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 elevate upper ribs scternocleidomastoid muscle elevate sternum 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