There are 3 main types of muscle contractions: Isometric, Eccentric, and Concentric.
Isometric: An isometric contraction is a static action where the muscle generates force but there is no movement. When a barbell is held at the midpoint of the bicep curl, the biceps exert force but do not change in length. When the athlete places resistance against the curling movement, the biceps neither overcome the resistance, nor are they overcome by the resistance. The muscle length does not change even though it is contracting.
Generally, we are strongest in our isometric contractions [ie; standing still with a loaded barbell on our back.