Underload Overload Bat Training Reviews | SKLZ vs AXE vs DIY

Underload Overload Training Bats

Written by: Just Bat Reviews


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Underload/Overload bat swing training is a series of swings a hitter performs with bats both lighter and heavier than their normal bat’s weight. At least two studies, done by real smart people, have measured the increases in overall bat speeds among elite to good players after a series of weeks in underload/overload programs. In all cases, bat speeds increased by as much as 8% or more.

Although the training regimen has been proven to work among elite players, Underload/Overload equipment programs sold by major manufacturers have yet to convince us they are worth the cost. For very cheap, no name heavier and lighter bats are easily acquired. This makes it easy to create an underload/overload program with old bats in the garage, a donut or heavily discounted old bats on eBay. Unless we were convinced the hitting program associated with the training bat purchase had a secret sauce, and we are not, then an underload/overload program could become a part of our training regimen for nearly free. If anything, we would need to buy two cheap bats on eBay to compliment the weight of the current bat in our bat bag. Standard training protocols are found in the clinical articles by sports fitness scientists and not proprietary to the manufacturers. We direct you to those below.

We should also note that no study has measured the success of overload/underload training on less than Highschool players. Claiming the training program works on young players is not an honest use of the data.

Underload Overload Training Article Table of Contents

Company Offerings: SKLZ vs Axe

Underload Overload Bat Training Reviews

Armed with this data, more than a couple companies have developed bats that implement this underload/overload technique. SKLZ, as one example, developed a product of three bats called the AMMO bat system. These three aluminum bats come in a normal weighted bat, one that is 20% heavier and another that is 20% lighter. An Ammo bat purchase comes with a program developed by SKLZ. Axe also offers a similar getup in a three bat selection. Axe’s program is developed by Driveline Baseball.

SKLZUnderload Overload Bat Training Reviews

SKLZ’s Ammo bats are offered in a number of sizing options. They include a Pro, Junior Barrel  (2 5/8), as well as a youth barrel (2 1/4) version of the three bats. Each comes with a middle weighted bat and then one bat that swings 20% heavier and another that swings 20% lighter. At the time of this writing, the bats price totalled between $359 and $400.


Axe’s only come in an adult version, but if that goes well for them, you can expect more in the future. They are composite bats that use the unique Axe handle. These currently price out at $599 on their website. Its major advantage, and we would guess their price justification, is the partnership with Driveline Baseball who develop and track the training protocol for your weighted bats. Driveline is a legit organization.

Price check the AMMO Bat System

Price Check the Axe Bat Training System

Underload Overload Bat Recommendations

Underload Overload Bat Training Reviews


Despite that high level data, there are several reasons running out and purchasing an underload/overload system may not be the right idea. We discuss in greater detail below, but a few reasons for this include that the data has yet to test young players, the clinical studies do not prove what the optimal weight differences among the three bats should be and maybe most damning, buying any three bats with different weights can be significantly less expensive than buying bats specially made for an underload/overload program.


With that said, there may be good reason to buy a program and set of bats, too. Most notably must be the training program that usually comes with the purchase of underload/overload training bats. For some, that program may do better at keeping you on track and holding you accountable. As well, although the optimal under/over bat weighting has yet to be agreed upon, the convenience of one purchase for the three bats, despite the price difference, may be preferred.

Underload Overload Sources

We read a number of real sources putting together this article. And by a number, we mean more than we have probably read for any article we have ever written. Much of these are in the sports science sections of major publications and have a pay wall behind them. But here they are nonetheless:

Effects of Weighted Bat Implement Training on Bat Swing Velocity, Journal of Strength and Conditioning

Effects of Baseball Weighted Implement Training: A Brief Review, Journal of Strength and Conditioning

Resistance Training to Develop Increased Bat Velocity, David J. Szymanski, PhD, CSCS,*D

Training Methods Using Various Weighted Bats and the Effects on Bat Velocity Journal of Strength and Condition Research

We also spent time on some vendor sites looking for their take on the data. Much of the information on these was gathered from the more technical articles above.

SKLZ Ammo System Training Introduction

Axe and Driveline Baseball Product Offering Information

SKLZ Amazon Reviews

There are also some dissenting opinions on the matter. Really just one, we found at baseballrebellion. In short, he thinks the ideas of underload/overload resistance training in baseball and softball swings are wrong. But his argument is all theory. He hasn’t looked at the data and the data is well documented and decisive. We cite his article here because it is often the first one on a Google search. Google gets this one wrong.

Underload Overload Data Issues & Suggestions

Non-Elite Hitters

Underload Overload Bat Training Reviews

Training models in the scholarly articles focus on high school players and above. A few of the articles focus on collegiate players and former pro players. Taking their data points and extrapolating down to freshman, travel ball and youth players is a serious stretch.

In other words, we do not doubt the effect underload/overload training has on elite players. But, it is quite possible youth players, or thereabouts, would find the same benefit from simply practicing their swing for twelve weeks with just about any bat.

For the record, we have our youth players do underload/overload training. But we make no pretenses that this is any better than just good old fashioned normal practice. And, at least for youth players, there is no data saying otherwise.

Optimal Weight

There is no definitive data on the amount bats should weigh compared to others in the training program. The industry appears to have settled on 20% over and 20% under. But, many of the studies used bats that were as much as twice as heavy and half as light. In other words, although the 20% band feels like a good number, let us not overthink the problem nor assume that a bat that is twice as heavy and one that is half as heavy will not have similar results.

DIY Underload/Overload

Underload Overload Bat Training Reviews

It is difficult for us to make the suggestion that an overload/underload hitting program costing upwards of $600 in some cases is worth the price. And when considering the underload and overload of bat weights don’t need to be exactly 20% lighter or heavier, it is nearly impossible to make the case.

Unless the hitting programs that comes with the bat purchases is highly valuable to you, we suggest you look on eBay for bats that are between 20 to 50% heavier and 20 to 50% lighter than your current bat. Those bats do not need to be new. And, better yet, they can complement the bat you already have in your bag and you only need to buy two used bats on eBay. No matter what you are swinging, we are confident you could pull off that feat for less than $60 total.

Another idea would be to add the hitting Jack-It to your bat—which ads weight on your bat and can be used during BP. Then, add something like the hitting knob which connects to the bottom of your bat and decreases the swing weight. Then you could use just one bat and have an underload/overload training apparatus.

Our Invention: Adjustable Weighted Bats

Underload Overload Bat Training Reviews

The technology to adjust a bat’s swing weight exist. Why, then, make three bats when you could make just one that has a 20% plus/minus knob? That seems like a simple solution that might drive down costs.

In the next six months (today is 2/7/2017), expect baseball bats with adjustable weights to be on the market. They will be usable in game. That technology combined with an Underload/Overload training system might make a ton more sense.

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