I run 50-60mpw and have lost a lot of my muscle lately. I have had dexa scans done recently and they told me I am 5.7% body fat. I am very skinny 5'9 127lbs with barely any excess body fat however I am unable to get any kind of abs or six pack(I do 10x1 minute planks everyday, along with 200 sit-ups). my body just looks flat. What should I do? try to gain muscle in my stomach by bulking up? or would some kind of others exercise help this?