If you only understand one piece of finger anatomy, it should be the A2 pulley.
It’s the most commonly injured structure in climbing, the most stressed in crimping, and the central redirector of the force line.
This article explains:
- what the A2 pulley does
- why it fails
- what movements stress it
- how joint angles influence its load
- how to protect it during training
Simple, clear, climber-focused.
1. What the A2 pulley actually is
The A2 pulley is a strong ligament ring that wraps around the base of the finger.
Its job is simple:
It keeps your flexor tendons tight to the bone when you pull.
If it didn’t exist, the tendons would “bowstring” outward, and you’d lose:
- strength
- precision
- control
- finger stability
The A2 is:
- the largest pulley
- the stiffest
- the one that takes the highest force
- the one most likely to get injured
It’s the Achilles tendon of finger climbing.
2. Why the A2 takes so much stress in climbing
Three reasons:
Reason 1 — Its position
It sits at the base of the finger, where bending force from the PIP (middle knuckle) is redirected.
This creates a high-angle redirect, similar to a tight quickdraw angle.
More bend = more stress.
Reason 2 — Crimp mechanics
In a crimp, the PIP bends sharply.
This bends the force line, which concentrates load at the A2.
Crimp = high redirect angle = high A2 tension.
Reason 3 — Small holds
On small edges:
- PIP bends more
- DIP unrolls
- force line becomes more angular
→ A2 receives amplified tension.
The smaller the hold, the sharper the angle, the higher the stress.
3. How the A2 fails: the three stress mechanisms
Understanding failure modes helps you avoid them.
Mechanism 1 — Sudden angle collapse
Examples:
- foot slips
- sudden deadpoint catch
- move goes wrong
- swing you didn’t control
The PIP collapses → force line spikes → A2 overloads.
Mechanism 2 — Fatigue-driven instability
When tired:
- DIP unrolls
- PIP collapses slightly
- grip becomes angular
- the force redistributes unpredictably
You don’t feel this until it’s too late.
Mechanism 3 — Too much intensity, too soon
Tendon adaptation is slow.
Pulley tolerance is slower.
Big jumps in:
- load
- edge size
- volume
- session frequency
…outpace structural adaptation.
A2 fails when the rate of load increase > rate of tissue adaptation.
4. What “A2 stress” actually feels like (climber language)
Climbers describe A2 overload in three ways:
- “sharp spot on the inside of the finger”
- “pinch feeling at the base”
- “tightness after training on small holds”
- “feeling of the tendon being too close to the skin”
This is NOT “normal training soreness.”
It is a mechanical warning signal.
A2 stress feels sharp.
Tendon stress feels deep.
Muscle fatigue feels dull.
Know the difference.
5. How training choices affect A2 load
Everything you choose in training affects A2 tension.
Grip
- Half crimp = safe balance
- Open hand = smooth load path
- Full crimp = highest A2 stress
Hold size
- 15–22 mm = safe training range
- <10 mm = steep load curve (not for strength training)
Angle stability
Stable PIP + DIP = safer A2 loading
Angle drift = amplified redirect force = unsafe
Load progression
2–5% increases are safe
10% increases are dangerous
“feeling good today” increases are trap triggers
6. The two myths that destroy A2 pulleys
Myth 1 — “Stronger fingers protect the A2”
No — stronger tendons load the A2 more.
What protects the A2 is:
- stable angles
- controlled progression
- hold size selection
- consistent volume
- not ego
Myth 2 — “I’ll crimp less, that’s safer”
Not always.
Half crimp on a safe edge
= safer than
open hand on a tiny edge
The edge dictates the angle → angle dictates A2 stress.
7. How to protect your A2 (concrete rules)
Here zijn de fundamentele regels:
- Train on stable edges (15–22 mm)
- Use half crimp, not full crimp
- Progress load slowly (2–5% per step)
- Stop the set if your DIP unrolls
- Never increase weight two sessions in a row
- Avoid small edges when tired
- Respect next-day warning signals
- Build long blocks, not random PR attempts
- Never change edge AND load in the same session
These rules are the difference between progression and injury.
Putting it all together
The A2 pulley:
- redirects force
- stabilizes the finger
- takes the highest stress
- fails when angles drift
- hates sudden load changes
- hates small edges under fatigue
- adapts slowly
- demands respect
If you understand the A2, you understand finger safety.