Although swinging may be perfectly acceptable on the playground or in the 60’s, it has no place at the gym while curling. Yes, you may be feeding your ‘ego’ because you can ‘do’ 20 lbs more than you can actually do with proper posture, you are not doing your self any good. Not only are you putting your back at risk of injury, you are not truly using your biceps (the soul purpose of a curl). To build up those biceps like you want to, lower the weight to something you can do with proper posture; Legs shoulder width apart, keeping your back straight while you curl upwards and downwards with control. The key is to remain in control.