Purpose. To study extraocular muscle (EOM) function, the local physiologic contraction and elongation (deformation) along human horizontal EOMs were quantified using motion-encoded MRI. Methods. Eleven subjects (healthy right eye) gazed at a target that moved horizontally in a sinusoidal fashion (period 2s, amplitude +/-20 degrees ), during MR imaging with an optimized protocol. In addition, EOM longitudinal deformation of two patients with Duane's syndrome type I were analyzed. The horizontal EOMs and the optic nerve were tracked through 15 time frames and their local deformation was calculated. Eight segments were separated along the EOMs and compared for left-to-right and right-to-left eye movements. Results. In healthy subjects, the maximal EOM deformation was situated at approximately 2/3 of the muscle lengths from the scleral insertions. The EOM deformations were similar for the entire movement range as well as in both movement directions. In two patients with Duane's syndrome type I, the abnormal innervation of lateral rectus muscle affected specific EOM segments only. The posterior muscle segments contracted and the anterior muscle segments relaxed during adduction. Conclusions. Motion-encoded MRI is a useful technique to advance the understanding of the physiology and pathophysiology of EOMs in humans during eye movement.