Hypermetria is a classical cerebellar symptom designating the overshoot observed when a patient is asked to make a very fast and accurate movement. The movement studied here was wrist flexion. Hypermetria was found to be increased by artificially increasing the inertia of the moving hand. A normal subject adapted himself to increasing inertia by increasing both his agonist activity (the launching force) and its antagonist activity (the braking force). In the same circumstances, a patient with symptoms of cerebellar system dysfunction was found to be able to increase his agonist activity but not his antagonist activity.