Then, you have to remember that when you go back to the starting position, you need to extend your biceps slowly. Remember that lowering the weight actually is more effective than the actual curl. Lower the weight slowly and not just drop the weight all the way down.
If the straight bar is putting a lot of stress on your wrists, which for some people does, then you might want to try the EZ bar curls. This will put your wrists in a much comfortable position and will help you lift the weights much easier and in a much comfortable way but will not decrease the overall impact on your biceps.
Standing alternate dumbbell curls are also a great way to build muscles in your biceps. What you do is hold one dumbbell on each hand at your sides with the palms facing each other. Then, curl one dumbbell at a time and at the same time rotate your palm forward as you lift. After you fully contract the biceps, lower the dumbbell with the arm rotating back to its original starting position. Do this with your other arm. That's basically it. You don’t need any machines or special benches to do these exercises.
For both exercises, you might want to do 10 repetitions with 3 sets.
The key here is to overload your biceps. After exercising your biceps, you have to let it rest for at least two days. You need to remember that when you lift weights, you are actually injuring your muscles in order for it to grow more muscles to support the weight you are constantly lifting. This means muscle gain.