In isometric contraction, muscle fiber length is constant, so muscle contraction occurs without joint movement. Isometric exercises are very useful when the strength of a muscle is to be maintained or increased but the movement of the joint is either contraindicated because of fracture instability or undersirable because of pain. This is the earliest type of strengthening exercise to use after most fractures because it has the least chance of disturbing the stability of the fracture site. Examples include contracting the quadriceps muscle while the leg is in a long leg cast or the bicep muscle while the arm is in a long arm cast. These exercises are also referred to as set exercises.
P R E C A U T I O N S : Isometric exercises can raise blood pressure significantly for the duration of the exercise. While it will return to a resting level soon after, this can be dangerous for people with hypertension or any form of cardiovascular disease. Even if you don't suffer from high blood pressure it is important to breath continuously throughout the exercises. Breath holding will only compound any increases in blood pressure with the glottis, nose and mouth closed leading to Valsalva maneuver.
Difference between isometric, isotonic, and isokinetic exercises Strengthening exercises are often classified as isometric, isotonic, and isokinetic. Isometric exercises involve contraction of muscles while they are at a fixed length, such that little or no joint motion is taking place. Isometric exercises are a reasonable initial approach for strengthening in patients who cannot tolerate repetitive joint motion, such as those with painful, inflamed joints. Although isometric contractions can be learned quickly and can result in rapid strength gains, functional benefits from isometric contraction exercises might be limited to a small range around the joint angle of training.
Isotonic exercises are often used to maximize strength. By definition, isotonic refers to joint movement through its ROM, against a constant weight or resistance. Isotonic work consists of concentric and eccentric contractions, depending on whether the muscle length is shortening or elongating during the contractions, respectively. Eccentric contraction is more stressful than concentric but also results in greater strength gains.
Isokinetic exercises involve constant speed of motion throughout the joint range during muscle contraction, whereas the amount of resistance may vary throughout the range. Isokinetic exercises are infrequently used because of equipment requirements and uncertain correlation to functional activities.
In summary, isotonic and closed-chain exercises are probably the most beneficial exercises in patients with osteoarthritis, but individuals who find these painful may perform isometric exercises. The goal should be to progress to isotonic exercises that include a combination of open- and closed-chain activities.
Strengthening is generally initiated with isometrics, advancing to concentric and ultimately eccentric exercises.
Treatment and Rehabilitation of Fractures edited by Stanley Hoppenfeld, Vasantha L. Murthy
Baker K, McAlindon T. Exercise for knee osteoarthritis. Curr Opin Rheumatol. 2000 Sep;12(5):456–463.