The A3 & A4 Pulleys: Small Structures, Big Influence
Climbers know the A2 pulley well, but its two supporting pulleys — A3 and A4 — quietly decide whether your grip feels secure, smooth, or unstable.
They rarely rupture, but they strongly influence:
- force distribution
- grip stability
- tendon alignment
- how “sharp” or “smooth” a hold feels
- how training loads translate to the finger
This is the simple, climber-focused breakdown.
1. What the A3 and A4 pulleys actually do
Think of the finger like a rope running along a curved path.
- The A2 pulley is the strong primary anchor.
- A3 and A4 are smaller anchor points that keep the rope (tendon) aligned so force stays smooth.
Their job:
They prevent tendon lift-off and keep the force line centered.
This matters because even tiny shifts in tendon position change:
- how force is distributed
- how stable the grip feels
- how pulleys share load
A3 and A4 don’t produce strength —
but they make strength usable and safe.
2. A3: the stabilizer of the PIP joint
Location: directly over the PIP joint (middle knuckle).
Role: stabilizes the tendon when the PIP bends.
A3 contributes to:
- consistent PIP bending angle
- smooth load redirection
- reduced tendon “bounce” under load
- supporting A2 during hard crimping
When does A3 get stressed?
- deep crimping
- PIP collapsing under fatigue
- unsteady grip on small edges
- sudden slips or dynamic catches
- long sessions that degrade technique
A3 rarely fails first —
but it often becomes irritated when A2 is overloaded.
3. A4: the pulley that controls fingertip mechanics
Location: mid-finger, close to the DIP joint (fingertip joint).
Role: prevents the tendon from lifting as the DIP curls.
A4 contributes to:
- secure fingertip contact
- stable drag and open-hand positions
- smooth force transfer to the fingertip
- preventing DIP unrolling under fatigue
When A4 struggles:
- the grip feels “slippy”
- the fingertip collapses
- load feels sharp rather than smooth
- the drag grip becomes unstable
A4 rarely ruptures —
but irritation is common on tiny edges and high-repetition sessions.
4. Why A3 & A4 matter, even if they rarely rupture
Because they influence quality rather than capacity.
When A3 & A4 are functioning well:
- tendon movement is clean
- load spreads evenly
- the grip feels smooth
- angles stay predictable
- A2 load is reduced
When they don’t function well:
- forces become uneven
- the tendon shifts microscopically
- grip feels sharp or insecure
- A2 ends up taking more load
- small edges feel “harsh”
This is the climber-familiar sensation:
“My finger feels off, but I can’t explain why.”
That’s A3/A4 instability.
5. What stresses A3 & A4 (climber-language)
1. Tiny edges (<10 mm)
→ DIP straightens
→ tendon tension increases
→ A4 works harder to control alignment
2. High-volume repeaters
→ increased tendon glide
→ friction rises
→ A3/A4 irritation
3. Poor warm-ups
→ tendons glide poorly
→ pulleys resist instead of guide
4. Joint angle drift
→ especially DIP unrolling
→ A4 stress spikes
5. Fatigue
→ PIP collapses
→ A3 catches more load
→ grip stability deteriorates
These are not “injuries” — they’re mechanical warning signs.
6. How to protect A3 & A4 (simple rules)
- Keep the DIP slightly curled → stabilizes A4
- Avoid small edges when tired → prevents A3/A4 overload
- Prioritize angle stability → reduces unpredictable load
- Use half-crimp for strength work → safer tendon alignment
- Increase load slowly (2–5%) → lets pulleys adapt
- Avoid extreme drag on very small edges → high A4 stress
- Warm up longer on cold days → improves tendon gliding
Most of these are already built into your Finger Strength training structure.
Putting it all together
A3 and A4 don’t produce force —
but they ensure the force line stays smooth and safe.
- A3 stabilizes the PIP
- A4 stabilizes the DIP
- Both prevent tendon shift
- Both reduce A2 stress
- Both keep grip mechanics efficient
They fail rarely, but they matter constantly.