Isometric exercise
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 isotonic movements).  Isometrics are done in static positions, rather than being dynamic through a range of motion.  The joint and muscle are either worked against an immovable force (overcoming isometric) or are held in a static position while opposed by resistance (yielding isometric).

# Definition
Isometric exercise is a form of exercise involving the static contraction of a muscle without any visible movement in the angle of the joint. This is reflected in the name; the term "isometric" combines the prefix "iso" (same) with "metric" (distance), meaning that in these exercises the length of the muscle does not change,  as compared to isotonic contractions in which the contraction strength does not change but the joint angle does.

# Resistance in isometric exercises
Resistance in isometric exercises typically involve contractions of the muscle using:
- the body's own muscle (e.g., pressing the palms together in front of the body)
- structural items (e.g., pushing against a door frame).
- free weights, weights machines or elastic equipment (e.g. holding a weight in a fixed position)
- pressure plate type equipment that have a digital readout of maximal force.
Depending on the goal of the exercise, the exertion can be maximal, or sub-maximal.

# History
Isometrics are thousands of years old and examples can be found in the static holds in certain branches of yoga or Chinese martial arts.
Isometrics was first brought to the modern publics attention in the early days of physical culture, the precursor to bodybuilding. Many of the great bodybuilders of the day used and incorporated isometrics into their training regimes. Perhaps the most famous of the isometric advocates was Charles Atlas. In his course he advocated a series of dynamic tension exercises which included isometrics. He had learned these exercises from the "Conscious Evolution" Course of Alois P. Swoboda. Atlas said "everything he knew he learned from A.P. Swoboda". Unfortunately, isometrics fell out of favor as it was discovered that many of the principal advocates were using steroids to enhance their gains. Charles Atlas' methods of self-resistance did not have traditional resistance limits as weight exercises do, as the tissue exercising against can provide greater resistance as it develops in tandem.  His system was flawed in that there was no method of measuring progression; users were not motivated by measurable progress as assessment of increases in strength subjective and lacked an empirical means of demonstrating progress.

# Isometrics today
Today many new training protocols exist incorporating isometrics once again. Isometric exercises are often made into parts of normal, isotonic exercises. For example, during a set of rows, some people 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. Other systems dedicate themselves entirely to isometrics. An example of an isometric would be holding or carrying something heavy.

# Medical uses
Isometric exercises can also be used at the bedside to differentiate various heart murmurs; the murmur of mitral regurgitation gets louder as compared to the quieter murmur of aortic stenosis.

# Isometric exercises in comparison with dynamic exercises
Isometric exercises have some differences in training effect as compared to dynamic exercises. While isometric training increases strength at the specific joint angles of the exercises performed and additional joint angles to a lesser extent, dynamic exercises increase strength throughout the full range of motion. Generally speaking however, people who train isometrically don't train through a full range of motion as the strength gained at the training joint angle is where they require it. While dynamic exercises are 5% better at enhancing the twitch force of a muscle than isometric exercises, isometrics are 32% better than dynamic exercises at increasing maximal muscle power.

# Isometrics and NASA
NASA has researched the use of isometrics in preventing muscle atrophy experienced by astronauts as a result of living in a zero gravity environment. Kenneth Baldwin, a professor in the Physiology and Biophysics Department at the University of California, Irvine, conducted studies in order to better understand how muscles work. Isometrics, muscle lengthening and muscle shortening exercises were studied and compared. The outcome showed that while all three exercise types did indeed promote muscle growth, isometrics failed to prevent a decrease in the amount of contractile proteins found in the muscle tissue. The result was muscle degradation at a molecular level. As contractile proteins are what cause muscles to contract and give them their physical strength, NASA has concluded that isometrics may not be the best way for astronauts to maintain muscle tissue.