how many repetitions does a 18 year old have to do with 20 pound dumbbells ?

Make sure to give yourself time inbetween reps, 5-10 minutes for a cool down. Also, work out every OTHER day, 3 times a week. It gives your body reovery time, which makes your workout more efficent than damaging.

Make sure you work out your shoulder muscles as well as your pectorial (chest), otherwise you will stand way too forward, or backward. Have you ever seen those guyse who walk around with there chests puffed out? Or hunched over like the Hulk? Thats what will happen. Another tip is to Make sure you work your traps, and triceps, if you only work your biceps it will make your arms still appear to be small and not as bulky. And over time your forearms may appear small in comparison to your arms, Get a smooth rod, pole or broom handle, aprox. shoulder lenght long. Tie a tough yet not bulky rope to the middle of the pole, with a weight at the very end of the rope. Now flip your arms around so your forearms are facing downwards, and your hands are palm down. Grasp each end of the rod and raise your hands so your arms are parallel with the ground. With one hand at a time but simitaniously, in a fast, upbeat mannor, wind the rope upward with just your hands.This will help build your forearms too.

depends