If an exercise causes pain, we discuss why simply waiting a minute and trying it again can work wonders.

I’ve written previously about why resistance training can help reduce pain over time.

What if an exercise causes pain in the moment however?

Strangely, just giving it a minute and trying again can often produce a different result.

It’s common to see this with knee pain on a leg extension machine:

• First light set → discomfort or apprehension
• Short rest
• Second set (sometimes heavier) → pain is reduced or gone

At first glance, this feels counterintuitive. If the knee was being “irritated” by load, more weight should hurt more. But that’s not what happens.

What’s actually changing between set one and set two

The knee hasn’t suddenly altered its structure.

What’s changed is how the nervous system is processing the movement.

1. The first set acts as a neural primer

That initial set:

• Increases motor unit recruitment in the quadriceps
• Improves coordination and timing
• Provides strong sensory input from the joint and surrounding muscles

In effect, it tells the nervous system:

“This movement is safe, predictable, and under control.”

That alone can reduce pain output.

2. Pain is being modulated, not eliminated

Pain isn’t related to tissue damage in this scenario — it’s a protective signal influenced by:

• Context
• Confidence
• Previous experience
• Sensory input

After the first set, several things happen quickly:

• Non-painful sensory signals “compete” with pain signals at the spinal cord
• Descending inhibitory pathways dampen pain perception
• The brain reassesses the threat of the movement.

Result:

The same movement now feels safer — even under more load.

3. Better muscle output = less joint threat

Early in a session, quadricep activation can be:

• Inhibited
• Inconsistent
• Guarded

That can increase the perception of threat at the knee.

After a brief activation:

• The quadriceps produce force more efficiently
• Joint motion becomes smoother
• Protective co-contraction decreases

Why load can increase while pain decreases

This can be somewhat confusing.

The second set doesn’t hurt less because it’s heavier — it hurts less because:

• Neural drive is higher
• Motor control is improved
• The nervous system is no longer on high alert

In many cases, a slightly heavier load actually:

• Makes the challenge clearer
• Reduces “hesitation” or guarding
• Feels more stable

What this means for training painful knees

A painful first set doesn’t mean:

• The exercise is harmful
• The knee is damaged
• Load should be avoided

It often means:

• The system hasn’t been primed yet

This is why warm-ups, isometrics, or gradual ramping sets can:

• Reduce pain
• Improve tolerance
• Restore trust in the movement

Summary

When pain decreases as load increases, it’s a sign that:

• The nervous system is adapting quickly
• The movement is being reclassified as safe
• Strength is acting as a regulator, not a threat

Sometimes the purpose of the first set isn’t to train the muscle – it’s to introduce the challenge, so by the second set an improvement in performance is possible.