Here is a great core exercise from my GLUTEUS FABULOUS catalog that is great for strengthening the glutes, hamstrings, adductors, rectus abdominis and erector spinae. 

 
Start by placing a resistance band above the knees. Lie on your back with legs hip width apart, knees bent and feet flat on the ground. Keep arms out to the side. 
 
Contract your abdominals and lift your hips off the ground until your knees, hips and shoulders form a straight line, abduct or open the band, close the band then lower back down to the floor. 
 
You can add single leg hip bridges to make it more challenging. Repeat for 2-3 sets of 10-12 repetitions.
Click on the video under Recent Posts on the right.