Isometric exercise or isometrics are a type of strength training in which the joint angle and muscle length do not change during contraction (compared to concentric or eccentric contractions, called dynamic/isotonic movements). Isometrics are done in static positions, rather than being dynamic through a range of motion. An isometric exercise is a form of exercise involving the static contraction of a muscle without any visible movement in the angle of the joint. The term "isometric" combines the Greek words "Isos" (equal) and "metria" (measuring), meaning that in these exercises the length of the muscle and the angle of the joint do not change, though contraction strength may be varied. This is in contrast to isotonic contractions, in which the contraction strength does not change, though the muscle length and joint angle do.
Isometric exercises are thousands of years old, with examples listed from the static holds in certain branches of yoga or oriental martial arts. Isometric exercises were first brought to the modern public's attention in the early days of physical culture, the precursor to bodybuilding. Many of the great bodybuilders of the day incorporated isometric exercises into their training regimens. Many of today's training protocols incorporate isometric exercises, which are often made into parts of normal, isotonic exercises. For example, during a set of seated rows, a subject can hold their position when the handles are closest to their chest in order to "squeeze" the muscle, in an effort to further strain the muscle. Contrary to popular opinion, The Charles Atlas "Dynamic tension" Course did not use any true isometric exercises, but rather dynamic self-resistance, that is, pitting one muscle group against another and calisthenics.