"The arm feels great, and it's good to get some positive results back," Wacha said. "I don't feel anything in the shoulder. It definitely feels strong, and it feels alive. I'm excited about getting good news and getting started back up here."
The timeline and specifics of Wacha's throwing program will be determined as the Cardinals see how his arm responds to each test. Wacha will throw on flat ground for several days, possibly until he has his next MRI, scheduled for Aug. 18. If he has not graduated to throwing off a mound by then, another positive reading should push him to that point.
"As long as there is nothing that shows up [on the MRI] in two weeks ... we'll just keep moving forward," general manager John Mozeliak said. "We'll take it day by day. Right now there is a lot of optimism."
If Wacha can avoid setbacks, the expectation is that he will be ready to pitch for the Cardinals in September. The organization still plans to build him up to carry a starters' workload, though Mozeliak has acknowledged that the Cardinals could use Wacha as a reliever down the stretch if that is the team's need.
Because Wacha has been active in the weight and training rooms while awaiting the green light to throw, he should be able to navigate through this throwing program quicker than he would during Spring Training. He has also spent the last several weeks targeting his lower body in workouts.
As the Cardinals try to find ways to prevent this injury from recurring, they hope that increased strength in that area will help.
"I feel really encouraged with where he's at that way," Mozeliak said. "I think from the pure physical standpoint, he's probably as strong as he's ever been."
"I'm building up all the muscles around it to get it stronger and trying to prevent it from happening again," added Wacha. "Hopefully, I took enough time here where it won't flare up for the rest of the season. [I'll] definitely have to still work on the rehab staff in the offseason, probably for the rest of my career."
Jenifer Langosch is a reporter for MLB.com. Read her blog, By Gosh, It's Langosch, and follow her on Twitter @LangoschMLB. Alex Halsted is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.