While the rest of the pitchers in his throwing group took part in their first live batting practice sessions of spring today, Ryan Mattheus remained sidelined with what he said is an upper chest injury. Mattheus has been shut down since Tuesday, he revealed today, after feeling discomfort in the chest following his second bullpen session of spring. He underwent an MRI and will wait on the results, which will reveal exactly what's going on that's causing the issues. "There's a joint in there that connects the cartilage to the sternum. I think I might have irritated the cartilage or an intercostal muscle or messed a rib up or something," Mattheus said. "I felt it after my second bullpen. They just kind of shut me down and said, 'We'll see how it feels the next couple days,' and it hasn't gotten any better. Hurts with deep breathing and things like that. It's really strange. Something I've never experienced before. It's painful." Mattheus said he didn't feel the discomfort in his chest, located specifically in his upper right pectoral area, until after his bullpen session was complete. "I threw a really good bullpen. I felt great," Mattheus said. "Didn't feel it on any one pitch or anything. And then was going over to do conditioning, running over with my bag, and I was like, 'Man, there's a little discomfort.' It wasn't bad. Got in here, told the trainers. They looked at it, they poked on the rib a little bit, because sometimes the ribs will come out of place. They thought that might be what it is. Then the next day I showed up and I couldn't take deep breaths. "I can't cough or sneeze or anything like that. It's crazy. So we'll figure it out tomorrow." The timing of the injury obviously isn't ideal for Mattheus, given that he's in camp competing for a spot in a crowded Nationals bullpen. There are seemingly two relief spots up for grabs with Rafael Soriano, Drew Storen, Tyler Clippard, Jerry Blevins and Craig Stammen in line to make the bullpen. A host of candidates for those two spots will battle it out this spring, but Mattheus has been set back as he tries to make a solid impression in camp.