While best known for his boxing talent, Muhammad Ali was also one of the greatest talkers of all time.

At a time when it was dangerous for black men to sass white people, Ali did just that. He spoke up and spoke back, using his tremendous wit and intelligence to challenge authority and undercut notions of what a black person in America could say and do. He earned his first nickname, “The Louisville Lip,” in tribute to his verbal gifts.

Sadly, however, Ali’s voice faded—literally and much too soon.

According to a new biography of the boxer and a study released Wednesday by speech scientists at Arizona State University, Ali’s articulation started to become less precise in the mid-1970s, when he was between 30 and 35 years old, almost certainly as a result of blows to the head suffered in the ring. The study suggests that changes in the quality of speech can be important early markers of neurological damage or disease. Ali died in June 2016 at the age of 74.

In writing Ali’s biography, one of us, Jonathan Eig, wondered when Ali’s speech began to decline and whether that decline could be tied to boxing. Eig contacted the other two authors of this piece—Julie Liss and Visar Berisha, scientists at Arizona State University who specialize in analyzing speech for signs of cognitive and motor impairment. Eig also enlisted CompuBox Inc., a boxing statistics company, to count the punches that struck Ali over the course of his 21-year professional career. The study is being presented Wednesday at Interspeech, a speech science conference in Stockholm, Sweden.