According to Louisiana State University Medical School, negative pressure in the lungs relative to the surrounding air pressure is what causes inhalation during breathing. This is accomplished by increasing the volume of the lungs, which decreases the pressure until new air passes through the trachea to equalize it.
Louisiana State University Medical School states that there are two areas of muscles that expand the chest cavity, and thus the lungs, decreasing their internal pressure. These include the diaphragm and the muscles that draw the ribs up and out. The diaphragm is a sheet of muscle between the organs of the chest and abdomen that lowers during inhalation. The diaphragm is the primary muscle powering inhalation. In both cases, inhalation requires that the muscles contract actively, while exhalation is caused by the relaxation of both the rib muscles and the diaphragm in quiet breathing. In forced breathing, such as during heavy exercise, other muscles cause exhalation to occur faster. These include the abdominal muscles, which decrease the abdominal volume, and opposing rib muscles, which quickly decrease the volume of the chest cavity. In either case, as with inhalation, exhalation is accomplished through changing the air pressure in the lungs relative to the surrounding air pressure.