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Analyzing Weight Shift and Ground Force in the Baseball Swing

I
n addition to being one of the most dramatic World Series in MLB history, the 2017 Fall Classic may have been a showcase of the evolution of hitting in baseball. Both the Astros and the Dodgers featured lineups loaded with impact bats which combined to produce a record-setting number of home runs in a World Series. Modern statistics such as exit velocity and launch angle were commonly referred to on the broadcast. The future of hitting has arrived, and it’s all about power.

Earlier this year, ESPN’s Sports Science analyzed the swing of MLB All-Star Robinson Cano using motion capture technology. While the feature offered a helpful insight into Cano’s power generation strategies, we found a small error in how they communicated their observations. We thought it would be helpful to highlight the point in order to clarify an important characteristic of baseball swings.

At the 0:36 of the ESPN video, host John Brenkus discusses Cano’s efficient mechanics, noting a key lower body move:

At footstrike he shifts all of his bodyweight into his front leg.

While the word choice may be inconsequential to a casual baseball audience, it is important to recognize the distinction between ‘weight’ and ‘bodyweight’.

Cano is applying tremendous vertical and horizontal force through his lead leg to help generate rotational speed, but his weight (represented by his center of mass) remains back. Cano isn’t shifting his center of mass significantly, but rather creating GRF with his lead foot, therefore shifting his center of pressure.

*‘Bodyweight’ is often used in display of vertical force to normalize force to a person.

Those who have followed BodiTrak for any time are likely familiar with distinctions between mass and pressure.  We see center of mass, we feel center of pressure.

The distinction may seem small, but it’s meaningful, particularly because it underscores characteristics in ground mechanics of baseball’s most powerful hitters.

As Cano’s bat approaches impact, you’ll notice that his trail foot is barely touching the ground. 

Scroll down the list of MLB’s most prolific batters and you’ll see a similar position near impact.

While it’s common for athletes and coaches of throwing or striking sports to emphasize the importance of force in the push off leg, force in the plant (or lead leg) is actually more closely tied to speed.

Driveline Baseball’s founder, Kyle Boddy, outlined the importance of the lead leg in pitching in this 2013 blog post.  In the post, he reviewed a 2015 paper which concluded that “Stride leg ground reaction forces during the arm-cocking and arm-acceleration phases were strongly correlated with ball velocity, whereas drive leg ground reaction forces showed no significant correlations.” 

We also covered this in golf with our analysis of long drive competitor Justin James.  As James (an ex-MLB pitcher) begins his downswing, he extends his left knee, exerting tremendous force into the ground.

Pushing into the ground with the lead leg is a proven strategy for generating speed in the golf swing.  According to research conducted by BODITRAK advisor Dr. Sasho MacKenzie, there is a strong relationship between club head speed and vertical force when the shaft is vertical on the downswing.

Jordan Spieth describes a similar move in his swing:

The toes of my left foot have rolled off the ground, proving that my weight has moved into my left heel. That allows me to straighten my lead leg so I can pivot my body and swing around that leg like a post.

In batting, pressure into the lead leg is similarly important and evidenced by observing that powerful hitters often pick their trail leg OFF the ground at or before impact.

Baseball hitting expert Bobby Tewksbary examined the concept of weight shift in hitting on a recent post on his site, Hitting Daily.

In terms of force into the ground, the front foot gets unweighted during the gather. As the hitter moves into contact, the rear foot becomes (briefly) unweighted. So we’re talking 100% back foot, then 100% front foot in terms of FORCE. But force and where you feel your weight can be extremely different.  For me, weight shift has more to do with WHERE your weight is.

Hitters are often told to “stay back” but also “shift their weight.”  While those cues might seem conflicting, Tewksbary does an excellent job of explaining how correct ground mechanics can accomplish both.

Hopefully, by clarifying that Cano’s move is not as much a transfer of weight as it is an exertion of force, we’ve been able to shed light on a key power generation strategy for rotational and throwing athletes.