Climbers think small edges are “harder.”
Mechanically, that’s not the full story.

Small edges change the geometry of the finger →
the geometry changes the force line →
the force line changes pulley load and tendon tension.

A 6 mm edge isn't just “a smaller hold.”
It’s a different mechanical environment — one where tiny angle changes multiply load.

This article explains why hold size is one of the strongest determinants of injury risk and finger loading.

The 3 main joints in the finger.

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1. The real reason small edges feel sharp

When the hold gets smaller:

• the finger bends more
• the PIP angle increases
• the DIP tends to unroll
• the tendon wants to straighten
• the redirect angle becomes steeper
• pulleys take more load

Small edges force your finger into a mechanically disadvantaged shape.

It’s geometry. Not weakness.

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2. The “curvature effect” — why angle matters more than force

The flexor tendon wants to take the straightest path possible toward the load.

On a large edge, this path is smooth.
On a small edge, this path is sharply curved.

Sharpened curvature =
→ increased A2 tension
→ increased A4 tension
→ decreased joint stability
→ higher risk of bowstringing
→ higher stress on tendon insertion points

The hold size determines the curvature.
The curvature determines the stress.
Not the weight.

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3. Comparing 15 mm vs 10 mm vs 6 mm

15 mm (the “safe zone”)

• moderate PIP bend
• DIP maintains slight curl
• pulleys load predictably
• force line is smooth
• excellent for training
• strong FDS/FDP cooperation

This is why almost all hangboard protocols start here.

10 mm (the “stress multiplier”)

• PIP angle increases
• DIP begins to drift
• tendon alignment becomes sensitive
• unexpected load spikes under fatigue
• force path starts to sharpen

10 mm is trainable —
but the finger must be stable.

6 mm (the “geometric danger zone”)

• extreme PIP angle
• DIP almost fully unrolled
• tendon path highly angular
• pulleys fight the load instead of guiding it
• bowstring tendency increases dramatically
• joint stability becomes fragile
• small technique errors → large load spikes

6 mm changes the physics, not just the difficulty.

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4. Why climbers get injured on small edges

Not because small edges require “more strength.”

But because:

• angles collapse faster
• tendons have less margin of error
• load becomes more focused
• muscles cannot react fast enough
• passive tissues absorb the mistakes

Small edges turn:

• tiny grip errors → big structural stress
• light fatigue → angle drift
• slight wrist collapse → tendon lift
• minor DIP extension → fingertip overload

Small edges punish instability.

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5. Why 15–20 mm is the ideal training range

15–20 mm has:

• best angle control
• predictable force distribution
• manageable tendon tension
• low curvature amplification
• early warning signals when technique breaks

This is the sweet spot where:

• FDP can produce high force
• FDS can stabilize angles
• A2/A4 stress remains predictable
• mechanics stay consistent

It’s where adaptation happens
without geometric danger.

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6. When to use small edges (and when not to)

Use 6–10 mm only when:

• technique is stable
• fatigue is low
• load is predictable
• warm-up is sufficient
• angle control is reliable
• progression is slow

Avoid small edges when:

• you are fatigued
• coming back from injury
• rushing progression
• changing grip positions
• testing max loads
• trying to hit PRs

Small edges aren’t “bad.”
They’re amplifiers — they magnify whatever you do.

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7. The real training lesson

You don’t get strong from small edges.
You get strong toward small edges.

The progression is:

stable angle → predictable load → consistent edge → smaller edge

Reversing the order is how injuries happen.

15 mm trains the system.
10 mm tests the system.
6 mm exposes the system.

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Putting it all together

Hold size changes the mechanics of your finger:

• smaller edges create sharper tendon angles
• sharper angles increase pulley load
• pulley load increases injury risk
• joint stability becomes harder
• bowstringing risk multiplies
• the finger becomes more sensitive to technique

A 6 mm edge is not just “smaller.”
It is mechanically different.

Understanding this is the key to structured, safe finger progression.

 

Want to see how your own crimp strength holds up on a real edge? Test it →