---
title: The Moment of the Swing Is Never Captured
url: https://oosioo.com/en/p/%ED%9C%98%EB%91%90%EB%A5%B4%EB%8A%94-%EC%88%9C%EA%B0%84%EC%9D%80-%EC%B0%8D%ED%9E%88%EC%A7%80-%EC%95%8A%EB%8A%94%EB%8B%A4
date: 2026-10-05T13:46:34+00:00
author: SYSOP
summary: The word in the notebook was "to swing." From Muybridge, who photographed a tennis swing with dozens of cameras 140 years ago, to radar built to chase missiles, sensors on the end of a bat, 300-frame-per-second cameras in every ballpark and swing analysis with a single phone, this piece follows the machines that measure a swing. It then sketches a blueprint for a Swing Mirror built from nothing but a webcam and a browser: which libraries and algorithms it would use, and what it would have to give up. Finally, it looks at where the technology can go beyond the stadium. The seventh piece in the A Page for Today series.
---
# The Moment of the Swing Is Never Captured

![The moment of the swing is never captured. One swing, cut into 30 frames per second. The backswing is densely sampled, but the moment the ball is struck vanishes into the gap between two frames](/uploads/273122449def3b85.webp =720)

I opened my idea notebook today and found a single word written in it.

> To swing

In Korean the word is *hwidureuda*, and the dictionary gives it several shades. To grip something and wave it in a wide arc. To push someone around until they can't think straight. To wield power or talent carelessly. You swing a bat, you jerk people around, you throw your weight around, and in Korean it is all the same verb. Today I start from the first meaning. I suspect the others will be waiting at the end.

Come to think of it, a surprising number of sports are about swinging. Baseball swings a bat, golf a club, tennis, badminton and table tennis a racket, cricket a flat bat, hockey a stick. Most of them are ball games. Nearly every culture has a game or two in which you hit a ball with a stick held in your hands. And everyone who plays one shares the same frustration: I can't see what's wrong with my own swing.

Today I follow the machines people have built to solve that problem. At the end, I sketch how I would design a tool that holds up a mirror to your swing using nothing but a laptop webcam.

## Swing analysis, 140 years ago

The first person to pin down a swing with a machine was a photographer. In 1887, with the backing of the University of Pennsylvania, Eadweard Muybridge published *Animal Locomotion*: 781 plates of sequential photographs of people and animals walking, running, throwing and swinging. He lined up 24 cameras alongside his subject and set 12 more at each end, so the same movement was photographed from several directions at once. White strings hung in a grid on the back wall let him measure how far the body had moved.

![Muybridge's 1887 sequential photographs. A woman in a long skirt strikes a tennis ball, captured in three rows of eight frames. The grid on the back wall serves as a measuring scale for the movement](/uploads/189a0aa289aab790.jpg =720)

*Eadweard Muybridge, Animal Locomotion, Plate 297, "Lawn tennis" (1887). Public domain. [Wikimedia Commons](https://commons.wikimedia.org/wiki/File:Woman_in_long_dress_playing_lawn_tennis_%28rbm-QP301M8-1887-297%29.jpg)*

Almost everything in today's story is already on this plate. Multiple cameras, the same instant seen from several angles, a measuring scale in the background, and the gaps between frames. Between one frame and the next lies time that was never photographed, and the faster the racket moves, the wider that gap seems to be. The twelve cameras standing in a Major League ballpark 140 years later do much the same job. Only the speed is different.

## Swinging like a whip

Before getting to the machines, it's worth pinning down what a swing actually is. Slow down the swing of someone who hits the ball a long way and you'll see the arms don't lead. The pelvis turns first; as it reaches its peak, the torso follows, then the arms, and finally the club or bat whips through. The biomechanist C. A. Putnam laid this out in a 1993 paper in the *Journal of Biomechanics*. When the segments near the body's center accelerate first, the outer segments lag behind and are then flung forward even faster by the forces passing between the segments. It is the same principle as cracking a whip: one big flick of the handle and the tip snaps.

