The secret behind Boston Red Sox reliever Joe Kelly’s 102 mph heater


Boston Red Sox reliever Joe Kelly might not be the first name that comes to mind when thinking of baseball’s elite fastballs, but the 29-year-old right-hander has turned heads this season by routinely throwing triple-digit heat.

To find out just how he does it, we’ve compared Kelly’s motion from 2012, his rookie season with the St. Louis Cardinals, to his current delivery. What’s powering the hard-throwing 2017 version of Kelly? It starts long before his triple-digit fastball leaves his hand on the mound.

Leg lift

2012: At full lift, Kelly’s hands moved toward his head, with his lead foot pointing toward shortstop and his torso arched out, causing his upper half to become disconnected from his lower body.

2017: At full tilt, Kelly now keeps his hands lateral, with his lead foot pointing toward third base and his upper body sitting directly over his lower body.


2012: Kelly’s upper body got stuck over his drive leg while both arms began to spread out.

2017: Kelly’s separation is on time with the beginning of his stride. He keeps his upper half stacked over his lower half and the inside of his drive leg, with both arms bent in a two-piece position.

Arm path I

2012: Kelly’s arm path began with both arms spread out and away from the body.

2017: Kelly’s arms remain in the two-piece position while his body moves forward down the slope of the mound.

Arm path II

2012: With his arms spread out in a one-piece position, Kelly’s arm path was inefficient and long.

2017: Kelly’s arms now work together in the two-piece position, allowing for leverage and an efficient delivery.

Arm slot at touchdown

2012: Kelly’s arm was positioned at approximately 90 degrees, far enough away from his head to create additional disconnect in the delivery.

2017: Kelly’s arm slot, with ball/hand hidden behind his head and inside a 90-degree angle, allows for a quicker arm path through the delivery.

Release point

2012: Kelly got to his release at a mid-three-quarter arm slot.

2017: Now Kelly’s release has slightly adjusted to a mid-high three-quarter arm slot.

Deceleration to finish

2012: Kelly finished his delivery with full extension of his upper half, and his throwing hand/elbow was outside his stride-leg knee.

2017: Kelly’s finishing position remains nearly identical to that in 2012.

Ready position

2012: At the final point, Kelly finished in an upright position and spun off to the first-base side of the mound.

2017: Kelly’s ready position remains nearly identical to that in 2012, even though the radar gun reading on his fastball has changed drastically.

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