This
exercise introduces some limb movement to the basic bridging
exercise. Because the muscles that elevate your thigh partially
originate from the front of your spine, your spinal stabilizing muscles
must work hard to prevent any movement from occurring at your spine.
If
you are having difficulty with this exercise, go back and work on
bridging
and maybe try the
bridging
with arms over trunk until you
can hold it for at least 30 to 60 seconds without any sagging or
shaking.