Researchers in the United States say doing sit-ups the old fashioned way provides the most strenuous workout.
This involves folding your arms across your chest, bending your knees and pulling your body up until you are sitting up straight.
They say full sit-ups beat more modern approaches, like "crunches" and sit-ups using exercise balls, when it comes to working muscles throughout the body.
Dr Raymond Chong and colleagues at the Medical College of Georgia tested six different types of sit-ups on 15 volunteers.
full sit-ups using a ball held steady by an assistant.
Sensors were placed on the bodies of each of the volunteers. These were hooked up to a computer to show exactly how much pressure was being put on key muscles.
The researchers found that full sit-ups were the most strenuous, impacting on muscles in the stomach, back, hips and legs.
They were followed by full sit-ups using an exercise ball without any assistance and full sit-ups using a ball with assistance.
Crunches were found to be much less effective and doing them on the floor was found to be the least strenuous.
Dr Chong said crunches were much easier.
"Crunches require less effort and less strain on the hip and lower back," he said.
They also put much more strain on the neck than traditional full sit-ups.
"When the body is vertical, the neck gets a break," he said.
How the sit-ups were rated
1. Full sit-ups on the floor
2. Full sit-ups using an exercise ball unassisted
3. Full sit-ups using an exercise ball with assistance
4. Crunches using a ball without assistance
5. Crunches using a ball with assistance
6. Crunches on the floor
The volunteers who took part in the study said they were surprised by the findings.
"I was quite amazed at the research results," said Laurie Adkins, a student at the Medical College of Georgia.
"It's nice to find out that different types of abdominal exercises target different muscles."
"I was very surprised with the results," said Kimberly Steele, who also took part.
"I've been an aerobics instructor for seven years and a personal trainer for three.
"I typically save the last 10 minutes of a class for abdominal work."
Dr Chong said the findings could help many people to reassess their exercise routine.
He suggested people who want the put their bodies through a strenuous workout may choose to do full sit-ups while crunches may be good for someone just starting to exercise.
"Exercise should be tailored to the individual," he said.
"The only bad exercise is one that's unsafe."