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In isometric exercises, you contract a muscle or group of muscles without moving a joint. You can perform isometric exercises by pushing or pulling an immovable object. These exercises differ from concentric exercises in which a muscle shortens to produce movement. When you hold a dumbbell in your hand and lift it toward your shoulder, you are concentrically contracting your biceps. If, however, you hold a dumbbell at elbow-height for several seconds, you are isometrically contracting your biceps.
Submaximal isometrics are isometric contractions in which you don't contract the muscle to your full ability. Instead, you contract the muscle below your maximum. Researchers and trainers typically refer to submaximal isometric contractions in terms of percentages. For example, you might be asked to contract a muscle to 50 percent of your full capacity.
You can perform submaximal isometrics by pushing your foot against a wall to produce an isometric contraction in the muscles around your ankle. You would not push as hard as possible. Rather, you would exert constant, steady pressure against the wall while keeping the ability to push a little bit harder. As another example, you could attempt to squeeze your thighs together on a hip adductor machine that is loaded to a weight that exceeds your capacities. Instead of trying your hardest to move the weight, you would squeeze a bit less than your maximum capacity.
Submaximal isometric exercises can help you increase strength. This type of exercise is typically used in rehabilitation settings, when moving a joint would be contraindicated. Sports coaches might also prescribe this type of exercise to increase strength in a muscle, at a particular point in the athlete's range of motion. For example, if a gymnast's hip flexors are particularly weak when she extends her leg as high as she can, the coach might ask the gymnast to contract the muscle at this particular point in her range of motion.
When performing submaximal isometric exercises, you must hold the contraction for a longer duration than you would hold a maximal isometric contraction in order to gain strength. Generally, you need to hold the contraction between 30 seconds and one minute, and you need to repeat each contraction several times in order to build strength. Even then, the isometric contraction only strengthens the muscle at the precise angle at which you held the position. Another disadvantage of submaximal isometrics is that it is difficult to gauge the intensity of the exercise. This requires highly specialized equipment that can measure your maximum capacity as well as the amount of force you are exerting. This type of equipment is not available at most gyms.