Strength training involves three different actions that cause your muscles to produce force.
When force is produced to overcome an external load and the muscle is shortened, the action is referred to as concentric. When force is produced while the muscle lengthens, the action is referred to as eccentric.
For example, picture a dumbbell bicep curl exercise. The lifting portion of the movement is the concentric action and the lowering of the weight back to the starting position is the eccentric action. Paired together, these movements create a dynamic exercise.
A dynamic exercise is any exercise that involves joint movement. When performing dynamic exercises, like the bicep curl, triceps dip or a squat for example, it’s important to move through what’s called the full range of motion (ROM). “By using the whole ROM, the whole length of the muscle is stimulated, leading to adaptations throughout the whole muscle and not just in parts of it,” says ACSM.
However, for exercisers with orthopedic injuries or other limitations, using the full ROM may not always be the best option. Sometimes it may need to be restricted.
An isometric contraction is the third type of action that can cause your muscles to produce force. Contrary to dynamic exercises, isometric exercises create no change in the length of the muscle.
According to ACSM, some research has shown that isometric training can lead to static strength gains, but that the gains are considerably less than five percent per week. “Increases in strength resulting from isometric training are related to the number of muscle actions performed, the duration of the muscle contractions, whether the muscle action is maximal or submaximal, the angle at which the exercise is performed, and the frequency of training,” says ACSM. Because there are so many factors involved, it’s hard to determine which might be most beneficial for increasing strength.
According to current research, it's recommended that exercisers who wish to include isometric actions as part of their strength training routine perform multiple positions of the contraction to ensure full ROM strengthening.
Although research shows that the most effective training programs include exercises involving concentric-eccentric repetitions (dynamic exercises), isometric exercises are a beneficial alternative for those with joint disorders where dynamic exercises might cause pain.
Soucre: ACSM's Resources for the Personal Trainer; Third Edition