USC opened training camp with good depth at running back, but several players are still recovering from knee injuries that will limit their participation.
Senior tailback Silas Redd, junior tailback D.J. Morgan and sophomore fullback Soma Vainuku are being held out of some drills.
Redshirt sophomore Tre Madden, who sat out last season while recovering from knee surgery, sophomore Javorius Allen and freshmen Justin Davis and Ty Isaac are taking the majority of repetitions at tailback, sophomore Jahleel Pinner at fullback.
"That's probably the deepest position we have, even though there's not a lot of real game experience, necessarily, in that group," Coach Lane Kiffin said of the tailbacks. "Top to bottom there's a lot of talent."
Davis, 6-foot-1, 200 pounds, enrolled at USC in January and impressed during spring practice.
The 6-3, 225-pound Isaac practiced for the first time Saturday.