The smooth, but steady, increase in muscular tension produced by increasing the number of active motor units is called a recruitment.
Muscle tension is the force produced when a muscle contracts (or when sarcomeres shorten). The two primary forms of skeletal muscle contractions, isotonic contractions and isometric contractions, are produced when a muscle contracts against a load that is not moving.
A load is transported as the length of the muscle varies during isotonic contractions, in which the tension in the muscle remains constant (shortens). Concentric and eccentric contractions are the two varieties of isotonic contractions.
When a muscle contracts isometrically, the angle of a skeletal joint remains the same while tension is produced in the muscle. Sarcomeres shorten and muscles tense up during isometric contractions, but the load is not moved since the force generated is insufficient to overcome the resistance provided by the load.
