Measurement of the mechanical stiffness in cyclotorsion of the human eye

We have measured the stiffness in cyclotorsion of the human eye using a scleral suction contact ring mounted on a shaft fitted with an eddy current motor to provide the torque to turn the eye and a shaft-position-encoder to register the torsion. The relation proved to be almost linear within the measuring range in four subjects with constant slope values of about 0.75, 0.60, 0.55 and 0.45 g·cm/deg. The average, 0.6 g·cm/deg, equals 0.5 g if applied at the radius of the globe. This value is rather low compared to the values obtained by other authors for horizontal eye movements. This may be due to different length-tension characteristics of oblique muscles compared to recti muscles.