De Groote, A., M. Wantier, G. Cheron, M. Estenne, and M. Paiva. Chest wall motion during tidal breathing. J. Appl. Physiol. 83(5): 1531-1537, 1997.We have used an automatic motion analyzer, the ELITE system, to study changes in chest wall configuration during resting breathing in five normal, seated subjects. Two television cameras were used to record the x-y-z displacements of 36 markers positioned circumferentially at the level of the third (S₁) and fifth (S₂) costal cartilage, corresponding to the lung-apposed rib cage; midway between the xyphoid process and the costal margin (S₃), corresponding to the abdomen-apposed rib cage; and at the level of the umbilicus (S₄). Recordings of different subsets of markers were made by submitting the subject to five successive rotations of 45-90°. Each recording lasted 30 s, and three-dimensional displacements of markers were analyzed with the Matlab software. At spontaneous end expiration, sections S_1-3 were elliptical but S₄ was more circular. Tidal changes in chest wall dimensions were consistent among subjects. For S_1-2, changes during inspiration occurred primarily in the cranial and ventral directions and averaged 3-5 mm; displacements in the lateral direction were smaller (1-2 mm). On the other hand, changes at the level of S₄ occurred almost exclusively in the ventral direction. In addition, both compartments showed a ventral displacement of their dorsal aspect that was not accounted for by flexion of the spine. We conclude that, in normal subjects breathing at rest in the seated posture, displacements of the rib cage during inspiration are in the cranial, lateral outward, and ventral directions but that expansion of the abdomen is confined to the ventral direction.