Finger Structure for Climbers: Bones, Tendons & Pulleys (Simple Overview)

Finger Structure for Climbers: Bones, Tendons & Pulleys (Simple Overview)

Most climbing injuries happen in the fingers — but very few climbers know how their fingers actually work.
You don't need medical knowledge. You just need a clear, climber-oriented understanding of:

  • the bones
  • the flexor tendons
  • the pulleys
  • how force travels through the finger
  • and why some structures fail before others

This is the foundation for understanding every training decision you make.

1. The basic architecture: three bones, two main joints

Each finger (except the thumb) has:

  • 3 bones (phalanges)
  • 2 main bending joints (PIP + DIP)
  • 1 base knuckle (MCP)

To keep it practical:

  • PIP = the big middle bending joint
  • DIP = the small fingertip joint
  • MCP = the base knuckle where your finger meets your hand

When you crimp, the PIP takes the biggest bending angle.
When you drag or open-hand, the DIP takes more curl.

Understanding these angles matters because they determine where force goes.

2. The flexor tendons: the engines of finger strength

Your fingers don't have muscles inside them.
All finger flexion comes from muscles in the forearm.

Two main tendons run through each finger:

  • FDP (flexor digitorum profundus) — the deep tendon that attaches to the fingertip
  • FDS (flexor digitorum superficialis) — the middle tendon that attaches near the PIP

In simple terms:

  • FDP = creates most pulling force
  • FDS = stabilizes the finger and helps share load

When you hang on small holds or train finger strength, both tendons load heavily, especially the FDP.

3. The pulleys: the structures climbers injure the most

Pulleys are small ligament rings that keep the flexor tendons tight against the bone.

Think of them as:
→ loops that keep the rope (tendon) in place against the pole (bone).

Without pulleys, your tendons would “bowstring” outward when you pull.

The key pulleys for climbers:

  • A2 → the big one at the base of the finger (most commonly injured)
  • A3 → smaller, near the PIP joint
  • A4 → near the middle of the finger

Why injuries happen here:

  • they take very high force in crimp positions
  • angles change tendon pressure
  • small mechanical mistakes multiply stress

This is why pulley knowledge is essential for training safely.

4. How force travels through the finger (climber version)

When you pull on a hold, the force doesn’t just go into the fingertips — it travels through a chain:

hold → DIP → PIP → A2 pulley → tendon → forearm muscles

Important translation:

  • Small edge = higher tendon and pulley force
  • More PIP bend = higher A2 stress
  • More DIP curl = higher FDP demand
  • Unstable angle = inconsistent force distribution

This is exactly why training models must be stable and predictable.

5. Why finger structure matters for training

Every good training principle comes from anatomy:

  • Short hangs (7–10 sec) → high tendon load, low metabolic fatigue
  • Stable joint angles → consistent pulley load
  • Same grip every session → repeatable stress
  • Small load increases → allow collagen remodeling
  • Edge size between 15–22mm → safe mechanical range

If you know how your finger is built, you instantly understand why all previous training rules exist.

Putting it all together

The finger is a simple but high-stress structure:

  • 3 bones
  • 2 major joints (PIP, DIP)
  • 2 flexor tendons (FDP, FDS)
  • a series of pulleys that keep everything aligned

Strength comes from the tendons.
Injury risk comes from the pulleys.
Control comes from joint angles.

This basic model is the key to understanding technique, training, and injuries.

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