the body.