Why cant I do any situps without someone holding my feet
At the Gym I use an abdominal machine to do stomach crunches and over time I've increased the weight and made steady progress. However I have just started a Taekwondo class and as part of the warm-up we have to do sit-ups, I cannot even do a single sit-up without someone holding my feet. I can lift my upper back of the floor but not the bottom half of my back. For what it's worth, in case there is a genetic component, neither can my son.
I can do the other exercises including press-ups, I'm surprised I cannot do them at all since I have made progress on the abdominal machine. Will doing them with my feet held strengthen the correct muscles so that over time I can do them unaided, or is there a particular muscle/mechanical issue that will continue to prevent me doing them unaided?
Try ankle weights, I'd say it's a compromise between unaided and fully locked down.
I think I do have long torso, actually if I have my legs flat I can situp but not easily and my feets/legs do leave the floor so I think Im not using my stomach properly, perhaps the ankle weights would help
The problem is not that your core muscles are not strong enough to lift your chest up. It looks like you are facing another (bigger problem) : physics.
Actually, it is not weird not to be able to do what you are trying to do. In the position that you are in (supposedly with your feet close to your butt), your center of gravity is located above the waist. So whenever you try to lift up your chest, you will simply more or less roll on your back.
Two ways of solving this :
Stick you feet under something heavy (furniture, children, wife, whatever is available)
Simply lift your feet like this
Actually, what you are currently doing will mostly target your upper abs. So you don't need to go all the way.
From laying on my back, I can sit up just fine with my legs flat on the ground. If I go slow I can keep the backs of my heels pinned to the ground, if I go fast, they pop up a bit. But in a knees bent (back of heels at my butt) position, there's no way I can sit up.
Well, that is explained easily by understanding how your legs position affect your center of gravity. With you legs flat on the ground, it is lower than with your knees bent. That is why you have a harder time sitting up.
You're not using *center of mass* correctly here. Lifting your legs as shown in the image will cause the same issue of really only being able to do a crunch rather than a full sit-up. ....
Good advise, but everyone else in the class seems able to do it so I dont understand what is the difference between them and me