![The kinematic sequence and the 3:1 tempo. Peak rotation speed arrives later and higher in order: pelvis, torso, arms, club. Ben Hogan used 21 frames for his backswing and 7 for his downswing](/uploads/dba8ce2c3ae0b193.webp =720)

In golf this ordering is called the kinematic sequence. Angular velocity peaks in the pelvis, then the thorax, the arms and the club, and each peak is higher than the one before. In 1992 the golf instructor Jim McLean observed that the longest hitters had their shoulders turned much farther than their hips at the top of the backswing, and named the difference the "X-factor." A 2001 follow-up study revised the idea: what matters is less the amount of coil itself than how much that coil widens again as the downswing begins. Skilled players stretched it by 19 percent, amateurs by 13.

Then there is tempo. In 2004 the authors of *Tour Tempo* counted the frames in video of tour professionals' swings at 30 frames per second. Ben Hogan used 21 frames for his backswing and 7 for his downswing. Three to one. Other pros, fast or slow, mostly clustered around the same 3:1 ratio. A swing has an order and a rhythm. So it can be measured.

## Three ways to watch a swing

The machines that measure a swing fall into three broad families: radar that watches the ball and club head in flight, inertial sensors attached directly to the tool or the body, and cameras that watch the whole body. What each one looks at decides what it measures well and what it misses.

![Three ways to watch a swing. Radar watches the ball and head, inertial sensors the tool and wrist, cameras the whole body. Each has its blind spots](/uploads/609eb474bb5862b1.webp =720)

### The radar that used to chase missiles

The most dramatic story comes from Denmark. In 2003 the golfing brothers Klaus and Morten Eldrup-Jørgensen founded TrackMan with the radar engineer Fredrik Tuxen. Tuxen had worked on tracking shells and missiles with Doppler radar, a technique that measures an object's speed by how much the frequency of a radio wave shifts when it bounces back. They pointed that radar at a golf ball. In a 2019 interview Klaus said the military technology was now being put to peaceful use. TrackMan joined the PGA Tour in 2006, moved into baseball in 2008, and by 2015 was installed in all 30 Major League ballparks. A technology built to chase missiles became one that chases balls.

![An indoor golf studio at a school in England. Behind the player in mid-swing, an orange TrackMan radar unit sits on the floor; two video tripods stand against the wall](/uploads/0b9b940b8d072b5b.jpg =720)

*TrackMan (the orange box on the floor) in the indoor golf studio at Culford School. Photo by Tommyraiks2, CC BY-SA 4.0, cropped. [Wikimedia Commons](https://commons.wikimedia.org/wiki/File:Max_Adams_Trackman_edited.jpg)*

Devices combining radar and cameras have since grown smaller and cheaper. In 2021 Garmin released the palm-sized Approach R10 portable radar launch monitor at $599.99, and in 2024 the R50, with three cameras and a built-in screen. Singapore's Rapsodo sells a unit combining two high-speed cameras and Doppler radar for around $600. The screen golf so familiar in Korea belongs to the same lineage: sensors in the ceiling and floor measure the ball and club head and use those values to launch the ball on screen.

![A dark indoor golf simulator bay. A player addresses the ball facing a fairway projected on the screen, with a sensor unit mounted on the ceiling](/uploads/505b6cc75e128198.jpg =720)

*A golf simulator in Oxford, Mississippi. Photo by Kjaskren, CC0. [Wikimedia Commons](https://commons.wikimedia.org/wiki/File:Golf_simulator.jpg)*

### Sensors on the handle

If radar watches the ball, sensors ride on the swinging tool itself. Attach an inertial measurement unit (IMU) the size of a thumbnail to the knob of a bat or the grip of a racket, and its accelerometer and gyroscope record the motion hundreds of times a second. The accelerometer measures how hard it was pushed, the gyroscope how fast it turned.

![A nine-axis inertial sensor board: accelerometer, gyroscope and magnetometer on a single chip, mounted on a red circuit board](/uploads/da272731c063b398.jpg =480)

*Nine-axis inertial sensor board (acceleration, gyro, magnetometer). Photo by SparkFun, CC BY 2.0. [Wikimedia Commons](https://commons.wikimedia.org/wiki/File:SparkFun_9DoF-Sensor-Stick_Acc%2BGyro%2BMag_13944-01.jpg)*

The thing that put this sensor into the most hands was a game console. The remote for Nintendo's Wii, launched in 2006, contained an accelerometer, and in the bundled *Wii Sports* people swung tennis rackets, golf clubs and baseball bats in their living rooms. In 2009 the Wii MotionPlus added a gyroscope and could read the twist of a wrist as well. By Nintendo's count, *Wii Sports* sold 82.9 million copies. There were even plastic rackets sold separately to slot the remote into.

