Variations on the crunch are often "go to" abdominal exercises. I get the attraction: crunches are convenient and relatively simple. Plus, they provide a good abdominal "burn," which always feels rewarding! The thing is, they are not overly functional or effective, AND, possibly more important, they are not the best exercise to do if you have lower-than-normal bone density.
Basically, the take-home message today is this: Stop relying on crunches to tone your midsection. Variety is the spice of life—mix things up! Everyone, but especially anyone over the age of 50, should transition away from relying on exercises that involve crunching. If you have been diagnosed with osteoporosis, don't gradually move away from crunching, STOP now. There are lots of fantastic core exercises that don't involve crunching forward.
Dead bug: Lie on your back with your arms straight above your shoulders and your legs in the air, bent at a 90 degree angle. Make sure your back is neutral (your lower back should not be excessively arched or pressed into the ground; instead allow for only a slight space so that your natural lordotic curve is maintained) and you have equal weight on both sides of your pelvis. Make a note of how this position feels. Your goal throughout the exercise is to not let the position of your back and pelvis change as you move your limbs.
Inhale to prepare, and as you exhale extend your left arm and right leg away from each other. Inhale and hold the extended position for one beat, and then exhale to bring the arm and leg back to the starting position. Do six reps and then switch sides.
Side dead bug: Assume the above starting position. This time, instead of straightening the arm and leg, let the opposite arm and leg move sideways. Inhale to prepare, exhale as you drop the arm and leg out to the side. Inhale with the limbs held out at the side, and then exhale to bring the leg and arm back to the starting position. Do 10 reps and then switch sides.
Pay attention to your pelvis. As your leg falls to the side, keep the opposite side of the pelvis on the ground.
Why shouldn't you do crunches if you have osteoporosis?
Osteoporosis is when one or more of your bones has lower-than-optimal bone density. If you have osteoporosis in your spine, the trabecular bone within the bodies of your spinal vertebrae will be affected. The trabecular bone (which can be thought of as dense "packing material" within your vertebral bodies) starts to lose its density. This means that under the wrong types of stress (e.g. a crunch forward) the vertebral bodies could start to collapse forward and begin to look more like wedges than squares.
Basically, when you flex forward into a crunch you are increasing the risk of a fracture by placing your body in a position that stresses the already weakened trabecular bone.
Be careful—often one does sit-up-like motions in life without being aware of it. For example, instead of doing a full crunch to get out of bed, try rolling onto your side and pushing yourself up with your arm. Or make sure you sit up tall when riding the upright bicycle so you don't slouch forward in a flexed, curled forward position.