 This is a test of upper motor neuron disease.If a forearm pronates, with or without downward motion, then the person is said to have pronator drift on that side reflecting a contralateral pyramidal tract lesion.In the presence of an upper motor neuron lesion, the supinator muscles in the upper limb are weaker than the pronator muscles, and as a result, the arm drifts downward and the palm turns toward the floor.