When many people think about strength training the type of training they are thinking about is isotonic strength training. Isotonic is where the joint moves and the muscle exerts force by either becoming longer or shorter against a resistance. Some examples of this are pushups, crunches, bicep curls, and basically any exercise where the joint is moving against a resistance. During an isometric contraction of a muscle, the muscle is still exerting a force, like during the isotonic contraction, however the muscle does not change length and the joint does not move. An example of this would be trying to push against a brick wall. You are exerting force, but no motion is occurring.
Isometric exercises are generally not included in very many workout programs, because they are slightly harder to use and can be harmful for people with heart problems or high blood pressure. For the most part isometric exercises are used only by professionals like athletes and weight lifters. This is due to the fact that isometric exercises increase the strength of the muscle faster than isotonic exercises. However, isomeric exercises only increase the strength of the muscle when it is in the exact position that is was when the isometric contraction was held. Doing isotonic exercises increases the muscles strength throughout the entire range of motion for that muscle when done correctly. This is why for most people it is more practical to do isotonic exercises.
However, there are certain situations where isometric contractions can be helpful, such as after an injury or surgery when the muscle is recovering. If the joint is healing and cannot be moved, isometric contractions can be used to strengthen the muscle and stabilize the joint with out moving it and risking further injury to the joint. People can also use self imposed isometric resistance to make a workout harder without having to actually life more weight. This means that even though the weight they are lifting is actually only a few pounds, they resist the weight like they are lifting a weight that weights 50 pounds. Isometric exercises can also be added into your daily life to increase strength in the abs and buttocks. While going through your day either driving, sitting at a desk, or standing in a line, you can contract the abdominal muscle bringing your belly button closer to your back. Hold this contraction for about 5-10 seconds then relax, repeating throughout the day. This will help strengthen the abdominal muscles and improve posture. A similar exercise can be done by contracting the muscles of the buttocks.