Which bicep tendon exactly?

Which bicep tendon are we trying to propare in the GB program? The long head, short head, or the one at the elbow? Thanks in advance.

All of them really, but the major tension will always be on the one at the elbow. The farther down a lever you go after the fulcrum, the larger the forces are. The elbow is way past the fulcrum, and the shoulder IS the fulcrum. Because of that, even though there is identical force applied by muscle contraction on both the biceps tendons in the shoulders and the biceps tendon in the elbows, the physics of the levers involved drastically change the forces acting on the tendons in each location.