If you get any soreness from sit-ups, it should be in your abs — not your tailbone. But, tailbone shapes and lengths vary, and if yours points in a certain way, it could make the action of lying on your back and crunching up and down from your hips extremely uncomfortable.
You can't change your anatomy, but you can change the way you do sit-ups to prevent pain from keeping you from your ab training.
Change the Surface
Sometimes, the surface you choose makes a difference in how sit-ups make your tailbone feel. If you do sit-ups on a wood floor, rough carpet or cement, you have no padding protecting a pointy or long coccyx bone.
Always do sit-ups on a gym mat. Keep in mind that not all gym mats are created equal, either. Yoga-style mats can be only 1 to 3 millimeters thick and may not offer enough protection. Look for general fitness mats that are usually 1 to 1.5 centimeters thick, or even a denser Pilates mat that could boast as much as 2-centimeter thickness.
If all you have are thin mats, pile a couple together to create a denser support for your tailbone, or fold a single one in half to double up the thickness.
Do Sit-Ups on a Stability Ball
Sit-ups don't have to be performed on the ground. Perform them on a stability ball to protect your tailbone and, as a bonus, you'll also get more activation for your abs. A study published in a 2007 issue of the Journal of Strength and Conditioning Research showed that sit-up exercises on a stability ball are more effective in stimulating the musculature of the abs than a sit-up done on the ground.
To use a stability ball:
Sit on a stability ball. Walk your feet forward until you are leaning on the ball with your low back pressing into it. Cradle your head in your hands.
Brace your abdominal muscles and curl upward, focusing on squeezing your rib cage to your pelvis.
Release to place your spine back in contact with the ball to complete one repetition.
Do Crunches Instead
A full sit-up puts more pressure on your tailbone, making you uncomfortable. Sitting up all the way may not be the most effective way to train your abs either -- so modify them to both save your tailbone and get a better workout. When you do a sit-up, you're using significant assistance from your hip flexors rather than just working your abs.
Crunches, however, leave the hip flexors mostly out of the exercise. To do a crunch, lift your head, neck and shoulders off the ground. Your abs work most when you lift up 30 to 45 degrees.
Stand to Work Your Abs
If you just can't escape tailbone pain, consider ditching the floor and flexing your spine from a standing position.
To do a basic standing crunch:
Stand with your feet hip-width apart. Bend your knees slightly -- so they aren't locked -- and place your hands behind your head. Keep your elbows pointed to the sides of the room.
Draw your abs in toward your spine as you bend you lift your right leg and curl your upper body to touch your right knee toward your nose.
Straighten back out and repeat with the other side to complete one repetition.