Will biceps grow more with straight barbell curls or EZ-bar curls?  Scientists used electromyography to gauge the response of the biceps brachii (the two headed muscle that's the main part of the biceps) and brachioradialis muscles during elbow flexion.  Using a 5 rep max, the results showed that optimum activation for the biceps brachii during the straight bar curls was far greater than with the curled bars during the concentric (bar raising) part of the movement, with no differences in the eccentric (bar lowering) phase.  The brachioradialis, however, showed no differences.  The bottom line for those seeking a greater fiber stress on your biceps, take the curl out of your curls, or better yet, alternate the movements from workout to workout.