![A toy tennis racket at a games trade show. The handle is hollow so a Wii Remote can be slotted inside](/uploads/ee168162167b42c9.jpg =600)

*Tennis racket accessory for the Wii Remote, gamescom 2009. Photo by D-Kuru, CC BY-SA 3.0 AT, cropped. [Wikimedia Commons](https://commons.wikimedia.org/wiki/File:Tennis_racket_for_Wii_Remote_at_gamescom_2009_PNr%C2%B00195.JPG)*

The same sensors soon moved onto real bats and clubs. In baseball, Blast Motion of the United States became MLB's official bat sensor in 2016. William "Buddy" Clark, an engineering professor at the University of Pittsburgh, was coaching his children's baseball teams and got so many questions about training gear from other parents that he started Diamond Kinetics; in 2022 its bat sensor was named an MLB youth development platform. Garmin released the Impact, which clips onto the bat knob, in 2017. Garmin golf watches also measure the 3:1 tempo described above, and the manual gives a 0.7-second backswing and 0.23-second downswing as an ideal example. Those are exactly Hogan's 21 and 7 frames converted to seconds.

Latvia's HackMotion measures only the wrist. Its founder originally set out to build a sensor suit for snowboarders, then changed course after interviewing about 100 golf coaches. A sensor on the back of the hand measures one thing: how much the wrist is bent at impact. Without outside investment, the company has sold 70,000 units in more than 70 countries. Choose the one thing worth measuring, and the equipment gets simple.

The market has not always been kind. In 2013 Babolat unveiled the Play Pure Drive, a tennis racket with a sensor built into the handle, predicting that before long every tennis racket would be connected. That racket is no longer sold. Sony also released an 8-gram tennis sensor that attaches to the end of a racket grip in 2014. Zepp, an American company that made sensors for golf, baseball, tennis and badminton, sold its assets in 2018 to Huami, the maker of Xiaomi's fitness bands, and in 2021 Huami renamed itself Zepp Health. The name of a company that measured swings lives on as the name of a smartwatch company.

| Product | Company, launch | Method | Measures | Status |
|---|---|---|---|---|
| TrackMan | Denmark, founded 2003 | Doppler radar | Club and ball speed, launch angle, spin | Became standard on tour and in MLB |
| Approach R10 | Garmin, 2021 | Portable radar | Head and ball speed, tempo and about 10 more | On sale, launched at $599.99 |
| Approach R50 | Garmin, 2024 | 3 cameras + screen | 15+ ball and club metrics | On sale, from $4,999.99 |
| MLM2PRO | Rapsodo (Singapore) | 2 cameras + radar | Ball and club metrics | On sale, $599.99 |
| Wii Remote | Nintendo, 2006 | Accelerometer (gyro added 2009) | Swing direction and force | Wii Sports sold 82.9 million |
| Blast Motion | US, MLB official 2016 | IMU on bat knob | Swing speed, efficiency | On sale |
| SwingTracker | Diamond Kinetics (Pittsburgh) | IMU on bat | Swing speed, power | MLB youth platform 2022 |
| Impact | Garmin, 2017 | IMU on bat knob + display | Bat and hand speed, time to impact, attack angle | Launched at $149.99 |
| HackMotion | Latvia | Wrist IMU | Wrist flexion and deviation | 70,000 units in 70+ countries |
| Play Pure Drive | Babolat, 2013 | Sensor built into racket | Power, impact location, spin | Discontinued |
| Smart Tennis Sensor | Sony, 2014 | 8 g sensor on grip end | Impact location, swing and ball speed | Discontinued |
| Zepp | US, 2010 | Clip-on IMU | Golf, baseball, tennis, badminton | Assets acquired by Huami, 2018 |

### A bat filmed at 300 frames per second

Measuring with cameras alone, with no sensors attached, was perfected first in the stadium. It started with tennis. Hawk-Eye, created in 2001 by the British engineer Paul Hawkins, reconstructs the ball's path in three dimensions using several cameras around the court. It made its first official line calls at the Miami tournament in March 2006, and the same year the US Open let players challenge calls with it. Sony bought the company in 2011.

