SAN FRANCISCO (AP) — Dwight Clark says he has Lou Gehrig’s disease and suspects playing football might have caused the illness.

Clark announced Sunday on Twitter that he has amyotrophic lateral sclerosis, a disease that attacks cells that control muscles. The former San Francisco 49ers wide receiver linked to a post on his personal blog detailing his ALS diagnosis, but the site crashed Sunday night, apparently from an overflow of traffic.

Clark says he’s been asked if his football career caused his ALS. He says, “I don’t know for sure. But I certainly suspect it did.” He encourages the NFL and the players’ association to work together in making football safer.

The 60-year-old Clark says he’s lost significant strength in his left hand and also has weakness in his right hand, midsection, lower back and right leg. He says he “can’t run, play golf or walk any distances” and that “picking up anything over 30 pounds is a chore.”

He adds “that the disease seems to be progressing more slowly than in some patients.”