What is the best exercise to define the split between the long and short head of the biceps?

Work the shoulder... The long head of the biceps originates from inside the shoulder joint whereas the short head of the biceps arises form the corocoid process (front of chest wall) the long head of the biceps can be engaged by performing modified shoulder flies (elbow slightly bent, start with your hands by your side and elevate them forward at a 45 angle to the sagital plane until area are parrallel to ground).
Aerobics. That's a trick question. You are talking about muscle definition, not muscle size or bulk. To really increase muscle definition, you need to perform aerobic exercise to thin out the layer of fat that overlies the muscle. The less fat that is hiding the muscle, the more the muscles will show.

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Please help! What is the best exercise to define the split between the long and short head of the biceps?

Loss of body fat. In general the best way to enhance muscle definition is reducing body fat percentage. Seeing definition between the heads of the biceps probably is not possible without a body fat percentage under 12%, and probably closer to 10%. I hasten to add that for health purposes one does not have to have body fat percentages this low, but i understand the drive to aesthetics. Read more...
Aerobics. That's a trick question. You are talking about muscle definition, not muscle size or bulk. To really increase muscle definition, you need to perform aerobic exercise to thin out the layer of fat that overlies the muscle. The less fat that is hiding the muscle, the more the muscles will show. Read more...