I sprained my thoracic/lumbar junction about 2 and a half years ago. I first picked up a disc 25 years ago, but I had awful form for a long time. One of the bad habits is throwing too hard. Following the injury, I had 3 total episodes where it spasmed the entire set of core muscles. FUN FACT: There are quite a few muscles that make up the core. I had a massage therapist try to work out my psoas following the injury. If you are a female and want to experience a taste of what it feels like to get hit in the balls, this is how you would do it.
The injury was caused by throwing hard, being off balance, which somehow hyperextended my back. The good news: You don't have to throw hard to throw far. Prior to the injury, on my best days, I could get a high speed driver to 425'. With the help of a local friend (#HUB), and keeping things slow, my good days are more frequent and my top distance is now 450-500 on those good days.
Here are some exercises I found to be very useful in preventing further injury and discomfort.
*NOTE: I AM NOT A DOCTOR* I highly recommend finding a good physical therapist who is an orthopedic specialist.
Exercise #1 Psoas:
I stretch the psoas because I work at a desk all day. I have a sit/stand desk to combat the effects of psoas being shortened by sitting all day. This helps relieve pressure on the lumbar portion of the spine.
Lay with your back down on a surface where you can dangle a leg. You can do this on the counter or a massage table. Your back down to the base of your spine should be supported. Pull one knee up to your chest and let the other leg dangle. You should feel this stretch in your groin/top of the hip. Stretch it for 60 seconds, about 3 times per leg. There are other supporting muscles that will prevent the psoas from stretching for a bit. That's why it's such a long stretch. I sometimes add weights to my ankles to make it a deeper stretch.
Exercise #2 Lumbar/Thoracic Strength/Stretch:
Get on your hands and knees. With one arm reach under your body and stretch out your hand with the back grazing the floor. You can also do this with a foam roller with the back of your hand resting on top of the roller to start. Bring your hand back to neutral. Do that 10x (start with one set, then move up to 2)
Exercise #3 Lumbar/Thoracic Strength/Stretch 2:
Still on your hands and knees, place one hand on the back of your hed. Stretch the elbow up towards the sky, hold it a second, then back down. Do this 10x (start with one set, move up to 2)
Beyond that, focus on exercises that strengthen and stretch your core. Yoga is great for this.