I am 16 yrs old and i workout every 3 or 4 days a week (lifting weight). I weigh 140, am slim/slender and my height is 5,8. I have seen results on my arms, chest, legs, back etc except for my six pack. I have been doing some sit-ups and crunches for the past 4 months and no results. What can i do to increase muscle structure in my belly? I would also like to know how can i build muscle faster in my whole body.
Thx