I've heard a million people say, "I don't have a glute ham raise at my gym... what do I do?" Well, here's an alternative. It helps if you have something to hook your heels under (as shown in the picture). Place your knees on the stability ball. Lower yourself under control until your knees are almost completely straight. Driving 
your heels under the ledge, pull with your hamstrings to raise yourself back to the top. This exercise is a little more difficult than a traditional glute ham machine because your calves are completely taken out of the exercise.