The Dejerine Percussion Hammer is a diagnostic instrument used to test deep tendon reflexes and muscle tone during neurological examinations, typically featuring a double-sided rubber head, a stainless steel handle, and sometimes a concealed pin or brush for additional sensory testing . Available in lengths ranging from approximately 20 cm to 24 cm, it is designed for assessing knee, bicep, tricep, and Achilles reflexes in clinical or veterinary settings .