Last season, it was an ailing back that kept James Harrison at less than 100 percent physically. This season, will it be a knee?

Harrison, a four-time All Pro, was among six Pittsburgh Steelers who began training camp on the physically unable to perform list. Each of the other five was known to be recovering from offseason surgeries.

Harrison's inclusion on the list could be viewed as the most alarming. He was limited to 11 games last season (four due to an eye injury and one because of suspension) and was bothered by a bad back that had required surgery. He turned 34 in May.

"We just want to make sure he's ready to go before we proceed," coach Mike Tomlin said. "We don't want to take any steps backward."

Running back Rashard Mendenhall, nose tackle Casey Hampton and offensive tackle Max Starks each sustained a torn ACL late last season and are in varying stages of recovery. Starks, whose contract ran out after last season, was viewed as healthy enough to have been re-signed last week. Mendenhall is seen as more of a concern than Hampton simply
because of the position each plays and the lateral cutting required by a ball carrier.

Two reserves -- linebacker Jason Worilds (wrist) and safety Damon Cromartie-Smith (shoulder) -- are the other players placed on the PUP list.

"In many instances, these might be viewed as precautionary measures," Tomlin said. "Some of these guys that are getting close to coming off of injury, we want to get them in this setting, look at them, work them out, be certain that it's something that's behind us before we progress."

Follow Steelers reporter Chris Adamski on Twitter: @CBSSportsNFLPIT and @buzzsawpgh.