![A Hawk-Eye camera mounted on the stands' railing at the Kremlin Cup tennis tournament in Moscow, with the court and a player visible below](/uploads/955053e1575858f3.jpg =600)

*A Hawk-Eye camera at the 2012 Kremlin Cup. Photo by JukoFF, CC BY-SA 3.0, cropped. [Wikimedia Commons](https://commons.wikimedia.org/wiki/File:Kremlin_Cup_2012_-_Hawk-Eye_cam.JPG)*

Since 2020, every Major League ballpark has had 12 Hawk-Eye cameras, five of which shoot 300 frames per second. In May 2024 bat-tracking data from these cameras was released for the first time. Bat speed was defined as the speed of the sweet spot, about six inches (15 cm) below the end of the barrel. The distance the bat travels from about 0.15 seconds before contact is called "swing length," and the league average is about 2.2 meters.

![A Yankees hitter lets go of the bat with one hand and looks toward first base the moment after hitting a home run, with the stands behind him](/uploads/3f236fdba4c2ab1f.jpg =720)

*Matt Holliday of the Yankees hits his 300th career home run. Statcast projected it at 446 feet (about 136 m). Photo by Arturo Pardavila III, CC BY 2.0. [Wikimedia Commons](https://commons.wikimedia.org/wiki/File:Matt_Holliday_hit_career_HR_No._300_a_Statcast-projected_446_feet.jpg)*

The numbers from the day of release are intriguing.

| | Average bat speed | Notes |
|---|---|---|
| League average | 71.5 mph (about 115 km/h) | Above 75 mph counts as a "fast swing" (about 22% of all swings) |
| Giancarlo Stanton | 80.6 mph (about 130 km/h) | First in the league; 98% of his swings were fast swings |
| Luis Arraez | 62.4 mph (about 100 km/h) | Last in the league, yet first in squared-up rate at 43.9% |

The hitter with the slowest swing squared up the ball most often. Swinging hard and swinging well are different things. Looking at those numbers reminded me again that there is a gap between what the machine measures and what we actually want to know.

The same year, Sony also bought KinaTrax, which mounts several high-speed cameras in the stands to capture players' joint movements without markers. It was the company that first collected in-game data, at the Tampa Bay Rays' ballpark in 2015. A single game produces roughly 1.4 TB of data, which belongs to the team and is reportedly handled like medical information. Every swing becomes part of a health record. Sony, having given up its racket sensor, ended up owning two camera companies.

## With a single phone

It didn't take long for twelve stadium cameras to shrink to a single phone. The credit goes to pose estimation, the technology that finds the positions of a person's joints in an image. In 2017 Carnegie Mellon's OpenPose opened the way by detecting the joints of multiple people in real time, and in 2020 Google released BlazePose, which finds 33 points on the body at more than 30 frames per second on a phone. Today it is available to anyone in a web browser under the name MediaPipe.

![A photo of a person sitting in a yoga pose, overlaid with points and lines connecting the shoulders, elbows, wrists, hips, knees and ankles](/uploads/1d6f5e4591e34bc5.jpg =600)

*Example output from the MediaPipe Pose Landmarker. Image by Google, CC BY 4.0. [Wikimedia Commons](https://commons.wikimedia.org/wiki/File:MediaPipe_pose_landmarker_yoga_example.png)*

A line of services has been built on top of it.

| Service | Sport | Cameras | What it does |
|---|---|---|---|
| Sportsbox AI | Golf | 1 phone | Turns 2D video into a 3D avatar viewed from 6 angles; measures turn, bend and sway. Founded during the 2020 COVID lockdown |
| SwingVision | Tennis | 1 iPhone or iPad | Automatic scoring, shot speed, line calls. 97% accuracy on balls within 10 cm of the line (human eye: 90%) |
| Uplift Labs | Baseball, golf and more | 2 iOS devices of the same model | 3D model of 30+ joints and segments |
| Mustard | Baseball pitching | 1 phone | Pitching mechanics analysis and corrective drills |
| KinaTrax | Baseball | Several high-speed stadium cameras | In-game 3D joint tracking; acquired by Sony in 2024 |
| Golfzon AI Coach | Golf | Simulator front and side + phone | Diagnoses about 50 postures from address to finish, 2021 |
| GolfFix | Golf | 1 phone | Diagnoses 45+ swing faults on-device in 5 seconds, 2021 |
| OpenCap | Research | 2+ phones | Mean joint-angle error of 4.5°, about 1/215 the cost of lab equipment |

