During the video"Foam Roller Flexibility Routine", it instructs to roll the lower back, knees, achilles. In my previous foam roller class, the teacher said: when we lied down on the ground, for those parts not touching ground should not be rolled, such as neck, lower back, knee, achilles. There is no big muscle there so the microvascular is easy to get injuried. I'm confused about we could roll these parts or not.