Fig. 32. A family of human medial rectus length-tension curves determined from indwelling strain-gauge measurements. The point on each curve at the eye position for that curve's gaze effort is the actual realized force in the muscle. Observe that the force rises beyond 15 degrees of movement out of the muscle's field of action. The loop shows that the force during a saccade is no more than 30 to 40 gram-force beyond that required for fixation. The small rise in force during the return saccade, when the muscle is inhibited as the antagonist, is due to viscosity. (Modified from Collins CC, Scott AB, O'Meara D: Muscle tension during unrestrained human eye movements. J Physiol [Lond] 245:351, 1975.)