According to Light on Yoga you do use the strength of your buttocks muscles in order to lift your legs. This is also recommended in a few other books as well. However with this said yoga has changed and been adapted for our Western culture that does not have the same lifestyle habits as those in India where yoga originated. If your buttocks contracts slightly as you lift your legs and there is no compression on your lower back then I would say great. If there is pain then what is probably happening is your large stronger buttocks muscles are taking over from the deeper back muscles such as the erector spinae and the quadratus lumborum. Ideally you have a balance during the posture where the deeper gluteus minimus and the larger maximus to a certain degree are engaged as well as the back muscles previously mentioned. May of us tend to solely use the buttocks muscles in the this posture and never develop the full potential and strength of the lower back.