Muscle contractions generally involve the shortening of a muscle while exerting a force and performing work. However, there are many different types of contractions, and some do not strictly follow that definition. Isometric contraction occurs when the muscle does not shorten, but it does exert force (e. g. pushing or pulling an immovable object). Isotonic contractions take place when the muscle length shortens and the force remains the same (e. g. lifting a weight at the gym). In an auxotonic contraction the force gradually increases while the muscle length is shortening (e. g. pulling on a rubber band). Conversely, a meiotonic contraction occurs when the force decreases as the muscle length shortens (e. g. depressing a key on a computer keyboard). Most muscle contractions involve a combination of two or more of the above contractions and are called mixed contractions. For example, when lifting a large bucket filled with water, there is first an isometric contraction, followed by isotonic shortening.
While skeletal muscle is resting, there is still a force exerted due to the tension created from the muscle’s connection to the bone on each end of the muscle. This force is called the resting force and is similar to the force of a rubber band that is stretched. Tests performed in the laboratory demonstrate that muscles have an optimal length where contraction produces a maximum active force. Maximum force usually occurs at the natural length of the muscle and is termed optimal length (L0).
Since cardiac muscle is not connected to bones like skeletal muscle, it functions over a greater range of lengths. Additionally, its maximum force ability is observed at a lower L0, giving it a “reserve” length. This allows cardiac muscle to contract more forcefully when necessary. The muscle is re-lengthened when the chamber of the heart fills with blood.
Smooth muscle does not make the typical isotonic contractions seen in skeletal and cardiac muscle. Most smooth muscle contractions of the digestive tract occur as a substance passes through the hollow tube that smooth muscle comprises; therefore, smooth muscle shortens against a decreasing load. On the other hand, smooth muscle in blood vessels maintains a partially isometric contraction where the force is held constant for an extended period of time, resulting in a particular blood pressure.