Q: I weight train with weights that are between 50-90 pounds.How much muscle could I gain with these light weights?

A: Unfortunately I can't tell you the answer to that without knowing more about you and the exercises that you do. A 90-pound bicep curl for reps is impressive for almost anybody. A 90-pound squat for reps may be the appropriate weight for beginner man or intermediate woman lifter. A 90-pound bench press is impressive for a woman, but not for a man. So to give you the answer, I'd need more information about you and the exercises that you do.

If your goal is building muscle then you should emphasize big compound exercises such as squat, deadlift, lunges, bench press, should press, pullups and pulldowns and rows. For any given exercise, pick such a weight that you reach muscular failure in 6 to 12 repetitions of the exercise. If your equipment limits you so you do not have weights that are heavy enough, don't despair and just do more reps or choose more challenging exercises so you reach failure in fewer reps. For example, substitute front squats or lunges for the regular backsquat, do slow one-handed dumbbell rows instead of two-handed barbell rows, etc.