In an advance that could eventually improve the mobility of thousands of people living with amputations, researchers said a 32-year-old man successfully controlled movements of a motorized artificial leg using only his own thoughts.

Aided by sensors receiving impulses from nerves and muscles that once carried signals to his missing knee and ankle, the patient was able to climb and descend stairs and walk up and down inclines much as he could with a natural leg, based on directions that came from his brain. Importantly, he...