Chris Carpenter is optimistic with his rehab program

Cardinals pitcher Chris Carpenter left an extended bullpen session feeling optimistic about his on-and-off rehab program.

After working four simulated innings and totaling 106 pitches counting warm-ups Friday, the 38-year-old Carpenter said it was the best he'd thrown and was something he could build on.

The next step likely will be facing hitters.

The former NL Young winner and longtime Cardinals ace was sidelined before spring training by a nerve injury that causes numbness and tingling on the right side of his body.

Lately he's been dealing with a lower back injury that briefly delayed the process.

The often-injured Carpenter pitched sparingly last season after undergoing radical surgery to relieve nerve compression