When eye movements, versions or ductions are evaluated clinically, reference is made to overaction or underaction of a muscle. Underaction of the extraocular muscles can be readily explained due to reduced rotation of the globe. This is from decreased innervation, loss of muscle substance, or because of an altered position of the muscle on the globe. Underaction can also result from tightness or tethering of the passive fascial structures and have no relation to how effectively the muscle can pull. When a normal muscle contracts in the presence of a restriction, a rise in intraocular pressure will result. For example, in cases of thyroid myopathy involving the inferior recti, patients have been treated for glaucoma because of the elevated intraocular pressure which occurred during attempts to look up. Underaction associated with paresis or paralysis of the agonist can be shown by observing a saccade. In this case, the saccadic velocity will be reduced and no increase in intraocular pressure will be noted when movement is attempted in the restricted field. Underaction of the superior oblique can result from a congenitally elongated tendon with a normal saccade. This will be discussed in detail later.