Measurements of eye movements have shown that centrifugal movements (i.e. away from the primary position) have a lower maximum velocity and a longer duration than centripetal movements (i.e. toward the primary position) of the same size. In 1988 Pelisson proposed that these kinematic differences might be caused by differences in the neural command signals, oculomotor mechanics or a combination of the two. By using the result of muscle force measurements that were made in recent years (Orbit 1.8 Gaze mechanics simulation, Eidactics, San Francisco, 1999) we simulated the muscle forces during centrifugal and centripetal saccades. Based on these simulations we show that the cause of the kinematic differences between the centrifugal and centripetal saccades is the non-linear force-velocity relationship (i.e. muscle viscosity) of the muscles.