A common problem with doing large numbers of sit-ups is rug burn on your backside. This friction burn is caused by a combination of moist skin from your sweat and the materials of your clothes repeatedly rubbing against the same spot. While rug burn is not a serious medical issue, it can be very irritating and painful. Fortunately, some quick tweaks in your choice of clothing, a thin mat and better technique can prevent you from getting rug burn the next time you try for 300 straight sit ups.
Wear clothes that won't cause friction. Your underwear should be tight-fitting and made of a fabric that will slide on rather than rub against your exercise shorts or pants.
Apply Vaseline or similar petroleum jelly to the affected area before you start exercising. This will prevent the skin from chafing from constant friction.
Use a mat that is firmly planted on the floor. A thin foam pad such as a yoga mat will work, as long as the mat does not slide around the floor as you complete each sit-up. You can also place a small gym towel under the mat to reduce its movement.
Brace your abdominal muscles and squeeze your glutes as you raise and lower yourself. By keeping your core tight and your backside firmly planted on the mat, you can prevent your body from moving around too much as you perform each sit-up.
Warnings
If you let the rug burn go untreated, it may start bleeding and eventually get infected. Clean the affected area with mild soap and cold water, then place a sterile gauze pad over it to let it stay cool and dry as it heals.