San Francisco 49ers linebacker Chris Borland, center, during an NFL football game in Santa Clara, Calif. Borland announced that he will retire after just one season to protect himself from brain injuries.
Marcio Jose Sanchez/AP
Kurt Busch drives during the NASCAR Sprint Cup Series auto race in Fort Worth, Texas, on Nov. 2, 2014. Busch was recently suspended indefinitely amid domestic violence accusations.
Kentucky's Aaron Harrison and coach John Calipari watch from the sideline during the second half of an NCAA college basketball game against Mississippi on Jan. 6. Kentucky won 89-86 in overtime.
Tiger Woods on the ninth hole during the second round of the Phoenix Open last Friday, in Scottsdale, Ariz. Woods, who'll turn 40 this year, is making Frank Deford wonder: How does an athlete know when to retire?