Muscles that most often experience spasms are muscles that contract in a shortened position. This is especially true of the muscles that cross the two joints, including the muscles that form your hamstrings, quadriceps, caviar, bicep shoulder, and long triceps head.
Normally, a person's muscles either strain completely or are completely relaxed. The degree of muscle tension is controlled, so a person is able to perform any actions: from physical heavy exercises to fine work under a microscope. Muscle spasm, or muscle block, is a state of muscle in which a section of the muscle is in constant tension and is not controlled by a person. Spasms affect both individual bundles of muscle fibers and entire muscles.