Situps are fine, but also do other core-strengthening exercises to prevent muscle adaptation.
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Figuring out the difference between a "good" and "bad" exercise is often a matter of who you ask. In general, doing any type of exercise is better than none, so long as you're doing the exercise safely. With that in mind, if you're wondering whether doing more than 20 situps is "bad," the short answer is no.
Situps are a muscle-strengthening exercise that primarily work the rectus abdominis muscles at the center of the abdomen. To do them correctly, place your feet under a couch or brace, or have a friend hold them. Lie down on the floor and place your hands behind your neck or head; just remember not to strain your neck during the exercise. Bend at the waist and hips and raise your upper body off the floor, touching your elbows to your knees if you so desire. Bring the upper back all the way to the floor before starting your next repetition.
As a general rule, each muscle-strengthening exercise you do, you should do "to fatigue," meaning your muscles will be exhausted and it will be difficult to do any more repetitions of the exercise. When you're using an exercise machine such as the abdominal curl machine at the gym, for example, you'd set the weight plates to a weight that will make your muscles fatigued at the end of a 12- to 15-rep set. When you're doing regular situps, meanwhile, you don't have the added weight to contend with, so working "to fatigue" means doing situps until you can't do any more.
As you might have gathered, the ideal number of situps is going to be different for each person, based on overall strength and conditioning, age and genetics. Your max might be 10 situps; for your friend, it could be 100. Once you've done that single set and worked your muscles to fatigue, you don't need to do any more that day. A single set of exercises working your muscles to fatigue is sufficient for most people, reminds MayoClinic.com. Another important thing to keep in mind: situps alone are not going to give you a flat belly. To lose the fat that's surrounding those muscles, you have to do calorie-burning exercises such as running, swimming, biking or any other type of cardio.
Time for Rest
As with any muscle-strengthening exercise, it's important to give your muscles time to rest and repair. Each time you do a muscle-strengthening exercise, your muscles get tiny tears -- that's where new muscle tissue will grow. To give your body time to generate that new tissue, you have to take at least 24 hours' rest in between strength-training sessions.