Lungs become stiffer, respiratory muscle strength and endurance diminishes, and the chest wall becomes more rigid. Total lung capacity is relatively constant across the life span but vital capacity (the volume of air that can be forcibly exhaled) decreases because our residual volume increases (the amount of air remaining in the lungs after maximum expiration). At age twenty, about 20% of our total lung capacity is residual air; at age sixty the residual air volume increases to about 35 %. In the normal aging lung, alveolar surface area decreases by up to 20% which reduces our maximal oxygen uptake (the volume of air that can be moved in and out by forced voluntary breathing) by as much as 55% by age 85.