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Using wearable sensors to improve performance

For everyone but the Royals and Giants, it's the offseason. How are advances in sensor technology helping baseball players train?

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For the Royals and Giants, today marks the climax of an eight-month season. But for 28 other teams, along with countless minor leaguers and amateurs, the focus is on improving for next year.

And while the professionals have armies of coaches and video-based systems and reams of data at their disposal, a handful of companies are developing more affordable options for serious and casual ballplayers alike. Sensor-based systems are not unique to baseball -- David Temple's recent article over at TechGraphs describes some options available to tennis players and golfers -- but have begun to proliferate in earnest over the past year.

Zepp

Zepp's device has technically been on the market the longest, debuting last October. For $150, users get a 0.2-ounce sensor and a 0.5-ounce mount that goes over the knob of the bat and keeps the sensor in place, along with a USB charger. The device (which can also be mounted to tennis rackets and golfing gloves) contains two 3-axis accelerometers (which tracks acceleration due to gravity and movement) and a 3-axis gyroscope (tracking sensor rotation and orientation). Using Bluetooth, the sensor pairs with an iOS or Android device to store information about swings; if the connection is lost, the device has enough on-board memory to store up to 2,000 swings.

The headline metric is the three-dimensional model of the swing plane that users can play through and rotate around to see where they made contact with the ball, how long their bat stayed in the hitting zone, and other components not visible to the naked eye. Additional parameters include hand speed, time to impact, and bat angle at impact. Each swing can also be tagged with the hit type (e.g., ground ball, line drive, or fly ball) and location then stored for later reference.

The company has been busy since their device hit the market, partnering with Perfect Game and the Durham Bulls to bring bat speed data to the home run derbies at the All-American Classic and Triple-A All Star Game, respectively. Zepp's CEO has also said in interviews their device is being used by "a number of MLB teams" but keeps the specifics under wraps.

Blast Baseball

Since the spring, Blast Motion has offered a competing product. Like the Zepp sensor, Blast Baseball (available only on iOS devices) retails for $150, weighs less than an ounce, and attaches to the end of the bat using a flexible mount. The heart of the system is the Precision Motion Sensor, which can also track golf and more general metrics. Information on what's inside the sensor is harder to find, but there is most likely a 3-axis accelerometer and 3-axis gyroscope, in addition to the low-power Bluetooth Smart transmission system and onboard memory to hold up to 250 swings. Blast claims their sensor is "dustproof, shockproof, and waterproof" and that their system is accurate to within "a few miles per hour and a few degrees" of more sophisticated motion capture systems.

The big number this sensor produces is the "Blast Factor," a combination of swing efficiency and power made up of a number of timing, rotation, and velocity metrics. Each swing can also be paired with a captured video, with the relevant metrics automatically synced to the key points in the swing. Unlike the Zepp system, which requires a specific calibration step before each session, the Blast system passively calibrates between swings. Blast's highest profile event this year was their partnership with Easton at the Little League World Series.

Diamond Kinetics' SwingTracker

Still in development -- but due out by the end of the month -- is the SwingTracker from Diamond Kinetics. Demonstrated at the 2014 Sloan Sports Analytics Conference, the SwingTracker will also be priced at $150. Like its competitors, the SwingTracker contains a 3-axis accelerometer and a 3-axis gyroscope and transmits data via Bluetooth to a nearby iOS device. According to Diamond Kinetics' patents, the SwingTracker was first developed for a fly-fishing application, but work with thousands of amateur (and some professional) hitters have helped the product evolve.

Because the device is not yet available -- and because the accompanying software is not yet available on the App Store -- it's hard to say exactly which metrics will be captured by the device. However, the company's website suggests a very data-intensive application, and demonstrations of the software suggest that a 3-D model of the swing (complete with swing plane and bat rotation) will be produced.

Motus Global Pitching Sleeve

But hitters don't get to have all the fun. Readers may remember Will Carroll's article from April on the Motus Global Pitching Sleeve. Since that article, Motus has released few additional details about the device conservatively dubbed "The Sleeve that Could Save Baseball." Currently in beta testing (and, according to Carroll's article, in use by the Pirates and Orioles), the device consists of a quarter-sized sensor (containing accelerometers and gyroscopes) embedded in a compression sleeve near the elbow. These sensors feed data to a smartphone application that in turn calculates a number of parameters relating to efficiency, fatigue, and torque on the infamous ulnar collateral ligament.

Conclusion

All of the companies referenced here express hope that their systems will make their way to the professional ranks, but in-game measurements are still much more likely to come from camera-based systems. Every athlete-led discussion of wearable sensors always gets back to making them as unobtrusive as possible. A one-ounce device might seem insignificant to the average user -- a typical golf ball weighs around 1.5 oz -- but to the professional who has been swinging the same bat for years, that ounce can make a huge difference in the bat's feel and balance. Witness Ted Williams, who kept his bats off the ground so excess moisture wouldn't make them heavier, and complained when his bat handles were thousands of an inch off.

That's not to say that the devices aren't useful: surely, the benefits of improved speed and timing outweigh any adverse effects from that extra ounce for the average hitter. Their price point and ease of use give amateurs a simple tool to track their progress over time.

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UPDATE: In addition to their existing relationships, Zepp today announced partnerships with a number of Major League hitters, including Mike Trout, David Ortiz, and Hunter Pence. Zepp also announced an updated version of their app, which allows users to capture videos of their swings and compare them to the professionals.

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Bryan Cole is a featured writer for Beyond the Box Score. He was in no way compensated by, nor does he endorse, any of the companies or products mentioned above; he's just fascinated by wearable sensor applications. You can follow him on Twitter at @Doctor_Bryan.