Did you know most people do them wrong? They make two deadly
1. They lower their legs all the way down, as in image 1 (left).
2. They don't lift their legs all the way up, as in image 3 (right).
You should lift your legs above your hips because the abs pull the
hips, no the legs.
That's also why you shouldn't lower your legs all the way down:
this works out your psoas and iliacus muscles (two muscles in your
hips), and not the abs.
Here's how to do them well:
For example, in this exercise, the lower abs are relaxed when your
legs are down (image 1). To work them out well, you'd better just
go from knees up (image 2) to knees to the chest (image 3) and back
to knees up (image 2). Never lower your legs completely (image 1):
this relaxes your lower abs.