I have a question for the personal trainers on this page. I am 5'1 and 105 lbs. How much weight should I be lifting? I normally lift 10-12 lb weights for 12 reps and three sets. I am petite but more of a mesomorph than an ectomorph. What sort of program should I be doing? i am 25 years old and starting to notice a decline in muscle tone in the back of my legs. I used to be a sprinter in school so this is unusual for me. Any suggestions?