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?