inferior rectus
Also called IR, inferior rectus muscle
The vertical extraocular muscle beneath the globe. It arises from the lower part of the annulus of Zinn, runs forward at about 23 degrees to the visual axis, inserts 6.5 mm behind the lower edge of the cornea, and its primary action is depression, with extorsion and adduction as subsidiary actions1.
With the eye turned outward by 23 degrees it becomes a pure depressor, which is its test position1,2. Its antagonist is the superior rectus and the inferior division of the oculomotor nerve supplies it2,3. It is the muscle most often stiffened in thyroid eye disease and the one trapped in an orbital floor fracture, and in both cases the forced duction test separates restriction from weakness.
More detail
It lies in the same plane as the superior rectus, so the same geometry applies: in primary position it depresses, extorts and adducts, and with the eye turned far inward it would act only as an extorter1,2. Its yoke in down-and-out gaze is the superior oblique of the other eye. A stiff inferior rectus restricts elevation and can imitate a palsy of the superior rectus or inferior oblique. On the three-step test a weak right inferior rectus gives a right hypertropia that increases on right gaze and on left head tilt5.
Sources (5)
- Kanski, Clinical Ophthalmology (2024), ch. 18
- von Noorden and Campos, Binocular Vision and Ocular Motility, ch. 3 and ch. 4
- Adler's Physiology of the Eye, ch. 7, Table 7.1
- Pickwell's Binocular Vision Anomalies, ch. 17
- Scheiman and Wick, Clinical Management of Binocular Vision, ch. 14
Reviewed October 2026