When performing push ups the concentric phase is?

when you push yourself away from the floor, the lowering phase is the eccentric phase concentric = shortening of muscles - i like to think of this as when the muscles are "working"

eccentric = lengthening of muscles - i like to think of this as when the opposite muscles would work to perform the action - e.g. when biceps brachii is eccentric in a bicep curl it is on the lowering phase (i.e. elbow extension - usually performed by triceps brachii)