Next, ask the patient to extend and raise both arms in front of them as if they were carrying a pizza. Ask the patient to keep their arms in place while they close their eyes and count to 10. Normally their arms will remain in place. If there is upper extremity weakness there will be a positive pronator drift, in which the affected arm will pronate and fall. This is one of the most sensitive tests for upper extremity weakness.
Pronator drift is an indicator of upper motor neuron weakness.
In upper motor neuron weakness, supination is weaker than pronation in the upper extremity, leading to a pronation of the affected arm. This test is also excellent for verification of internal consistency, because if a patient fakes the weakness, they almost always drop their arm without pronating it. The patient to the left does not have a pronator drift. Provide resistance at the wrist. Repeat and compare to the opposite arm.