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.
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.
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.
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.
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.
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.
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.
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.
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.