In humans, inhalation is an active process requiring intercostal and diaphragm muscles, while exhalation is passive.

“Now, an important principle to understand is that in humans, and in some other species, but really what we're talking about now is humans, when you inhale, that's an active process. You really need to use those muscles of the intercostals and the diaphragm in order to inflate the lungs. But the whole process is made easier when air pressure outside your body is higher than it is in your lungs because then they're going to fill up really readily. Exhaling, at least for humans, is a passive thing. You just have to relax the diaphragm and relax the intercostals and let the rib cage kind of fall back to its original position. So inhaling is active, and exhaling is passive.”