You’ve averaged the same 61 mph bat speed for six straight weeks. You lift and take extra cage rounds, but the number won’t move. Overload underload bat training is one of the most direct ways to break that plateau.
The fix is simple. Alternate a bat about 20% heavier and a bat about 20% lighter than your gamer with your game bat, in short max-intent sets of 5 swings, and measure every set. Below you’ll get the bat weights, the exact protocol, the Blast metrics that prove it’s working, and the mechanics to protect while you add speed.
1. Why Weighted and Light Bats Work
Your coach hands you a heavy trainer bat and a light one and tells you to swing both. Fair question: how do two opposite bats make your game bat faster?
Each bat trains a different side of bat speed. The overload bat builds swing-specific strength, because your body has to produce more force through the same movement. The underload bat lets you move faster than your game bat allows, so you practice speed your body doesn’t own yet.
The research backs this up. In a 1995 12-week study of 60 male university players published in the Journal of Strength and Conditioning Research, DeRenne and colleagues had groups alternate overweighted, underweighted, and standard bats in batting practice or dry swings. All groups improved, and the alternating groups improved bat swing velocity significantly more than a control group that used only a standard bat.
Train both ends. Heavy builds force. Light builds speed. Your game bat is where you cash it in.
2. Pick Your Bats
You grab a teammate’s bat that’s way heavier than yours as your “overload” bat. By the third swing, your swing turns into a slow, looping push.
A bat that’s too heavy or too light changes the swing you’re trying to speed up. Aim for these targets:
- An overload bat about 20% heavier than your game bat
- An underload bat about 20% lighter than your game bat
For a 30 oz game bat, that’s roughly a 36 oz overload bat and a 24 oz underload bat.
No trainer bats? Wrap athletic tape around the barrel of an old bat until it hits your overload weight on a kitchen scale. A lighter youth bat works for underload.
Next step: Weigh your game bat today and do the 20% math before your next session.
3. The Protocol: Sets, Order, Intent
You take 100 swings with a heavy bat, then wonder why your game bat feels slow and your hands feel dead. Short, fast sets beat long, tired ones.
Run this order every session. It matches the protocol in our guide on how to increase bat speed.
- Take 3 sets of 5 swings with your underload bat.
- Take 3 sets of 5 swings with your game bat.
- Take 3 sets of 5 swings with your overload bat.
- Finish with a few swings on your game bat.
Starting light gets you moving fast before you add load. Finishing on the game bat brings your feel back to the bat you’ll actually use in a game.
Every swing is max intent. Rest between sets so each set stays fast. If bat speed drops inside a set, end the set, because tired reps train a slower swing.
Track bat speed for each phase, every time. A sensor makes this easy, and the next section shows you how.
Drill: Run the full protocol in your next session and log your average bat speed for each of the 4 phases.
4. Measure Every Set With Blast
Two weeks into overload underload bat training, your swing feels faster. Feel can fool you. Without a number, you can’t tell if the speed is real.
Start with a baseline. Before your first session, record at least 10 swings with your game bat and note the average bat speed. That’s your starting line.
Blast Baseball puts a Gen 3 sensor on your bat knob and pairs it with the mobile app. It tracks bat speed, attack angle, connection, rotation, time to contact, and power on every swing. Blast iQ scores each swing and flags green, yellow, and red focus areas. Air Swings let you run the protocol as dry swings in your garage.
Compare your numbers to these benchmarks from our guide on what bat speed is in baseball:
| Level | Bat Speed |
|---|---|
| Middle School | 46–62 mph |
| HS Varsity | 60–70 mph |
| Pro | 66–78 mph |
Watch Rotational Acceleration too. It’s the “0-60 time” of your swing, showing how quickly you get the bat up to speed.
The metric that matters most is your game bat average climbing over your baseline. Auto-edited video with metric overlays shows you what changed when it does.
Next step: Record your 10-swing baseline before you pick up a trainer bat.
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5. Protect Your Swing While You Add Speed
Your bat speed is climbing, but you’re popping up fastballs and rolling over pitches away. Speed you can’t control won’t help you in a game.
Check your attack angle on every set. If it shifts on the overload bat compared to your game bat, the heavy bat is changing your path. Then check these two targets:
- On-Plane Efficiency of 70%+ is optimal.
- Early Connection targets 90°, with an 80–105° range, measured at the Transition Event.
If either number slips on the heavy bat, look for casting, where your hands push the barrel away from your body early. Fix it with the Stop at Contact Drill: swing, freeze at contact, and check your position. Pair it with the cue “Short to it, long through it.”
Low bat speed can also come from a separate problem: insufficient hip-shoulder separation. Connection and hip-shoulder separation are different metrics, so fix them separately. Use the Torque Drill plus the Knob to Knee Drill, and load your pelvis with the cue “Back pocket down the line.”
For more mechanics fixes, read our data-driven guide to improving bat speed.
Keep On-Plane Efficiency at 70%+ and Early Connection inside 80–105° while your bat speed rises.
6. When to Use It (and When Not To)
It’s the night before a doubleheader, and you want one more overload session. Skip it.
Overload underload bat training is an off-season build. Use it on training days when you have time to recover. Keep it out of your pre-game and on-deck routine, where you want your timing locked to your game bat.
In season, shift to maintenance. Run fewer sets on non-game days so you hold the speed you built.
Retest every couple of weeks. Take a fresh 10-swing game bat test and compare it to your baseline. Then check Time to Contact. If bat speed rises and Time to Contact holds or gets quicker, the speed is transferring.
If your game bat average stalls across 2 retests, pause the protocol and fix your mechanics first.
Next step: Put your first retest on the calendar 2 weeks from your baseline session.
Supporting Resources
- Blast Baseball shows how the sensor and app track every swing.
- What Is Bat Speed in Baseball? breaks down bat speed benchmarks by level.
- How to Increase Bat Speed covers drills and training beyond weighted bats.
Heavy and light bats give you a research-backed way to train bat speed. Measuring every set turns that work into progress you can see. Set your baseline, then let the data tell you when the speed shows up in your game bat.
