Hey guys i want to strenghten my core, and give some good workout to my ligaments. I was thinking to get a medicine ball and work out on my own, i got this whole book of exercises, but i`m having trouble picking the right weight.. I don`t want it too heavy cause i`m coming of a small shoulder injury. Anybody knows what would be a good beginners weight? My weight is 165 with a 5'11 hight..