I was just wondering what muscles you were working when running and walking fast on a threadmill ?

I thought it was only the muscles in my legs that were working out. But I found that after a work-out, I had pain (a burning sensation mostly) at the bottom of my back. I also have pain just below my breast (on my ribcage) and also in the region of the stomach (mostly on the side of it). What are those muscles and is this normal they are exerciced as well during running and walking ?

The pain in your back and sides sounds like you're feeling running in your core muscles (stomach and low back). These help keep you stable, balanced, and upright, so it makes sense you'd be working them.

The under rib pain, though, I don't know about - that isn't a muscle you'd typically feel pain while running.

I'm not really sure what kind of pain you are talking about. It could be muscle but it could be something else too. But when running/walking, you use your entire body. Legs, core, upper body. It all works when your are running.

The rib pain and stomach pain sound like a cramp. Cramps usually happen while you are running, not afterwards, but sometimes you might get one for a short time after you finish running. Was it a sharp pain and did it go away fairly quickly or more of a dull soreness that has continued for a day or so? Sharp and quick=cramp, dull and continuing means you've probably worked a muscle you haven't worked in a while. You do have to engage your core muscles when you run.

Also, the impact of running can sometimes cause odd pains. I broke my collarbone as a kid and sometimes when I run, the impact will make my collarbone ache a bit. But it stops when I stop running.

The impact is also hard on you back, especially your lower back. Make sure you stretch out well after you run so that your back doesn't get tight. If I don't stretch properly after I run, I can really feel it in my lower back.

The first time I ran, it felt like I was sore all over. I think my eyelids even hurt. I could barely move for days. But that went away pretty quickly once I started exercising regularly. I've never been that sore since.