Concentric contractions are the primary means by which muscles move us. Muscles, however, don’t always shorten. If you hold the weight steady without moving it up or down, your biceps will still try to shorten but won’t actually change its length, an isometric muscle action. Isometric muscle actions can be challenging, but it is even harder to lower the weight very slowly by extending your elbow. This sort of eccentric muscle action requires your biceps to fire as it lengthens.
