The mound session was Carpenter's second since facing hitters during a live batting practice last Friday. He was scheduled to have another session on Monday but had to cancel it because of recurring weakness in his shoulder.
Though Carpenter resumed throwing on Wednesday, that bout of weakness prompted the decision to visit Dr. Gregory Pearl, a specialist in thoracic outlet syndrome, a condition that can involve weakness, numbness and tingling due to pain in the neck and shoulder. Though the condition is considered rare, it is more common in athletes who use an overhead motion during activity.
The diagnosis didn't come as a surprise to the Cardinals, who anticipated that this was the explanation for Carpenter's nerve-related issue during Spring Training. Surgery can correct the problem, but the Cardinals are going to see if Carpenter can pitch through the issue before going that route.
"In a perfect world, we would address it this offseason," said Cardinals general manager John Mozeliak, inferring the likely possibility that Carpenter will undergo offseason surgery.
"I would say we had a pretty good idea that this was coming," Mozeliak added. "Can he pitch or not? At some point, we have to determine that. The fact that he's started throwing again is good news, but as far as the probability of what we can expect with him this year, I think it's a little too early to tell."
The amount of time missed after such a procedure has varied greatly among Major League players.
Jeremy Bonderman, for instance, underwent surgery for thoracic outlet syndrome in June 2008, missed the rest of the season and then missed much of the '09 season because of the resulting damage around the nerves in his shoulder.
Matt Harrison, on the other hand, had surgery in August 2009 and resumed throwing bullpens in less than a month.
For now, though, Carpenter and the Cardinals are encouraged by the improvement he has felt since Monday. Carpenter threw about 40 pitches in this latest bullpen session.
"It was good to go down and get an opinion of why, the other day, it kind of went backwards," Carpenter said. "I felt good again today. We're going to go by the way that I feel."
Carpenter likely won't throw again until Monday -- assuming his arm continues to feel strong -- and it has not yet been determined whether that next session will take place in the bullpen or out on the field against hitters.
"They seem to have found a title to what he has, which I think is comforting to him, comforting to all of us," Cardinals manager Mike Matheny said. "But there's really not a game plan or protocol to what he has to follow. It's going to be mostly how he feels. There's not a timetable right now."