FDP vs FDS: What Each Tendon Really Does in Climbing

FDP vs FDS: What Each Tendon Really Does in Climbing

Climbers often talk about “finger tendons” as if there is only one.
In reality, you rely on two different pulling systems:

  • FDP (Flexor Digitorum Profundus)
  • FDS (Flexor Digitorum Superficialis)

They share force, stabilize the finger differently, and get stressed in very different ways depending on grip type, joint angles, and hold size.

This article breaks down their roles with the clarity climbers actually need.

1. The core difference between FDP and FDS

FDP = the deep tendon

  • attaches at the distal phalanx (fingertip bone)
  • controls the fingertip curl
  • active in every climbing grip
  • carries the largest portion of pulling force

FDS = the superficial tendon

  • attaches at the middle phalanx
  • stabilizes the PIP joint
  • reduces sudden angle shifts
  • adds extra pulling force only in certain grips

In simple terms:

  • FDP = primary engine
  • FDS = stabilizer + secondary engine

2. FDP’s role in climbing (the real workhorse)

FDP is responsible for:

  • loading small edges
  • fingertip stability
  • force transfer in open hand, half crimp, drag
  • maintaining grip under fatigue
  • “bite” on sharp holds
  • tension during lock-offs and pulls

Because FDP reaches the fingertip, it:

  • sees higher tension
  • compensates when grip angles degrade
  • is heavily overloaded when the DIP unrolls

This is why DIP collapse = FDP overload.

3. FDS’s role in climbing (the hidden stabilizer)

FDS attaches to the middle phalanx, so it does NOT control the fingertip.

Its main functions:

  • stabilizes the PIP joint
  • smooths the force line
  • supports the A2 pulley
  • reduces tendon shift
  • helps resist PIP collapse
  • works harder in the half crimp than in open hand

This is why PIP collapse = FDS overload.

FDS is also the reason half crimp feels more “locked-in” than open hand.

4. How grip type changes FDP/FDS load

Every grip recruits these tendons differently:

Open Hand

  • FDP = dominant
  • FDS = minimal
  • low PIP bend = smoother force
  • high fingertip demand → FDP heavy work

Half Crimp

  • FDP + FDS both active
  • FDS stabilizes the PIP
  • force line is redirected more steeply
  • higher pulley load but more joint stability

Full Crimp

  • FDP = extremely high
  • FDS = high
  • both pulleys and tendons are stressed
  • dangerous if angle collapses

Drag Grip

  • almost pure FDP
  • low redirection angles
  • safer for pulleys
  • but high fingertip tension

If FDP = engine
and FDS = stabilizer
dan bepaalt de grip hoeveel beide bijdragen.

5. Why tendon dominance matters

Most climbers unconsciously develop one of two patterns:

FDP-dominant climbers

  • rely heavily on the fingertip
  • feel strong in open hand and drag
  • struggle with PIP stability
  • higher risk on tiny edges
  • higher DIP overload risk

FDS-dominant climbers

  • feel secure in half crimp
  • stable PIP mechanics
  • lower fingertip precision
  • difficulty with pure open hand
  • great on medium edges (12–22 mm)

Knowing which you are explains:

  • why some grips feel “unnatural”
  • why certain holds feel sharp
  • where your injury risks lie

6. How training influences FDP/FDS balance

Training can shift dominance:

Training that increases FDP dominance

  • repeater volume work
  • drag grip sessions
  • fingertip recruitment
  • small edge hangs
  • high-tension open-hand

Training that increases FDS stability

  • half-crimp isometrics
  • slow controlled loading
  • PIP angle-stability drills
  • mid-edge work (15–20mm)
  • reducing DIP unrolling

The best climbers have:

  • FDP strength
  • FDS stability
    in sync.

7. Why misunderstanding FDP/FDS leads to injury

Common problems:

  • DIP collapse → FDP spike → fingertip pain, distal tendon irritation
  • PIP collapse → FDS overload → A2 stress increases
  • fatigue → one tendon compensates for the other → inconsistent load
  • uncontrolled full crimp → both overloaded at once → worst-case scenario

Good finger training =
controlling which tendon carries which force, at which angle, on which edge.

Putting it all together

FDP and FDS form one integrated pulling system, but they serve different mechanical roles.

  • FDP = main engine (fingertip → forearm)
  • FDS = stabilizer + secondary engine (middle phalanx)

Grip type, joint angle, fatigue, and hold size determine which tendon is loaded more.

Understanding this is essential for:

  • safer hangboard training
  • better grip selection
  • diagnosing finger discomfort
  • building stable, predictable finger strength

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