First, you have to make sure that you supinate your wrist fully when performing any biceps workout or exercises. The complete contraction offered by supinating your wrist allows you to generate a strength of muscle contraction that you are unable to do otherwise. With the long head of the biceps at a mechanically better advantage to do this, you will target this head even more by doing so.
Next, you want to be sure that you understand how your grip width will impact your biceps peaks. To start, you have to realize that the long head of the biceps is the one that is responsible for the height of your peak. This can be preferentially hit by keeping your grip narrow on barbell curls or any other biceps exercises. By narrowing your grip and holding your elbows close to the body you will be able to shift the focus to the outside of the biceps muscle more and therefore get more out of that long head.
Third thing you want to do when trying to build bigger biceps peaks is choose exercises that place the long head on stretch by positioning the arm behind the body. This happens very naturally with exercises like the incline dumbbell curl and the barbell drag curl. If you keep the arm behind the body at some point during the exercise, you will put the long head on stretch by virtue of the fact that it is the only head that crosses the shoulder joint.
Fourth, you can actually make the biceps muscle more prominent by training the muscle that lays beneath it; the brachialis. This muscle wraps around the humerus and is positioned underneath the two heads of the biceps. That said, it can be most visibly seen when looked at from behind as it situates between the triceps lateral head and the long head of the biceps. Build this muscle up and you will effectively push up the overlaying muscle group and get a fuller upper arm.
Finally, if you want to build big biceps peaks you have to try to minimize the contribution of the forearm to your biceps exercises. For instance, when you are doing barbell curls, the tendency is to cheat the weight up by allowing the wrist to curl as you lift the weight. This engages the muscles of the forearm and diminishes the work that the biceps are doing. Over time, this will lead to less strength in the biceps and less bicep peak and size overall.
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