When an eye moves excessively into the field of action of an extraocular muscle on version testing, that muscle is traditionally described as being overacting. Consider the patient in Figure 1. On elevation in right gaze, the left eye elevates higher than the right eye. This phenomenon is typically referred to as an overaction of the left inferior oblique muscle. In this context the term overaction can be misleading because it assumes an etiology based solely on an instant clinical observation. In fact, many conditions exist in which such a motility pattern may be caused by factors completely unrelated to the left inferior oblique muscle.1 These conditions include Duane syndrome of the left eye, dissociated vertical deviation in the left eye, craniofacial syndromes, antielevation syndrome after inferior oblique muscle transposition in the right eye,2 mechanical restriction of the right inferior rectus muscle, right superior rectus muscle paresis, pseudo-inferior oblique muscle overaction with a Y or V pattern,3 or pulley heterotopia.4,5 In addition, when an abnormal motility pattern like that seen in Figure 1 is caused by the left inferior oblique muscle, multiple different pathophysiologic situations can make the muscle seem to overact. The abnormal elevation of the left eye on adduction can be caused by an increase in the elastic force (decreased elasticity) or an increase in the contractile force (increased strength) of the left inferior oblique muscle.1 This latter condition can be a result of increased innervation to or sensitivity of the muscle, an increase in muscle bulk (cross-sectional area), or changes in the types of muscle fibers within the muscle.1 Consequently, the term overaction can lead to misunderstanding as to what is causing a perceived motility disturbance, which can then lead to inappropriate treatment. For example, a routine weakening procedure on the left inferior oblique muscle may not benefit the patient in Figure 1 if the elevation of the left eye on adduction was caused by any of the aforementioned mechanisms that did not involve an abnormality of that muscle.