As the diaphragm and intercostal muscles relax, the lungs and thoracic tissues recoil, and the volume of the lungs decreases. This causes the pressure within the lungs to increase above that of the atmosphere, causing air to leave the lungs.Muscle contraction and relaxation are not the same; contraction is the tightening or shortening of muscles, while relaxation is the return of the muscle fibers to a low-tension state.