Muscle growth and fat accumulation continue into the early 20's. Higher proportion of the male body is muscle, not fat, making men stronger. Emerging adults are better at running up a steep hill, lifting a heavy load, or gripping with max force. Serious disease is uncommon and chronic illnesses are less troubling. By age 20, the immune system is developed and strong. Blood pressure is normal, teeth develop no new cavities, heart rate is steady, brain is fully grown, lung capacity is at it's largest.