A 45-year-old female with bipolar I disorder taking aripiprazole 15 mg daily reports an overwhelming sense of inner restlessness and an inability to sit still that developed over the past two weeks. She describes constantly shifting positions, pacing, and an inability to relax. She notes the symptoms are present throughout the day but worsen in the evening. She denies leg discomfort, crawling sensations, or an urge to move specifically in her lower extremities. Her husband reports that she has no unusual leg movements during sleep. On examination, she is observed rocking from foot to foot and appears unable to remain seated. Which finding on assessment most reliably distinguishes akathisia from restless leg syndrome in this patient?