Rotational action of deep back muscles.

Splenius (cervicis and capitis) and Erector spinae mm.: these muscles diverge (origin closer to midline that insertion; e.g. spinous process to ribs), so unilateral contraction causes ipsilateral flexion and rotation.

Ipsilateral flexion: since they do not cross the midline (spinous processes of vertebrae), they must flex the same side on which they are located.

Ipsilateral rotation: since the inferior attachments are more medial than the superior ones, rotation occurs to the same side on which the muscles are located.





Transversospinalis mm.: semispinalis (cervicis and capitis), multifidus, rotatores (br. and lng.). These muscles converge since they run from a transverse process to the spine of a vertebra 1-3 (depending on the muscle) segments superior. Therefore, unilateral contraction causes ipsilateral flexion but contralateral rotation.

Ipsilateral flexion: for same reason as erector spinae mm.

Contralateral rotation: since the inferior attachments are more lateral than the superior ones, rotation occurs to the opposite side on which the muscles are located.





Pictures from http://moon.ouhsc.edu/dthompso/namics/trunkmm.htm, Dave Thompson, PT.