Abdominal muscle use during quiet breathing and hyperpnea in uninformed subjects.

Although there is electromyographic evidence for abdominal muscle activity during quiet breathing in standing subjects, several studies have shown, or assumed, that subjects normally breathe on their relaxation characteristics. This latter observation would by itself suggest that abdominal muscles do not contract during quiet breathing. To test this assumption we observed abdominal and rib cage displacements with magnetometers in 17 uninformed subjects. During quiet breathing most subjects showed evidence of tonic or phasic abdominal muscle contraction while standing and sitting but not supine. Subjects studied during hyperpnea immediately following exercise-showed evidence of greater abdominal muscle contraction than at rest. We conclude that most subjects standing at rest normally contract their abdominal muscles.