Two of the most prominent ab exercises were roman chairs and leg raises. Leg raises require only a pull up bar while the roman chair needs a specialized chair and therefore makes leg raises the cheaper of the two. Leg raises were popular during Roman times for both its pure simplicity and effectiveness. Also leg raises succeed at where crunches fail. Leg raises strengthen your abs instead of just activating you abs. What I mean by that is that crunches can activate the muscles but will not trick your body to create more muscle cells. All you will do is burn calories and have the six pack of skinny person, all show no actual use. Leg raises on the other hand requires strength and causes your body to both burn the fat and create muscles. It will not only show a six pack but that six pack will bulge out and take blows, maybe even injure the other person. So what type of six pack do you want? If you want the abs of a martial artist get a pull up bar and start your leg rasies, otherwise if you just want to burn fat stick with 2000 crunches.
Leg raises only require a pull up bar. Start by hanging and contract your abs. Any hanging exercise will be easier if you contract and activate your core because it helps stabilize the body. While you are still hanging slowly raise you legs until your legs are parallel to the ground. While you are raising your legs slowly breathe out and contract the abs. By breathing out your abs will contract even further allowing for more dependency on your own ability. One thing I suggest is to never swing your legs back. Swinging only cheats yourself out of the movement and will not result in the six pack that you want. Instead take your time enjoy your workout music and do it slowly. Trust me. While your legs are parralel to the ground slowly lower them back to the initial position. Remember to breathe in as well as keep your abs contracted. Your abs need to be contracted during the whole exercises to help generate power to the core and stabilize the body.
Leg raises require both strong foreamrs and strong core. In order to strengthen your forearms just keep hanging. Hanging is the best way to strengthen the muscles in theforearms since it causes tension in the muscles. Your core on the other hand can only be strenghtened through more ab exercises. I suggest starting by doing v-sit up as it will work your core in a similar manner to leg raises. Once you are comfortable start with some knee raises. Grab a bar and bring your knees to your chest. The motion of bringing your legs up to a certain height will allow you get used to both the slower cadence as well as the overall motion. After placing a couple of sets of hanging knee raises under your belt we will start slowly lifting your legs parrallel to the ground. I suggest starting slow by either lifting you legs a little at a time or doing the motion with bent knees. If you try the first method remember to try to increase the height every time you attempt a new set. If you try the latter method then slowly get used to straightening out your legs unitl they are perfectly straight. I have seen people do it with both so it just depends on the person. After you achieve you first one then you now have an exercise that will help you achieve your dream of a six pack.