Exercises can be done in different ways to adapt to your abilities and needs!
Isometric exercises are exercises where the muscle contracts but the muscle length does not lengthen or shorten, meaning no movement of the limb. This is typically done against a wall or another stabilized firm object. Isometric exercises place low stress on the muscles and joints and are typically the safest exercises for anyone, but may not provide as many benefits as the other two forms of exercises. An example of an isometric exercise is a shoulder abduction against the wall, where the shoulder is being activated, but no movements are made.
Concentric exercises are exercises that the muscle contracts and the length shortens, causing motions such as bending and extending of the limbs. These exercises are more functional movements, and help train the body in developing the strength and muscle mass to perform these activities.Concentric exercises place some stress on the muscles, and are effective in training for specific movements and coordination of multiple muscles. An example of a concentric exercise is a bicep curl, where the bicep shortens to lift a weight. The movement of the bicep curl is functional in lifting up other objects in a similar fashion.
Eccentric exercises are exercises that the muscle contracts and the muscle lengthens. This uses the muscle to stabilize and support the movement against gravity. Eccentric exercises have been shown to be more effective in increasing strength and muscle mass than concentric exercises, as well as reduction in pain during rehabilitation. However, eccentric exercises cause more tears into the muscle fibers, thus individuals may find themselves feeling more sore after an eccentric exercise. An example of an eccentric exercise is a negative pull up, where muscles are used to slow down the descent of a pull up rather than performing a concentric pull up. This is great for those who have not developed the strength to perform a full pull up yet.