From el Rippo
This is because the degree of supination of the forearm and hand directly affects the amount of bicep in contraction, as discussed earlier. The EZ Curl bar does in fact take the stress of supination off of the wrists and elbows, but it does so at the expense of a quality bicep contraction. The camber of the bar is specifically intended to decrease the supination of the forearm, and anything less than full supination results in a less-than-complete biceps contraction.