you shouldn't be contracting your abs during exercises like that, because if you are it means you are rounding your back. its possible to feel a slight pump or soreness but contracting (eccentrically, concentrically) your abs during the movements is dangerous because you need to keep your spine stabile. the stretch could be because you are hyperextending, which is also dangerous once you start getting up there in weight. make sure you are bending from your hips and stop just short of where your back starts rounding. to get deeper start stretching your hammies but always make sure your spine is stabile.
"The greatest obstacle to knowledge is not ignorance but the illusion of knowledge." -Barry Marshall, Nobel Laureate
I think I may have misinformed you when I said that my abs contract - I just meant that they feel stretched a little as I extend to the upright position; so this is normal, right? And when you mention the back 'rounding', exactly what do you mean? Are you referring to when the back is in the fully upright position? Or when one is leaning back? Sorry for the ignorance, and thanks for the help!
he means rounding, like your bending your back hence contracting your abs. if this is the case, I hypothesize that you probaly have to much weight and cannot keep your back straight on the pull up, or you are locking your knees. remember keep your knees SLIGHTLY bent.
if your tensing them to pull your body upright then thats wrong form too. your shoulders and back should be locked so the strain is pushed onto your glutes and hams
another tip to be sure youre hitting glutes and hams... focus on the hips/pelvis moving back as you bend, rather than how far down the bar goes. it's better to have the hips move back slightly further, even if the bar doesn't hit say shin level. then as you extend back up, focus on pulling the hips/pelvis all the way back to the starting point, concentrating on the hams. this will also help to keep your head up and back straight. just a tip i picked up on... hope it helps