Field testing the motusTHROW

After a year on the market, Motus Global's mThrow has been supplanted. The Massapequa, N.Y.-based company announced an overhaul to the device this winter, rebranding as the motusBASEBALL system. The new system includes new accelerometers and gyroscopes with higher ranges, an improved sensor and sleeve design, and updates to the software that allow the system to track metrics related to both hitting and pitching.

The new sensor first went on sale this spring, coinciding with the news that MLB had approved the motusTHROW system for in-game use. Shortly after, Motus Global announced their first endorsement deal with a Major League pitcher, bringing Yankees setup man Dellin Betances on board.

As part of the new sensor's rollout, Motus Global stopped supporting the mThrow. Users who had purchased the first-generation system were given the opportunity to upgrade free of charge to the new system. The upgrade didn't require users to ship back their old mThrow, though, setting up a natural experiment.

We went up to SmartKage headquarters in Tyngsboro, Mass., and put the sleeves on Nate, a local high school junior. We then fired up Motus Global's bullpen mode – first in the new motusTHROW app, and then in the legacy mThrow app*. At the app's direction, Nate threw a mix of fastballs, curveballs, and changeups. A voice from the iPhone instructed him which pitch to throw, whether to throw from the windup or the stretch, and what part of the strike zone to aim for. Each of the two sessions consisted of 21 pitches, simultaneously tracked by SmartKage's PITCHf/x system.

* - At this time, the Motus Global apps are still compatible only with Apple devices.

Both sensors successfully recorded all of Nate's 21 throws. But of the other four metrics reported (torque, arm angle, shoulder angle, and arm speed), two showed substantial discrepancies. The graph below shows some disagreement between all four metrics, with the largest discrepancies between the shoulder rotation angles and arm speeds.

The shoulder rotation angle – defined as "the maximum angle by which the forearm rotates back before the arm moves forward to the plate" – ranged from 126 degrees to 135 degrees with the new sensor and from 140 degrees to 165 degrees with the old. Granted, this isn't a one-to-one comparison: Nate could wear only one sleeve at a time. But the difference here is so great that it suggests either Nate substantially changed deliveries between bullpens, or that Motus Global substantially changed the way they calculate shoulder rotation between apps.

Even greater is the difference reported between arm speeds. The motusTHROW app defines arm speed as "the peak rotational velocity of your forearm measured as RPM," which leaves some room for interpretation: is it the vector magnitude of the three gyroscope axes? Or maybe the greatest of the three gyroscope readings? Or have the gyroscope readings undergone some rotation to make one axis correspond to rotation towards home plate?

Regardless, there is a 50 percent discrepancy between the two sets of measurements. Whereas the new sensor reported an average arm speed of 953 RPM – with a maximum of 1050 RPM – the mThrow never went higher than 770 RPM. In the absence of some external "gold standard," it's hard to say which is right. But the figure below shows that the new motusTHROW measurements showed a higher correlation – albeit still a small one – with the start speed reported by PITCHf/x. Your correspondent, no expert on pitching mechanics, finds this intuitive. When asked to throw a changeup, a typical Major League pitcher will not vary his arm speed, because this will alert the batter that a changeup is coming. But a high school pitcher, with much less experience with the pitch, probably will.

When asked about the difference via Twitter, Motus Global emphasized that they had ceased supporting the mThrow. But the company said the layout of the printed circuit boards inside the devices had changed, along with the physics engine that produces the metrics from the sensor. A future update to the app will include fingertip velocity, which should correlate better to release speed across pitchers than the current arm speed metric.

"A child with low rotational inertia has similar arm speeds as college pitchers with heavier arms," the spokesperson said in another tweet.

In Other News

• Bat sensor maker Diamond Kinetics announced in April that they had integrated their SwingTracker system with HitTrax, a camera-based data capture and simulation system. Data from the SwingTracker sensor on the knob of the bat will be transferred to the HitTrax system. The end result is a system that combines measurements of swing speed, power, and control with distance and exit velocity. It's easy to envision new metrics that depend on inputs from both systems, but no metrics were announced as part of the initial rollout.
• Shortly after receiving MLB approval for on-field use, Blast Motion inked a deal to become the official swing sensor of the Houston Astros. Representatives for Blast were reluctant to give details about how the Astros planned to use the system, other than to say that "the Astros purchased sensors for player development and metrics analysis," and that the team worked with Blast to determine which metrics were included in the company's new professional apps. To our knowledge, this is Blast's first such deal with an MLB organization; competitor Zepp announced deals with the Angels, Diamondbacks, Padres, and Rays last summer.
• Zepp also made improvements to their sensor this spring. In addition to improvements to its internal components, the Zepp 2 casing has been completely redesigned for compatibility with the "smart bat" unveiled during Spring Training. The new sensor is significantly smaller and comes with a new magnetic charger that will let users "unscrew" the sensor from a smart bat. The app has also been beefed up, with a diagnostic feature and a number of new training programs to help users increase bat speed or focus on other areas of their swing that need improvement.

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Bryan Cole is a featured writer at Beyond the Box Score, specializing in wearable sensors and other sports science technology. You can follow him on Twitter at @Doctor_Bryan.