Somewhere on the wave, you straightened up. You didn't decide to — it just happened, the way it happens to almost everyone once the immediate panic of the takeoff passes. Legs long, spine tall, arms settling by your sides. On land, that's just standing. On a surfboard, it's riding with the suspension removed.
A tall body is a rigid body
A wave face isn't smooth. It's textured with chop, steps, and the constant small accelerations of water doing water things. Something has to absorb all of that, and your legs are the only shock absorbers you brought.
Bent knees soak up the bumps the way a bent elbow catches a ball — softly, with room to give. Locked knees have no give at all, so every bump transmits straight up through your skeleton and arrives at your center of balance as a shove. That's why upright riding feels so strangely precarious even on friendly waves: you're receiving every jolt at full strength, and your only remaining correction tool is frantic arm circles.
The fix is specific, because there's a trap here. Told to "get lower," most people fold at the waist — back rounds, head drops, hips shoot back — which is just the poo stance with extra steps. The shape you actually want: knees bent to roughly 45 degrees (a bend, not a squat), plus a slight forward hinge from the hips so your chest rides over the board. Not bolt upright, not folded over. Think "chest proud, knees soft." From that shape your legs can pump for speed, absorb chop, and adjust trim — none of which is available to a locked-out body.
That hinge is visible and measurable from the beach: your trunk angle while riding. Bolt-upright shows up as a nearly vertical torso; CoachVision reads that angle from the sand, and a session's clips make the pattern obvious — most upright riders pop up into decent shape and then drift vertical two or three seconds in, as soon as they relax.
Make the low shape your landing gear
The reason you straighten up is simple: standing tall is your body's default, rehearsed millions of times since you were one year old. The bent-knee hinge has had, what, a few dozen rehearsals? The fix is volume — and the drill is Surf-Stance Burpees, 3 sets of 10.
It's a burpee with a different ending. Drop to the floor, chest all the way down, then explode up — but instead of finishing tall the way a gym burpee does, you finish in your surf stance: feet shoulder-width or a touch wider, back foot behind, knees bent about 45 degrees, hips hinged slightly, chest over your imaginary board, eyes on the horizon. Stick that landing for a full beat before the next rep. The stick is the rep.
Three sets of ten, and make the landing shape non-negotiable even when you're tired — especially when you're tired, because tired is exactly when your body reverts to tall. You're building two things at once: the explosive push of the pop-up, and a new default posture waiting at the top of it.
Give it a few weeks and the wave version arrives on its own: you'll pop up, relax — and notice you stayed low without being told. That's the rehearsals cashing in.