Fatigue resistance of human extraocular muscles.

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Abstract
Human subjects performed 2 unusual and strenuous eye movement tracking tasks to assess the fatigue resistance of the extraocular muscles. In the 1st tracking task, they made 60.degree. horizontal saccades (.+-. 30.degree.) at a rate of 1/s for 31 min. In the 2nd, they looked as far eccentrically in the horizontal direction as possible and held the eccentric gaze for five 2-min periods, each separated by 1 min of 60.degree. test saccades. After 31 min of 60.degree. saccades, the average peak saccadic velocity decreased by < 10% in all 3 subjects. Since the peak velocities could usually be raised to control values by encouraging the subjects, it seems unlikely that even this modest reduction represented true muscle fatigue. Extreme eccentric gaze produced no consistent change in peak saccadic velocity across subjects. The eye did drift toward the midline following the saccade and a 2nd, much smaller corrective saccade followed. The maximum drift never was more than .apprx. 7% of the size of the net movement. No drift or increase in the incidence of corrective saccades occurred in the other tracking task (31 min of continuous saccades). Apparently, even extremely demanding tracking tasks can be accomplished by the extraocular muscles with only modest reductions in their level of performance. In normal usage, these muscles apparently never show symptoms of fatigue.

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