Television and radio broadcasts frequently note the exit velocity of a batted ball, but other than clamoring about its ferocity, there isn't contextual discussion.

As such, next up on the survey of advanced stats is an overview of exit velocity. One of the underlying themes of this series is using expected stats to help gauge future performance. This topic veers from that a bit, but exit velocity is a component of other expected stats, so it makes sense to understand exit velocity.

On the surface, exit velocity seems routine; it’s the velocity the call comes off the bat, and average exit velocity (AEV) is the average of all batted ball events. It’s the application that gets a little hairy.

Average exit velocity encompasses all batted ball types (ground balls, line drives and fly balls). There is a lot of work trying to use exit velocity to project other stats, most notably hits and homers. Sometimes it’s not AEV, but maximum exit velocity or 90^th percentile exit velocity.

The main issue comes with utilizing exit velocity to project homers. The only element that matters is the exit velocity of fly balls, and sometimes line drives. Let’s take a step back and think about high school physics.

Maximum energy is transferred when the path of the swing is most closely aligned with the trajectory of the pitch. All pitches have a downward orientation, so optimal energy is transferred with an uppercut swing. If the ball is squarely hit, this produces fly balls and line drives. If it is topped, grounders ensue, and if the bottom of the ball is struck, popups result.

One of the repercussions of an uppercut swing is that the bat and ball are simultaneously in the hitting zone for less time than a flatter swing. This is why sluggers often carry a high strikeout rate. It also explains what seems to be a disconnect as to why a better strikes out a lot, but when he does make contact, he hits it hard. It’s often the consequence of an uppercut swing.

The caveat is an uppercut swing is not always the best approach. If the player doesn’t generate a lot of bat speed, the resulting fly balls will not travel far enough to clear most fences. A fly ball that isn’t a souvenir is caught around 87% of the time while ground balls generate outs at a 76% clip with just 37% of line drives being caught.

Some hitters are much better off generating grounders and line drives. A flatter swing achieves this objective and stays in the hitting zone longer. This is the reason high contact hitters generally produce more grounders and liners. However, their AEV may not be as high since the swing path isn’t as close to the downward flight of the pitch.

A fact holding true regardless of the type of batted ball is the harder it is struck, the better the chance of it becoming a hit. I know, duh, but the effect of exit velocity isn’t linear, and funky things occur at the low end of the EV spectrum. A hitter has a better chance of beating out a slowly hit grounder than one of medium velocity. A weakly hit fly ball has a better chance of falling in than one with a bit more exit velocity. Even so, when you get to the upper end, harder hit is much better.

BABIP Per Range of Average Exit Velocity

This isn’t an ideal presentation since the AEV isn’t parsed into ball types, but it serves the intended purpose. Other than observing the quirks at the lower end, the chief observation from the data is the big spike in BABIP starting in the 95-100 mph AEV range. This is the reason the formal designation of a hard-hit ball is anything with an AEV of at least 95 mph. A hard-hit rate is the percentage of contact with a minimum 95 mph exit velocity.

As suggested, there has been a lot of research using AEV as a gauge of home run potential. The treatment of expected home runs is a topic for a future essay and will focus on the fly ball component of exit velocity. In this introductory piece, I’d like to issue a warning on some flawed uses of AEV.

My biggest pet peeve is incorporating maximum exit velocity into analysis. As it seems, maximum exit velocity is player’s hardest batted ball event. It’s one among hundreds of batted balls. That alone should raise eyebrows. Is one event predictive? Sure, those with the highest maximum exit velocities generate the most bat speed, so chances are they’ll fare well when considering maximum exit velocity in an algorithm, but it’s still just one event, manifesting from the perfect confluence of other factors.

You’ll probably cringe at the thought, but let’s go back to Physics 101. Let’s say you wanted to chop down a tree. Where would you stand relative to the trunk? Hopefully, it is a bit in front of you. You want to minimize the number of times you swing the axe, so you want to hit the tree as hard as possible each time. If it is parallel to your body, you can’t generate as much force with your swing upon contact. The maximum velocity of the axe is in front of you, then it slows down so you can stop the swing. Even so, it’s at its apex in front of you, hence that is where you position your body.

This is of course an analogy to a baseball swing where the ball is hit before it is parallel with the hitter’s body. Maximum energy is transferred to the ball when the swing is at its hardest, hence when the ball is in front of the batter. This is why pulled contact is hit harder than opposite field contact. The hitter doesn’t generate full bat speed on balls hit the other way. There are ways to counter this, such as standing in the bucket, but in general, the hardest contact is of the pull variety.

Bringing this back to maximum exit velocity, this singular event was the perfect confluence of swing path and timing (where the ball was struck), not to mention having it occur when the batter is swinging the hardest. Maybe I’m missing something, but I can’t rely on this single, perfectly times event to help predict the future.

There are also studies using 90^th percentile exit velocity instead of maximum to project power. OK, this introduces more events, but there are still flaws. If a hitter puts 300 balls in play, 90^th percentile is the average of the top 30. Chances are not all 30 are fly balls. Power emanates from fly balls, ergo when projecting homers, I prefer to focus on the exit velocity of balls hit in the air. Admittedly, the data is harder to come by, but it’s there (Statcast has the combination of fly balls and outfield line drives).

That’ll do it for this week’s discussion. Hopefully it will set the framework for some of the impending discussion on expected stats, as well as be useful when reading the Player Profiles which will soon populate the Fantasy Index Baseball Draft Kit, available now.

Todd Zola is an award-winning fantasy baseball writer and 2020 inductee into the Fantasy Sports Writers Hall of Fame. He's the content provider for the 2024 Fantasy Baseball Index Draft Kit, available now. To purchase, click HERE.