« Back to Glossary Index These are the additional muscles to those typically used for breathing, that contribute to breathing in or expelling air. The accessory muscles of inspiration are the external intercostal, scalenus, sternocleidomastoid, pectoralis, and trapezius. The accessory muscles of expiration are the rectus abdominis, external abdominis obliques, internal abdominis obliques, transverse abdominis, and internal intercostals. « Back to Glossary Index