In a normal stable patient, the diaphragm does much of the muscular work of respiration. But in a patient with a respiratory problem, they have to work harder to get adequate oxygenation, and this may tire them out.
As they tire, they compensate for fatigue by using secondary muscle groups to help them maintain adequate respiration. For example, patients in acute respiratory distress may use their trapezius muscles or their abdominals (aka belly breathing) to try to catch their breathe.
accessory muscles of respiration - in labored breathing, the use of muscles other than the diaphragm and intercostals. the sternocleidomastoid, spinal, neck, and abdominal muscles and even the platysma, may be used. their use is a sign of an abnormal or labored breathing pattern. (page 15, taber's cyclopedic medical dictionary, 18th edition, published in 1997 by f.a. davis company)