If your body shakes the first time you do crunches, there's no need to panic. This quivering is your body's natural reaction to the lengthening and shortening of your muscles. You're putting demands on your abdominal muscles that they're not used to. Although you're working on strengthening them, they might not be strong enough yet to smoothly work through the motion. As you continue doing abdominal exercises, your muscles will get stronger and more capable of withstanding the exertion. To avoid the uncomfortable, involuntary shaking, modify your crunches to make them easier, as you get stronger, gradually increase the challenge.
Once you've grown accustomed to regular crunches, increase the challenge by doing them on an incline bench or while holding weight plate behind your head.
Consult a doctor before starting a new exercise regimen, especially if you have a health condition or injury.
Bend your knees and place your feet flat on the floor rather than dangling your bent legs in the air and using your hips to control them. Alternatively, raise your legs straight up, bend your knees 90 degrees and rest your feet on a chair or bench to really isolate your abdominals.
Extend your arms along your sides so there's less leverage to lift as you crunch. The higher your arms, the more challenging the exercise. As you get stronger, cross your arms in front of your chest to increase the challenge. Once you master this, place your fingertips on the sides of your head.
Perform no more than one set of as many crunches as you can with perfect form. Aim for anywhere from 10 to 25 crunches. Perform the crunches slowly -- go up on a count of three, pause one second, and lower on a count of three. Really press your back into the floor and use your abdominals to raise your shoulder blades off the floor. Stop doing crunches as soon as your form starts to suffer.
Rest for 24 hours after working your abdominals so your muscles can repair themselves and get stronger.
Repeat the exercise, aiming to complete as many crunches as you can with perfect form. Keep doing this every other day. Over time, you'll gradually get stronger and will be able to do more repetitions without shaking. This is when you can add one more set. Once this is manageable, add a third set, making sure to always rest 30 to 60 seconds between sets and to work your abs on nonconsecutive days.
Kimberly Caines is a well traveled model, writer and licensed physical fitness trainer who was first published in 1997. Her work has appeared in the Dutch newspaper "De Overschiese Krant" and on various websites. Caines holds a degree in journalism from Mercurius College in Holland and is writing her first novel.