Muscle body tension refers to the condition in which muscles of the body remain semi-contracted for an extended period. Muscle tension is typically caused by the physiological effects of stress and can lead to episodes of back pain. When the body is stressed, muscles tense up. Muscle tension is almost a reflex reaction to stress—the body’s way of guarding against injury and pain. With sudden onset stress, the muscles tense up all at once and then release their tension when the stress passes.