The back of the legs and buttocks have more flesh than other areas of the body. Your partner will love a little increased penetration. You will both agree that deeply massaging the tissue in this area is appropriate.
When executing a massage of the back of the legs your partner will be most comfortable with their legs outstretched and slightly apart. Be cautious around the back of the knees. Brushing this ticklish area can be quite alarming and can spoil the gentle mood.