Sportsbox AI, founded by the former golf journalist Stephanie Wei and the former LPGA player Jeehae Lee, says in its published validation that its angles differed from sensor-based motion capture by a little over 2 degrees on average. SwingVision began in 2016 as an Apple Watch app. In Korea, screen golf caught this wave early. In 2021 Golfzon launched an AI coach that diagnoses posture using simulator and phone cameras, and the same year GolfFix, built by graduates of Seoul National University's computer science program, hit No. 1 in popularity on Google Play.

The change in the lab is even bigger. OpenCap, published by Stanford researchers in 2023, measures a person's 3D movement with two phones. Compared with lab equipment that requires markers stuck to the body, its mean joint-angle error was 4.5 degrees. Equipment costing more than $150,000 shrank to roughly 1/215 of the price, and measuring one person went from about two hours to five minutes.

## The limit of 30 frames per second

So could you build a swing analyzer with just a laptop webcam and a browser? The ingredients are all public. The problem is speed.

A webcam typically shoots 30 frames per second. Take an amateur male's driver head speed as roughly 45 meters per second, and the club head travels 1.5 meters between one frame and the next. At the tour average of 113 mph, or 50.5 meters per second, it's 1.7 meters. That's longer than the driver itself. The moment of impact almost always falls into the gap between two frames. Even while the shutter is open for a sixtieth of a second, the head covers 75 cm, so anything that is captured is just a long smear.

![How many frames per second determines where the club head gets captured around impact. 30 fps leaves 1.5 m between frames, 60 fps 75 cm, 240 fps phone slow-motion about 19 cm, 300 fps Hawk-Eye 15 cm, 1,000 fps 4.5 cm](/uploads/fe95e9b55be4cf00.webp =720)

| Frame rate | Head travel between frames (45 m/s) | What you can trust |
|---|---|---|
| Webcam 30 fps | 1.5 m | Torso, pelvis and shoulder rotation; tempo |
| 60 fps | 75 cm | The above, plus the path of the hands |
| Phone slow-motion 240 fps | about 19 cm | A rough shaft path |
| MLB Hawk-Eye 300 fps | 15 cm | Bat speed, swing length |
| High-speed camera 1,000 fps | 4.5 cm | Head and ball at the moment of impact |

Television cameras run into the same limit. Japan's Dentsu, working with Rhizomatiks, built a system that draws the path of a fencing blade's tip on screen precisely because 30-frame-per-second broadcast cameras kept losing the tip.

So an analyzer you can build with a webcam has to measure the body, not the club. The club head slips away, but the pelvis and shoulders turn far more slowly. The kinematic sequence, the X-factor and the 3:1 tempo can all be computed from the movement of the torso and hands alone. The swing itself is never captured, but the body before and after it is captured clearly. The machine looks at what came before and after and infers what happened in the gap. A coach's eye has always worked the same way.

## The webcam Swing Mirror: a blueprint

This time I won't build the tool; I'll only design it. I've named it the Swing Mirror. It isn't a coach that hands out correct answers but a mirror that reflects how your body moved. I set three principles.

1. It runs in a single browser. No installation, no server.
2. Video and coordinates never leave your computer.
3. It doesn't measure anything 30 frames per second can't be trusted with.

### Which library?

There are several open-source libraries that find human joints in a browser.

| Library | Released | Points | Runs on | Notes | In the Swing Mirror |
|---|---|---|---|---|---|
| MediaPipe Pose Landmarker (BlazePose) | Google, 2020 | 33 | Browser (WASM, GPU), phones | Returns both image coordinates and 3D coordinates in meters. Lite, full and heavy variants | **Primary** |
| MoveNet | Google TF.js, 2021 | 17 | Browser (WebGL) | Fast: Lightning 104 fps, Thunder 77 fps on a MacBook | Alternative for slower devices |
| OpenPose | Carnegie Mellon, CVPR 2017 | 18–25 | Server GPU | Multiple people at once; where pose estimation went mainstream | Reference |
| RTMPose (MMPose) | OpenMMLab, 2023 | 17–133 | Server, some mobile | High accuracy, including hands and face | If a server version is built |
| VideoPose3D | Facebook, CVPR 2019 | Lifts 2D to 3D | Server | Lifts 2D keypoint trajectories into 3D | When more 3D is needed |

I chose MediaPipe as the primary library. It runs directly in the browser and, above all, it returns 3D world coordinates. Even with a single front-facing camera, it can roughly estimate how far the shoulders have rotated toward or away from the lens.

