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When you swing up into pike position, do you keep your trail leg straight or bent? I've seen straight (bubka) and bent where both thighs rest on the chest and calves are at 90 degree angle (hartwig and many others)?

Swing with your leg straight. All of those guys do. What you are seeing is whether they allow their legs into their chest after they have initially swung. That is a pretty individual thing and one way doesn't generate any more force than the other.

Barto

There's only one world class guy that i can think of that doesn't actually swing his leg straight, and thats Hysong. He actually swings his leg bent the whole way but he's got an incredible take off which allows him to get away with it. However, many people are different in what they do at the top of their jump with their trail leg. I personally like having my leg straight the whole way. When a vaulter tucks their legs in up top it usually means they are behind the pole and getting in the bucket. If the leg swings straight up to the top the hips will already be high when its time to extend off the top of the pole. But its a lot easier said than done. The higher the grip the harder to swing. A soft pole and a good take off thats not under also make it hard to swing.

true you want swing thruogh with a straight leg, and if you can turn up keeping it straight then i definately would go for it. that looks like the smoothest to me(a tucker) but your naturally going to do one or the other i would stick with the one you do naturally. bending your leg in the tuck speeds up the rock back and helps get your hips back higher, a good swing comes from a good takeoff and good timing.