Role of the diaphragm in trunk rotation in humans

Anna L. Hudson, Jane E. Butler, Simon C. Gandevia, Andre De Troyer

THE MECHANICS of a skeletal muscle is essentially determined by the anatomy of the muscle and the structures it displaces when it contracts. Given that the muscle fibers of the costal portion
of the diaphragm run cranially and dorsally from their insertion on the lower ribs, it would therefore be expected that isolated contraction of one hemidiaphragm would both lift these ribs
and pull them backward relative to the upper ribs.

In addition, even though the diaphragm is the main inspiratory muscle, it receives corticospinal inputs (e.g., Gandevia and Rothwell 1987; Murphy et al. 1990) and is involved in a range of nonrespiratory contractions, such as stabilization of the trunk prior to rapid arm movements (Hodges et al. 1997), flexion of the upper and lower extremities (Kolar et al. 2010), and trunk extension (in patients with complete cervical spinal cord injury, Sinderby et al. 1992; see also Hodges et al. 2001).

On these grounds, we hypothesized that the muscle would participate in ipsilateral rotation of the trunk. Thus the right hemidiaphragm would contract during rotation of the trunk to the
right to pull the lower rib cage on the right side dorsally relative to the spine, whereas it would remain silent during rotation of the trunk to the left (contralateral rotation). Conversely, the left hemidiaphragm would contract during rotation of the trunk to the left to pull the lower rib cage on the left side dorsally relative to the spine.