In case you missed John Quint's explanation of how to decrease shoulder pain and increase bench press performance, read Optimize Biomechanics for a Bigger Bench.

In the following video, John breaks down the anatomy of the triceps, and demonstrates with a dumbbell how to emphasize the long head in training—the one head that is particularly more difficult to train effectively.

When performing the DB overhead extension, I focus on pressing/extending my elbow towards the mid line of my body which emphasizes the long head of the triceps. 

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