### Seven stages

![The webcam Swing Mirror blueprint. Seven stages — capture, find joints, clean up, angles, segment, metrics, display — all run inside the browser](/uploads/375444c572a38c50.webp =720)

1. **Capture.** Receive webcam video through the browser's getUserMedia. Filming from the front shows pelvis and shoulder rotation well; filming from the side shows spine tilt. The tool also accepts 240 fps slow-motion clips shot on a phone.
2. **Find joints.** Run the MediaPipe Pose Landmarker in VIDEO mode. Each frame yields coordinates for 33 points.
3. **Clean up.** Discard points whose visibility falls below 0.5 because they're hidden, and filter out jitter with a One Euro filter. This filter smooths heavily when movement is slow and lightly when it's fast, which suits a motion like a swing that is slow and then suddenly fast. Divide by shoulder width to normalize body size.
4. **Angles.** Compute angles from a handful of points. More on this below.
5. **Segment.** Split the swing into address, top, impact and finish using wrist height alone. Start with simple rules; later, swap in a model trained on GolfDB, a public dataset of 1,400 golf swing videos annotated with eight events.
6. **Metrics.** Each swing becomes one row in a table: tempo, maximum X-factor, the order of rotation peaks, and how far the head slid sideways.
7. **Display.** Draw the skeleton over the video, and show metric cards and a timeline. History is stored only in the browser's IndexedDB, so you can overlay a swing on your best one.

### From 33 points to angles and rhythm

Of the 33 points, only about ten are actually used: both shoulders (11 and 12), both wrists (15 and 16), both hips (23 and 24), the nose (0) and both ankles (27 and 28).

![From 33 points to angles and rhythm. Pick the shoulder, wrist and hip points; measure the direction of the shoulder and hip lines in the plane seen from above to get the X-factor; find address, top and impact from the wrist-height curve](/uploads/d05f6b695c004441.webp =720)

Rotation is measured in the plane seen from directly above. From MediaPipe's 3D coordinates, a single function, atan2, gives how far the line joining the right and left shoulders has turned compared with address. The hip line is found the same way, and the difference between the two is the X-factor. If the shoulders have turned 90 degrees at the top and the hips 45, the X-factor is 45 degrees.

Tempo comes from the wrist-height curve. The frame where the wrists start moving after being still for about 0.3 seconds marks the end of address (A), the frame where the wrists are highest is the top (T), and the frame where they come back down to address height is impact (I). Tempo is (T−A) divided by (I−T). Twenty-one frames to seven gives 3.0. At 30 fps, I can be off by one frame either way, about 33 milliseconds. With a seven-frame downswing, one frame of error swings the tempo value anywhere from 2.6 to 3.5. So the Swing Mirror shows tempo to only one decimal place and displays the average of several swings alongside it.

| Metric | Calculation | Trustworthy at 30 fps? |
|---|---|---|
| Shoulder and hip rotation | Direction of each line in the x–z plane of the 3D coordinates (atan2) | Yes. Movement near the top is slow |
| X-factor | Shoulder rotation − hip rotation, maximum at the top | Yes |
| X-factor stretch | Increase in X-factor over the first few frames of the downswing | Roughly. It happens within 2–3 frames |
| Tempo | (top − start) ÷ (impact − top) | Usable when averaged |
| Order of rotation peaks | Order of the frames where pelvis, shoulder and wrist angular velocity peak | The order only; exact timing is hard |
| Sway | Side-to-side shift of the nose and hip center from address (cm) | Yes |
| Club head speed, moment of impact | — | Not measured |

### The prototype screen

Here is what that design looks like on screen. On the left, the skeleton and the wrist path are drawn over the webcam video, and a vertical dotted line marks where the head was at address. On the right are cards for tempo, X-factor and sway, the order of rotation peaks, a timeline of this swing, and recent history. Below the video, the same sentence is always displayed: Video never leaves this computer.

