When my work out begins, I normally begin strong. For example, I can lift up to 30 Lbs doing bicep curls, I can feel the burn, but it's not difficult at all to do the curls in good form; but as the workout progresses, I find myself having to lower my weights in each set. For example, by the second set of bicep curls, I have to lower the weights to 27 Lbs to maintain good form. By the third set I'm doing 25 Lbs, and so on.
It's not that I start feeling tired, low energy or anything, it's just that my muscles simply can't take it. What am I doing wrong? I was hoping I could maintain the same number of pounds throughout the entire workout. What do I need to change? Should I start with lower weights even though they'll feel too light?