When I do leg raise abdominal exercises I feel burning in my hips, why?

When you perform any type of leg raises you have to activate your hip flexors in order to lift your legs. Your abdominals do not do this motion, so you are really exercising your hip flexors. These muscles run under your abdominals, connecting on your spine. The burning sensation you feel is your hip muscles fatiguing. Use caution when doing leg raise exercises. A safer, alternate abdominal exercise is the plank, ball crunch, or reverse crunch.