Marcus Lattimore stays in training room during practice

San Francisco 49ers running back Marcus Lattimore, whose 21-day window to either be activated to the team's active roster or be placed on season-ending injured reserve opened Wednesday, sat out practice Friday after two days of hitting.

Niners coach Jim Harbaugh said Lattimore "hung back" in the trainers' room during practice but would not elaborate on if the running back suffered a setback.

Lattimore has not played since blowing out his right knee in college at South Carolina on Oct. 27, 2012. The 49ers made him a fourth-round draft pick the following April and he has yet to be on the team’s 53-man roster.