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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. Plank  is one of the most popular isometric exercise, as it challenges all core muscles.
Overcoming versus yielding
In an overcoming isometric, the joint and muscle work against an immovable object. In contrast, in a yielding isometric, the joint and muscle are held in a static position while opposed by resistance.
In the context of the bench press, an example of a yielding isometric would be holding the bar at a given place even though it could be pressed higher, and an overcoming isometric would be pressing the bar up into the safety guards of a squat cage that prevent pushing the bar any higher.
The distinction is that in a yielding isometric, one is pressing roughly the exact amount of pressure needed to negate the resistance, neither dropping or lifting it. Whereas in an overcoming isometric, one can be exerting more force and simply unable to move it. The yielding wavers slightly into concentric and eccentric actions due to inexact control, whereas the overcoming isometric is more purely isometric and can involve more variation in the force used, since one can press harder without the bar moving.
In overcoming isometrics, subjects can safely do 100 percent effort, and continue with 100 percent available effort as strength is depleted, allowing longer time under (maximum volitional) tension. This is thought by experts such as Ken Hutchins and Drew Baye to be best for building muscle. However, subjects depend on a subjective perception of effort and progress, unless a metered system is available.
Yielding isometrics allow measurable progress. Free-weight enthusiasts tend to believe the "back pressure" of real weight is superior for building strength, possibly triggering a productive "fight or flight" response. However, the extremely heavy weights needed by advanced subjects can be an inconvenience, and present a risk of injury.
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.
Resistance in isometric exercises typically involves contractions of the muscle using:
- The body's own structure and ground
- Structural items (e.g., pushing against a fence)
- Free weights, weight machines, or elastic equipment (e.g., holding a weight in a fixed position)
- Pressure-plate-type equipment that has a digital display of maximal force.
Depending on the goal of the exercise, the exertion can be maximal or sub-maximal.
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.
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. They can also be used to prevent disuse syndrome in a limb that has been immobilized by a cast following a fracture.
Seated isometric exercises
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One benefit of isometric exercise is that it can be done pretty much anywhere using your own body weight. According to Sit Sense, a book on seated exercises, you can get a significant workout with seated isometric exercises and significantly lower your blood pressure. Seated isometric exercise is ideal for geriatrics or rehabilitation.
Comparison with dynamic exercises
Isometric training maximally increases strength over all joint angles the exercises are performed at, similar to how dynamic exercises increase strength throughout the full range of motion, but with less risk of injury than weight training (no weights). More recent research has confirmed the finding that every skill has a specific strength component that must be practiced with the skill itself. 
NASA has researched the use of isometrics in preventing muscle atrophy experienced by astronauts as a result of living in a zero gravity environment. 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 concluded that isometrics may not be the best way for astronauts to maintain muscle tissue.
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