Muscle contraction is the activation of tension-generating sites within muscle fibers. In physiology, muscle contraction does not mean muscle shortening because muscle tension can be produced without changes in muscle length such as holding a heavy book or a dumbbell at the same position.he termination of muscle contraction is followed by muscle relaxation, which is a return of the muscle fibers to their low tension-generating state. Muscle contractions can be described based on two variables: length and tension.