Isaac, 6-foot-2 and 215 pounds, chose the Trojans over Michigan and Auburn. He was the 2011 ESPNChicago.com Offensive Player of the Year and is ranked No. 68 in the ESPN 150.
"We've been to a lot of schools and done our homework," Isaac's father, Tyrone Isaac, said Tuesday. "We dwindled it down to one school. It was about what they were able to offer academically and what they offered athletically. It came down to the right fit for him. It was about the tradition, and he fit with the players and coaches."
Isaac rushed for 2,629 yards and 41 touchdowns on 203 carries and caught 16 passes for 369 yards and six touchdowns during his junior season. He rushed for a state-record 515 yards and six touchdowns in a 70-45 loss to Montini in the Class 5A state championship.
"First of all, he's very elusive," Joliet Catholic coach Dan Sharp said. "He has great vision and ability to cut in space. He's strong and extremely fast. Put all those components together, and he's a great running back."
Joliet Catholic has produced a number of quality running backs over the years. Most recently, Illinois freshman running back Josh Ferguson and Northwestern incoming freshman running back Malin Jones played for the Hilltoppers. Former NFL fullback Mike Alstott also attended Joliet Catholic.
USC now has six Class of 2013 commitments.