Overload vs Underload Training: What Actually Increases Bat Speed?

Intro

One of the most common debates in bat speed development is whether hitters should train with heavier bats, lighter bats, or a combination of both.

Overload and underload training methods are both widely used, but they are often misunderstood. When applied correctly, each can contribute to bat speed gains. When applied incorrectly, they can disrupt timing and mechanics.

In this Bat Speed University lesson, we’ll break down the difference between overload and underload training, explain how each affects swing development, and outline how to use them safely and effectively.

If you’re new to Bat Speed University, start with our first lesson, The Science of Bat Speed: Why It's the #1 Competitive Edge You Can Train, where we define the fundamentals that this lesson builds on.

What Is Overload Training?

Overload training involves swinging a bat that is heavier than a player’s game bat. The goal is to increase strength specific to the swing pattern.

When used properly, overload training can help improve force production and strengthen key movement patterns in the swing.

However, excessive overload can cause:

• Mechanical breakdown.

• Slower swing tempo.

• Altered barrel path.

• Poor timing carryover.

Overload should challenge strength without significantly changing movement quality.

The ProVelocity Bat functions as a controlled overload trainer, adding resistance that challenges force production while preserving proper sequencing and barrel path.

What Is Underload Training?

Underload training involves swinging a bat that is lighter than a player’s game bat. The goal is to train the nervous system to move faster.

Underload training can help improve swing speed, coordination, and intent.

But if the bat is too light, hitters may:

• Overswing and lose control.

• Change the swing plane.

• Develop timing habits that do not transfer to game conditions.

Underload works best when it reinforces proper acceleration rather than reckless speed.

The ProVelocity Quick Stick is an effective underload training tool because its lighter design allows hitters to train faster acceleration and reinforce high-intent swings without overloading the movement pattern.

Which Method Actually Increases Bat Speed?

Both overload and underload training can increase bat speed when used strategically.

Overload improves force production.

Underload improves movement velocity.

The most effective programs combine both, while protecting swing mechanics.

The Key Is Transfer

Bat speed gains only matter if they transfer to a player’s game bat.

Training tools should reinforce clean sequencing, early acceleration, and barrel control.

If swing mechanics change dramatically when switching back to a game bat, the training stimulus was likely too extreme.

How to Use Both Safely

Rotate between overload, game-weight, and slight underload swings in short, controlled sets.

• Monitor swing quality carefully.

• Prioritize smooth acceleration over forced effort.

• Keep volume moderate to prevent fatigue-related breakdown.

The goal is progressive adaptation, not exhaustion.

When to Use Each Method

Overload training may be emphasized during off-season strength phases.

Underload training may be emphasized when refining speed and intent.

In-season, both should be used conservatively to maintain rather than aggressively build bat speed.

Key Takeaways

• Overload builds strength.

• Underload builds speed.

• Both can increase bat speed when mechanics remain stable.

• Transfer to game swings is the ultimate test.

• Balanced programming produces sustainable results.

If you haven’t read the previous Bat Speed University lesson, it provides important context. In How Bat Speed Training Improves Timing and Pitch Adjustability, we explain how increased bat speed expands timing windows and improves pitch recognition. Understanding that relationship makes overload and underload training more purposeful and effective.