One of the biggest problems in pole vaulting is over-emphasizing the bottom arm or even worse...locking it out. First reaching out with your bottom arm too much going into the plant can cause your step to be under. Your feet go to under your hands. If your hands go out front, your feet will do the same. If you reach out to put your plant up, you need to get your body back under your top hand before you hit your plant.
The other biggest problem with locking out or pressing too much with your bottom arm is it stops your shoulders from moving forward. That causes your hips to get out in front of you too soon. This causes your lower back to absorb all the force of the plant. This causes lower back pain and injury. Many vaulters that do this kick out with their bottom foot. This just emphasizes the pressure put into the lower back resulting in worse lower back injury.
Another problem of pressing too hard with the bottom arm is it causes an uneven bend in the pole. Look at your pole when this happens. If there is more bend near your top hand, you're pressing too hard. On the other hand...I've seen vaulters hold on or hang by their bottom arm. This keeps the top of the pole from bending. You can see the pole bend low in this case with the top of the pole staying straight.
Vitally Petrov said that his research and experience showed that trying to the top of the pole to horizontal too quickly causes the vaulter to stall out over the bar.
There is one way to check to see if a vaulter is pressing too much or not pressing enough with the bottom arm. The vaulter should come to full extension of the body with the trail foot pointed (or tapping) the box. The body should be at a 45-degree angle to the runway at this point. This is the transition between driving forward and trying to get inverted or rising.
See "How to Get Past Vertical"
http://www.advantageathletics.com/polevault/molnar.html
The vault on that page is 14' 10".
Look at Kira Costa's new Indoor Girl's High School Record
http://www.advantageathletics.com/polevault/costa.html
Look at Shayla Ballentine's High School outdoor record
http://www.advantageathletics.com/polev ... ntine.html
Look at beginner pole vaulter Albert Abbene's bottom arm
http://www.advantageathletics.com/polevault/abbene.html
He's a 10th grader vaulting on a 14' pole 30 lbs over his weight.
Still not convinced see
http://members.aol.com/actstunt/bubka.html
Bottom line...Stop focusing on your bottom arm.