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Why does my high jumper's takeoff leg stay bent instead of straightening at the top of the jump?

The takeoff leg is meant to snap straight in a fraction of a second, and whatever extension goes missing there, height goes missing with it.

The takeoff leg is supposed to straighten hard into the ground and drive the jumper up. When it stays bent through that instant instead, the jumper still gets over the bar sometimes, just not as high as the approach and speed suggest they should.

A bent takeoff leg is a power leak, not a style issue

A high jump takeoff is one short, violent extension: the leg goes from bent to straight in a fraction of a second, and however much of that extension is missing, that is roughly how much height goes missing with it. A jumper can have a fast approach, a good curve, and a correct plant position, and still lose height right here if the leg does not finish opening.

What is usually behind it

• The plant foot lands too far out in front of the body, which leaves no time to extend the leg before the jump is already committed.
• The jumper is bracing rather than driving, landing stiff and absorbing the speed instead of converting it, often from being told to plant hard without being told what happens after.
• Simple strength: the leg cannot yet produce enough force fast enough to fully extend under that much speed, which is a training issue rather than a cue issue.

The first two are technical and coachable with drills. The third needs time in the weight room, not more reps at the bar.

What the high jump setting actually shows

The Film Room reports hip rise as a share of the athlete's height, takeoff angle, and knee fold and extension through the jump. A jumper with a good takeoff angle but a knee fold number that shows the leg never getting close to straight is losing height at the exact moment the approach and plant already did their job correctly, which points the coaching conversation at the leg itself rather than at the run-up.

A single side-on camera can see how far the knee opens. It cannot measure how much force the leg actually produced doing it, so it cannot tell you on its own whether a jumper who stays bent is choosing to brace or is simply not strong enough yet to finish the extension. That distinction still needs a coach's eye and, ideally, time under a bar in the weight room.

Related

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