There are two types of muscle movement—concentric and eccentric. Concentric contraction is when the muscle contracts and shortens. Eccentric contraction is when the muscle contracts and lengthens. Both movements work the muscles, but in different ways. Both types of movement are important to muscle development.
In fact, both concentric and eccentric movement are necessary not only for developing the strength of a muscle, but also in developing the neuromuscular control of a muscle. Eccentric movement tends to be more challenging for us and in doing very slow movement, deficits will show up as shakiness or jerkiness.