Straight-leg sit-ups are a variation of sit-ups performed with both legs flat on the ground as opposed to the traditional version with both knees bent. This version of the sit-up is different from the traditional, bent-leg sit-up.
While the original version of the sit-up with bent knees is a great ab exercise, it has certain drawbacks. It can be very difficult to perform without a partner holding your feet down.
A Physics Advantage
If you do a sit-up with bent knees you will have a tendency to fall backwards because most of your body weight is distributed towards your upper body, especially in men. By straightening out your legs you distribute more weight towards your legs, which will help you stay balanced when you roll upwards into a sit-up.
If you have very strong abs and a lot of experience with sit-ups you might be able to do bent-knee sit-ups even if you have nothing weighing your feet down. However, people with weaker abs or less experience will get more out of a straight-leg sit-up because they will be able to go through the entire range of motion, as opposed to falling backwards halfway up in a bent-knee position.
Straight-Leg Sit-Up Form
To perform a straight-leg sit-up, start in a sit-up position. Then straighten your legs out. Flex your abs and begin rolling up towards your legs until you are in a seated position. You don't have to go too far forwards with this exercise, just high enough that your torso is upright at the top. When performing this exercise you can either hold your arms in front of you like a zombie or fold your arms across your chest. Check out the video below for a demonstration.
Read more: How to Do a Correct Sit-Up
Which Sit-Ups are Right?
You might wonder if you should just stick to regular old bent-leg sit-ups or try straight-leg sit-ups.
Both versions of the sit-up offer benefits. In fact, according to this study published in the Archives of Physical Medicine and Rehabilitation, performing either type of sit-up will give you a good ab workout, as long as you don't have anything holding your feet down.
Another study showed that the straight-leg sit-up and bent-knee sit-up are essentially the same in terms of muscle activation but the bent-knee sit-up uses more of the lower rectus abdominis (the muscle that forms the "6-pack") and hip flexors. The straight leg version used a little more of the upper rectus abdominis.
One of the major concerns with bent-knee sit-ups is that they can be bad for your back. Stuart McGill has published a study which shows that "no biologically significant differences were found between the bent knee and straight leg sit-up techniques." This means that the bent-leg and straight-leg sit-ups put equal strain on the lower back.
It seems that the bent-knee sit-up and straight-leg sit-up are fairly similar. So, how should you decide which one you want to do?
The best approach would be to try both variations to see which one you feel more in your abs. Beginners will probably find that the straight-leg version is a better exercise because they can get the full range of motion. If you have very strong abs the bent-knee version will probably be a better challenge.