![The Swing Mirror prototype screen. On the left, a skeleton and wrist path over the webcam video, with shoulders at 88 degrees and hips at 47 degrees. On the right, cards for a 3.1:1 tempo, a 41-degree maximum X-factor and 6 cm of sway, the order of rotation peaks, a timeline and a table of recent swings](/uploads/fdec951b65342edc.webp =720)

I'll also state plainly what this tool can't do. It doesn't know where the ball went, how fast the head was moving, or whether the face was open at impact. If you need those values, put a radar device's numbers alongside it. The Swing Mirror reflects only the body.

## Swinging beyond the stadium

The technology for turning body movement into numbers started with the swing, but there's no reason for it to stay there. Of the Swing Mirror's seven stages, usually only stages 4 to 6 need to change: which angles to watch and how to divide the movement into segments.

| Field | Examples | What is measured |
|---|---|---|
| Remote rehabilitation | Kemtai (by its own description, tracks 111 points on the body via webcam or phone), Exer Health, Sword Health | Range of motion, repetitions, incorrect form |
| Research and clinical | OpenCap (Stanford, 2023) | Gait and joint angles with two phones, measured outside the hospital |
| Workplace safety | Research on automating RULA and REBA assessments | Musculoskeletal load scores for lifting and twisting postures |
| Home fitness | Peloton Guide (launched 2022, discontinued 2025), Tempo Studio (depth sensor) | Recognizing exercises, counting reps |
| Sign language | Google's American Sign Language competitions (two in 2023, $300,000 in total prizes) | Recognizing signs and fingerspelling from MediaPipe hand and body coordinates alone |
| Gymnastics judging | Fujitsu's Judging Support System | All 10 men's and women's apparatus since the 2023 World Championships |
| Fencing | Dentsu and Rhizomatiks' "Fencing Visualized" | Draws the blade tip's path on screen; used in live competition in 2026 |
| Music | VioPose (WACV 2025) | A violinist's 3D posture estimated from sound and video |
| Animals | DeepLabCut (*Nature Neuroscience*, 2018) | Label around 200 images and it tracks a mouse's paws or a fly's legs |

The closest application is rehabilitation. When a patient exercises at home after knee surgery, no physical therapist is standing beside them. Israel's Kemtai follows the patient's movements through a webcam or phone camera, counting angles and reps and pointing out incorrect form. The researchers behind OpenCap were aiming at the same thing: measuring the movement of patients who can't get to an expensive lab, outside the hospital.

In factories and warehouses, the same technology protects workers' backs. RULA and REBA are assessment methods that score the musculoskeletal load of a worker's posture, and until now an expert has filled in the checklist by watching on site. A number of studies already have cameras extract joint angles and score these automatically. Lifting a box is, in the end, also a motion that transfers force from the body's center outward. The eye that corrected swings becomes an eye that spots the postures that hurt backs.

There is a lesson in the other direction too. Peloton released the Peloton Guide, which recognized exercise movements with a camera, at $295 in 2022 and stopped selling it in 2025. It went the same way as Babolat's racket and Zepp's sensors. Having the technology to measure doesn't mean people want to measure every day. That's why, in designing the Swing Mirror, I added the third principle: don't measure what can't be trusted.

## Swinging, and being swung

Let's return to the dictionary from the start. The Korean word for swinging also meant pushing people around, and wielding power carelessly. In front of the machines we followed today, those meanings overlap in a curious way.

Sensors and cameras turn a swing into numbers: bat speed, attack angle, X-factor, tempo. Once there's a number, people want to push it up. Yet in Major League Baseball, the hitter with the slowest swing made the most solid contact. Numbers measure only part of a swing. Let yourself be pushed around by them and you can lose your swing trying to fix it. The fact that KinaTrax data belongs to the team and is handled like medical information raises the same question. Whose hands will the numbers measuring my body's movement end up in, and whom will they be used to push around?

That's why the Swing Mirror's blueprint had two lines drawn into it from the start. Video never leaves your computer. The numbers are a mirror, not an answer. In Muybridge's sequential photographs, in Hawk-Eye's 300 frames per second, and in a webcam's 30, the moment the ball is struck always slipped into some gap. What fills that gap is still the feel of the person swinging.

And that's how *A Page for Today* starts from a single line in a notebook and arrives at one page of today's thinking.