You may be thinking that you should "keep your bottom arm straight" in order to provide some resistance against the pole ... in order to bend it.
This is NOT TRUE!
If you're trying to do the TUCK/SHOOT model, then yes ... straight bottom arm all the way to the tuck. Personally, I don't recommend you try to jump that way.
It's very difficult to do, and this technique is loaded with booby traps. Unless you have a really, really good coach that can steer you around these traps, stick with the Petrov Model.
With the Petrov Model, the technique is easier to learn, and easier to coach. Heck, your coach just needs to buy BTB2, and follow it page by page!
Well, OK, it's not QUITE that simple.
But what reference book does a tuck/shoot vaulter or coach have?
In the Petrov Model, you just have to stretch your bottom arm as high as you can, to get the best pole angle on takeoff. But once the pole hits the box (arguably just before or just after), you shouldn't hold your bottom arm stiff with the elbow straight.
Mike, it sounds like you already have sufficient strength to hold the weight of the pole up as high as you can reach. That's all that's needed!
Kirk