According to WebMD, push-ups primarily develop the chest and triceps, resulting in improvements to strength and mass. Other muscle groups affected to a lesser degree include the shoulders, abdominals and even legs. The muscles of the back are not usually worked by the motions in the push-up.
WebMD states that a proper push-up uses form that is concentrated in the chest and arms. The upper back, shoulders and arms are used to lift the body slowly in a straight line. When coming back down, it is important to stop a few inches before touching the floor, creating more tension. If a basic push-up is too difficult, an alternative is to leave the knees on the ground and raise the rest of the body.