Re: GH Machine
Lots of options. One, to increase difficulty, is hold a slam ball or wall ball or weight with fully outstretched arms and just do the ab mat sit ups with the added weight. It is really tough if you hold the weight with outstretched arms the whole time. Make it easier by holding the weight to your chest. Another exercise would be to grab a bed post or table leg with your outstretched arms and, with straight legs, raise you legs straight up over your head--that is like a toe to bar on the floor.
Here are some other ab exercises:
v-ups (dead cockroaches)
hollow rocks (awesomely difficult when repping them out)
leg raises or leg holds
russian twists--hold a weight for added difficulty
planks in push up position or on forearms