Roughly 80% of throwing velocity comes from rotational mechanics below the shoulder. When the body sequences correctly, the arm comes along safely. Below is every checkpoint our engine measures on a delivery, the reference window for each, the fault that shows up most often, and one drill that fixes it.
A delivery is a relay. The pelvis reaches top speed first, hands off to the trunk, which hands off to the elbow, which hands off to the hand. Each segment peaks just after the one before it. That order, called proximal to distal, is the single most predictive thing in a delivery and the easiest to see once you know where to look.
| Segment | Peaks | What it means |
|---|---|---|
| Pelvis | 1st, around foot strike | The hips start the delivery. Nothing above them can add speed they did not create. |
| Trunk | 2nd | The chest rotates only after the hips have opened, releasing stored separation. |
| Elbow | 3rd | Elbow extension ramps sharply just before release. |
| Hand | 4th, last | The hand is fastest at the end. If it peaks early, the arm is doing the work. |
Why the order matters so much: a 20% decrease in kinetic energy coming from the hips and trunk requires a 34% increase in shoulder rotational velocity to produce the same result (Kibler and Chandler). The arm cannot create energy, it can only pass along or replace what the lower half failed to deliver, and replacing it is where stress accumulates.
The fault to look for is simultaneous peaking. When the pelvis and trunk hit top speed together, the delivery is rushed, separation never loads, and the hand has to accelerate from nothing.
These are the areas our pitching framework scores, in weight order. Every window below is a reference range, not a template: interpret it against the athlete in front of you, because a limit at one joint is often mobility rather than technique.
| Checkpoint | What good looks like | Most common fault |
|---|---|---|
| Sequencing weight 20 |
Pelvis peaks, then trunk, then elbow, then hand, with clear separation between peaks. | Trunk fires with or before the hips, so the arm becomes the engine. |
| Hip to shoulder separation weight 20 |
40 to 60 degrees in 3D, peaking during the drive phase. Our 2D read shows roughly 60% to 75% of that, so 25 to 40 degrees on video. | Peak separation arriving at or after release, which means the elastic window was missed entirely. |
| Lead leg block and stride weight 15 |
Front leg lands flexed near 135 degrees and extends past 150 degrees by release. Stride near 85% to 100% of standing height. | Knee flexion increasing from foot strike to release, absorbing the stride instead of bracing against it. |
| Trunk and posture weight 15 |
Quiet head, forward flex over the front leg, lateral tilt of 15 to 25 degrees at release. | Staying upright, which shortens the acceleration path. Past 35 degrees of tilt the cost flips to arm stress. |
| Arm action and timing weight 15 |
Elbow near 90 degrees at shoulder height when the front foot lands, arms roughly mirroring each other, hand speed peaking last. | Hand speed ramping before the hips rotate, the signature of flying open. |
| Deceleration and finish weight 15 |
Speeds decay smoothly and settle within roughly 0.3 to 0.5 seconds, chest continuing over a braced front leg. | Recoil: the body snapping back upright after release instead of finishing forward. |
The most measurable of the six is the front leg. Each additional degree of lead knee extension between foot contact and release is worth roughly 0.47 m/s, about 1 mph, of throwing velocity (Dowling et al. 2024), so a front knee that holds its brace is worth real velocity on its own.
Two timing references tie it together. A full delivery runs about 0.6 to 1.2 seconds from first movement to release, and foot contact inside roughly 1.05 seconds reads as efficient. Well past 1.15 seconds reads as slow and segmented. The drive phase should show sharp acceleration: a gradual ramp usually means the pitcher is muscling the ball rather than using stored energy.
Each of these makes the correct pattern the only way to finish the rep, which is why they transfer faster than a verbal reminder. Work at 60% to 75% effort until the feel is repeatable.
Stand in the stride position a foot from a wall on your glove side. Drive the lead hip toward the wall first and let the trunk and arm stay back, then rotate through.
Cue: lead with your belt buckle, not your shoulder.
Pause the delivery exactly at foot strike and freeze. The hips should face the target while the chest still faces the dugout. Hold two seconds, then finish the throw.
Cue: hips to the target, logo to the dugout.
Throw at 75%, then freeze the finish for three full seconds with the front leg long and strong. If the knee keeps drifting forward after release, the block is leaking energy.
Cue: land soft, finish long, stick it like a gymnast.
Set up with both feet on a foul line and deliver at 75%, landing the front foot on the line. A partner stands behind and calls out any time the cap drifts off the line.
Cue: keep your cap on the railroad track.
Shadow the delivery facing a mirror and freeze at foot strike. Check the throwing elbow bent near 90 degrees at shoulder height with the glove arm mirroring it, then repeat with eyes closed.
Cue: make a goalpost when the foot lands.
Throw at 75% and immediately walk two steps toward your partner. If you cannot walk forward naturally, the finish stopped short or recoiled.
Cue: let the throw pull you down the hill.
Teach the pattern early and the intensity late. A 10 year old can learn sequencing, a quiet head, and a firm front leg at low effort years before chasing velocity is appropriate. The order of operations matters more at that age than any single angle.
The trap is the template. Growing athletes land in different parts of every reference window because of limb proportions and mobility, so a joint that will not reach a range is often a mobility limit rather than a technique error. Score the quality of the pattern, not conformity to one number, and re film every few months because the athlete you measured is not the athlete in front of you.
Everything above is measurable from a single camera, with one honest caveat: a single camera produces a 2D approximation, and any system worth trusting labels it that way. Our engine attaches the basis landmarks and a confidence value to every number, and omits anything it cannot measure confidently rather than estimating it.
Three things a full body clip genuinely cannot give you:
One practical filming note that changes results more than anything else: shoot at 120 frames per second or in slow motion mode against a clean background. At 60 frames per second a fast release blurs badly enough that the engine cannot reliably pin the release moment.
Send one bullpen clip and we will return the full breakdown, so your staff judges the engine on your players rather than a demo reel.
Request a breakdownWhere velocity actually comes from, and the four mechanical leaks that cost the most.
The same chain applied to position player throws: crow hop, transfer, and arm action.
The hitting side: load, sequence, contact, balance, and timing.