Measuring Mike Trout and Giancarlo Stanton's swing plane with the Zepp sensor

It's been a big week for Zepp's MLB partners. Their recent update included three-dimensional swing data and videos from some of the biggest names in baseball. And just hours after one won the American League Most Valuable Player award, another put the finishing touches on a record-breaking 13-year, $325 million contract.

When he was in high school, Mike Trout was challenged by his coach to hit more balls to the opposite field. To do this, Trout developed an "inside-out" swing that allows him to scatter his hits all over the field. We can see Trout demonstrate how to achieve this by keeping the barrel of the bat inside his hands until late in his swing, giving Trout better bat control in the hitting zone.

If we compare the trajectory of Trout's swing to Hunter Pence's, we can also see that Trout keeps his hands slightly above the barrel of his bat, keeping the bat at a lower vertical angle. The Zepp sensor measured Trout's vertical angle at -25 degrees, whereas Pence's swing is much flatter, with a vertical angle around -18 degrees.

Courtesy of Zepp

We can also contrast Trout's approach with Giancarlo Stanton's, whose quick swing and vicious home runs put him on the verge of the biggest contract in baseball history with the Miami Marlins. According to ESPN's Home Run Tracker, Stanton's 37 home runs came off the bat an average of 3 miles per hour faster than Trout's, and flew an average of two feet farther. Stanton talked about his approach to hitting the curveball in his session with the Zepp sensor.

Stanton stresses "staying back on the ball" in the video, keeping his hands back as his weight goes forward. Once his hands start moving forward, his more direct path to the ball (as illustrated below) allows him to complete his swing faster than normal. Stanton's time to impact was measured as .11 seconds, as compared to Trout's .14 seconds, even though his bat speed at impact (91 mph vs. 96 mph) and maximum hand speed (32 mph vs. 34 mph) were both slower for this particular swing. But notice the red part of the swing, where the ball impacted the bat: Stanton's is farther in front of the plate than Trout's, meaning Stanton will pull this particular pitch while Trout will hit his the other way.

Courtesy of Zepp

The videos and training sessions are part of Zepp's exclusive licensing agreement with the MLB Players' Association, as announced last month. Trevor Stocking, Zepp's product manager for baseball and softball, said that working with the players was eye-opening for both the players and the developers.

"Right off the bat, we got a pretty colorful response," Stocking said. "We've heard from a lot of teams who want to use [the device], from front offices to scouts to players."

Stocking said working with players allowed them to discover the importance of metrics such as time to impact, and the players appreciated the instant feedback on their swings.

"Players work on their swings all the time," Stocking explained. "It's kind of their brand. And with this sensor, they could make adjustments so quickly. When you work with video, you have to take a swing, then upload the video, then the coach breaks it down with you, then you go back in the cage. It's a five-minute breather. But with this [system], they can take two or three swings and see their results in seconds."

Zepp's sensor contains two accelerometers and one gyroscope, allowing Zepp to track the bat's path through six degrees of freedom. CEO Jason Fass explained that having two accelerometers allows the sensor to track the large, high-frequency accelerations that happen around impact while still accurately tracking the lower-frequency accelerations as the bat moves through the zone.

Establishing the sensor's accuracy was another challenge.

"We had to establish a ground truth for these metrics," Fass said. "There wasn't a very good bat speed benchmark before this."

Zepp used an array of high-speed cameras, as well as Doppler radar-based devices such as Trackman and FlightScope, to verify their metrics. Fass claims the barrel speed measured by the Zepp sensor is accurate to within three to five miles per hour, and the hand speed to one to two mph. The measurement of the bat's position is said to be accurate to "within a few inches."

Fass says their experience working with thousands of players of different ability levels is part of what sets their system apart.

"Our goal is to help the user capture meaningful data, make that data actionable, and take you on a guided experience to deliver high-level, premium data," Fass said.

Fass sees Zepp as part of a future where "every single piece of equipment will be connected in some way."

"It will be better for the players, but also for the fan experience," Fass said.

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Home run statistics courtesy of ESPN's Home Run Tracker. Swing data courtesy of Zepp Baseball.

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. You can follow him on Twitter at @Doctor_Bryan.