Muscle contractions also occur apart from the arm movement in a pushup. These are known as isometric contractions. The muscles are tensioned and exert force, but do not change in length.The muscles of your core, including your rectus abdominis and obliques, isometrically contract during a pushup.uscle contractions also occur apart from the arm movement in a pushup. These are known as isometric contractions. The muscles are tensioned and exert force, but do not change in length.