Force-Velocity Relationship

Image description
Force-velocity relationship according to Hill (1938), image reproduced with kind permission of Dr. sc. nat. Marco Toigo

Muscle force decreases as shortening velocity increases. The higher the concentric shortening velocity, the lower the muscle force. Force increases the slower the movement speed is, or the greater the external resistance is, the more slowly it can be moved.
In eccentric contraction, the muscle has more force than in concentric or isometric contraction.
If your goal is to build muscle strength, the movement speed should be slow, because that's when the internal force is greater!

Sources: Hill AV. (1938) The heat of shortening and the dynamic constants of muscle. Proc R Soc Lond B Biol Sci 126:136–195.