What’s the better way to do sit-ups? Feet hooked under something, or feet free? I usually do sit-ups without hooking my feet, but tried the other way yesterday and found they weren’t any easier or more difficult, but my legs (particularly my quads) were pretty sore afterwards.
What’s your experience?
I always use something, while in the Army we took turns holding each others feet even for the PT test. Guess I just got used to it.
I use an inclined bench so I pretty much have to hook my feet under the pole, but even without a bench I would use something. I always feel that I get the best effect out of my workout if the muscles I’m not working on are stabilized against something, so I can focus totally on the muscles I am working on and make sure they are doing all the work.
Anytime I try without my feet held down, I notice I tend to “rock” myself to swing forward. They seem harder because I work harder to keep myself in the right form, but I could see that they would feel easier if I allowed the rocking motion to help.
Mostly I use an incline board, or that big ball (Bosu?) both of which force my body into a better posture and feel as though they work my abs more.
I would suspect that the quads get more use keeping your body posture right, thus the additional soreness there.
I’ve heard that crunches give the same result as a full on situp and you don’t need to worry about rocking or bracing your feet.
That said, the best way I’ve found to sit up is to sort of roll halfway over and push back up into a sitting position using my arm. It’s just a lot easier.
If you hook your feet under something, you are working your hip flexors.
You don’t need to come up very far. Once your shoulder blades are off the mat, your abdominals are as contracted as they are going to get.
How do jackknifes compare?
If I don’t hook my feet, when I try to do a sit up or even just a crunch, my feet will go up in the air while my shoulders remain firmly planted on the ground. I think I am unevenly weighted. Although I admit I am far from being an expert in sit ups. 
For a long time I’ve heard crunches are better than sit-ups, tho I’m not sure exactly why.
When I do crunches I generally put my feet on top of something. I feel it isolates the abs better, and prevents any reliance on the hip flexors.