Challenging ab exercise

[Razor X]

Has anyone ever tried this one?

Lie on your back, hands behind hour head,  knees bent like you're ready to do situps.  Have a work-out partner stand on your feet.  Then do a situp and propel yourself up as far as you can and touch your partner's shoulders.  Do as many reps as you can.  You can really work up a good sweat doing this one.

TIP:  Try to find a work-out partner who isn't too tall.   

[bulls_fan]

Can't say that i've tried that one before.

I like the leg lift exercise. Lying on your back with your legs straight, bringing them up into the air than back down without touching the ground

[StumpyDave]

That's one we use sometimes in martial arts classes.  It starts to burn remarkably quickly.

Here's another:
In your regular situp position interlock your ankles with a partner.
Alternate sitting up so that you can touch your partners knees.

With only limited resistance on your feet you can't use your thighs as much to pull up so your abs have to do all the work.
After 3 sets of 20, stay interlocked, hold your arms straight out to the side and then raise to 45 degrees and hold for 10 seconds.  Drop down an inch and hold for 10 seconds.  Keep dropping an inch and holding until you are back on the floor or unconcious.