During an isometric contraction, the muscle is contracted, but does not shorten, like during a isotonic contraction. In an isometric contraction, the muscles pull on the tendons, stretching the tendons and the muscle tissue.
During isometric contractions, the contraction does not create any movement. Although the muscle may bulge, the arm or leg does not produce any movement. The muscle is flexed, but it is not used to produce movement. Isotonic contractions produce movement, such as pulling or pushing open a door or lifting weights. The muscle in these contractions actually changes size from the attachment site, whereas isometric contractions produces no change in the muscle length.