30DS L3D5 done.
Right, those dodgy abs exercises. There was someone on the Jillian Webchat who thought the high impact moves had exacerbated her pre-existing back problem to the point that she ended up in hospital. I'd put money on it being one, some or all of these three moves!
Pike Crunches - Natalie is a case-study in bad form on this one, unfortunately. Every time her arms and legs go down, you can see daylight under her lower back. This should never happen in any abs moves, or you're asking for pain and injury. Beginner, intermediate, or advanced, please stick with Anita on this one - don't lower your legs all the way down to the ground. For a slight upgrade, you can add in the arms, but those abs must stay engaged (see Errol's video upthread) and the lower back should stay pinned to the floor.
Scissor Crunches - do not, under any circumstances, put your hands under your bum as they do in the video. It puts your pelvis in an unstable position and creates a greater pull on your lower back. Keep them down by your sides on the floor or, for more advanced, raised by your sides with your head and shoulders off the floor, as per the photo I have attached. Keep the abs flat and engaged and that lumbar spine pinned to the floor.
Sit Ups - Jillian asks at one point in the video 'why doesn't anyone do sit-ups anymore?' Because they're no more effective than ab crunches, Jillian, they can put a lot of undue strain on the neck and it's almost impossible to do them without 'popping' the rectus abdominus, thereby building this muscle incorrectly, as I've mentioned in some PPs. Poor old Natalie is an example of bad form again in this one in that you can see that rectus popping right up every time she does a sit up. The centre of her stomach from sternum to pubic bone underneath her belly button elevates, such that there are hollows either side of the muscle. This will never train your abs to sit flat and support your spine correctly. Furthermore, do not try sit ups with legs out straight the way Anita does them. The psoas muscle which attaches to the top of the femur crosses over the pelvis and joins onto the lumbar spine. With legs out straight, your lower back will be pulled off the floor. See pike crunches above for why this is a bad thing. Interestingly, Anita clearly has mighty core strength - I suspect she's done truckloads of Pilates as she looks like a dancer in a lot of the moves she does. Quite apart from the enviable appearance of her abs, she manages sit ups without popping, and her lower back never lets a chink of light through. If you have core stability like that, go ahead and do the exercises as given. But you probably don't, so either stick with ab crunches or keep your arms stretched out (as per the attached photo) and concentrate on not popping your abs while going through the full range of movement.