A push-up is a compound exercise that strengthens the upper body, including the chest, shoulders, triceps, and core. The primary action at the shoulder that is strengthened during a push-up is shoulder flexion. In addition, shoulder horizontal adduction also plays a role, particularly as the push-up progresses and the elbows begin to move closer to the torso.
During a push-up, the humerus moves in three planes of movement: sagittal, frontal, and transverse. In the sagittal plane, the humerus moves from extension to flexion as the person lowers their body towards the ground and pushes back up. In the frontal plane, the humerus moves from adduction to abduction, as the arms move out to the sides and back towards the body. Finally, in the transverse plane, the humerus rotates as the hands turn outwards during the descent and then turn back inwards during the ascent. These complex movements of the humerus during a push-up contribute to the overall strengthening of the upper body.