Fresno, CA - the Hitting Performance Lab is releasing the findings of a baseball hitting experiment they did May 15th, 2015 using the Zepp Baseball app.  The study looked at whether the bending of the back knee during the baseball swing had any effect on ball flight.  

100 swings were taken off a batting tee using a bend in the back knee during the swing, and another 100 swings were taken off a batting tee using a straightened back knee during the swing.

There was a 27% difference in ground-ball percentages, and a 24% difference in fly-ball percentages.  The line-drive percentages remained virtually unchanged.  

There were higher fly-ball and lower ground-ball percentages when using a bend in the back knee during the swing.  This increases a hitter's power numbers.

There was also a 3-degree difference in the attack angle.  More up with a bend in the back knee during the swing.  According to the Zepp app, the Attack Angle is the direction the bat barrel is moving (Up or Down) at impact. A positive number would mean your barrel is going UP at impact, zero is LEVEL and a negative number is the barrel going DOWN at impact.

The Hitting Performance Lab used Oakland A's slugging catcher Stephen Vogt as a model of someone who has an above average fly-ball percentage, and a below average ground-ball percentage.  And as of this release, Stephen Vogt is 5th among the Major Leagues in Slugging Percentage.