Normal, Quiet Breathing Muscles of inspiration diaphragm external intercostals Muscles of expiration passive process Exercise Breathing Exercise → ↑ respiratory rate, ↑ tidal volume Muscles of inspiration diaphragm contracts downward increasing intrathoracic cavity volume elevates lower ribs external intercostal muscles elevate lower ribs accessory muscles scalenus muscles login to view 1 more bullet scternocleidomastoid muscle login to view 1 more bullet Muscles of expiration internal and external oblique muscles, rectus abdominis, transversus abdominis compress abdominal cavity and push diaphragm upward internal intercostal muscles pull ribs downward and inward