Hand Tendon Anatomy: Flexors, Extensors, and Why It Explains Your Symptoms

Written by Nigel Chua, hand occupational therapist · 21 years treating fingers, hands, wrists, forearms and elbows · Singapore

Your Fingers Have No Muscles In Them

The muscles that bend and straighten your fingers sit in your FOREARM, not your hand. They pull on long cords — tendons — that run down through the wrist and along each finger.

Make a fist and watch your forearm move. That’s where the engine is.

Once you understand that your fingers are operated remotely by cables, most hand symptoms stop being mysterious — including why a problem at the wrist stops a fingertip working, and why a cut in the palm makes a finger useless.

The flexors: the cables on the palm side

Each finger has TWO flexor tendons, stacked one behind the other.

Flexor digitorum profundus (FDP) — the deep one. Attaches to the last bone and bends the FINGERTIP
Flexor digitorum superficialis (FDS) — the superficial one. Splits in two, passes around the deep tendon, and attaches to the middle bone. Bends the MIDDLE joint

The thumb has only one: flexor pollicis longus.

Two tendons per finger is why doctors test each joint separately after a cut. Hold the middle joint straight and bend only the tip — that’s FDP alone. Hold the other fingers down and bend the middle joint — that’s FDS alone.

A finger that still moves after an injury has not proved both tendons are intact. It has proved at least one is.

Pulleys: why tendons don’t bowstring

Running along the palm side of each finger is a series of fibrous bands — the annular pulleys, A1 through A5 — holding the flexor tendons tight against the bone.

Without them, bending a finger would lift the tendon away from the bone like a drawn bowstring, and you would lose enormous power and range.

Two pulleys explain a large share of finger problems:

A1, at the base of the finger. When this thickens, the tendon catches passing through it — that’s trigger finger
A2, over the first bone. The strongest and the one climbers rupture — see climbing pulley injury

The tendon is the cable. The pulley is the guide it runs through. Almost every finger tendon problem is a fault in one or the other.

The extensor mechanism: not a tendon, a net

Straightening is where the hand gets genuinely elegant.

On the back of each finger, the extensor tendon flattens and spreads into a triangular web of fibres called the extensor expansion — and the small muscles inside the hand weave into it.

It divides into three:

Central slip — attaches to the middle bone, straightens the MIDDLE joint. Rupture this and the finger buckles into a boutonnière deformity
Two lateral bands — run down each side, rejoin, and attach to the last bone to straighten the TIP. Rupture this and you get mallet finger

The whole net is a single interconnected system, which is why an injury at one joint changes the position of the joint next door. The slack has to go somewhere.

Holding the extensor tendon centred on the knuckle is a thin sling called the sagittal band. When it tears, the tendon slides into the valley beside the knuckle — boxer’s knuckle.

Where the tendons cross the wrist

On the palm side, nine flexor tendons and the median nerve squeeze through the carpal tunnel. Fixed walls, no room to expand — the reason swelling there produces carpal tunnel syndrome.

On the back, the extensors pass through SIX compartments, each a separate tunnel.

1st compartment — thumb tendons. Inflammation here is De Quervain’s
3rd compartment — EPL, the thumb-lifting tendon, which turns a sharp corner around a bony ridge and can fray after a wrist fracture
6th compartment — ECU, on the little finger side, which can snap in and out of its groove

Numbered compartments sound like trivia. They’re actually a map: tell someone which compartment hurts and you’ve named the diagnosis.

Why tendons heal slowly

Tendons have a poor blood supply and get much of their nutrition passively, from the fluid in their sheaths. That fluid only circulates when the tendon MOVES.

Hence the central paradox of hand rehab: a repaired tendon must move to heal, but move too hard and the repair ruptures.

Every flexor tendon protocol you’ll encounter is an attempt to solve that one equation.

Gliding distance matters too — a flexor tendon travels several centimetres with a full fist. A tendon stuck down by scar loses that travel, and the finger loses range even though the tendon is perfectly intact.

The short version

Muscles in the forearm, cables in the hand
Two flexors per finger, tested separately
Pulleys keep the cables down; A1 catches, A2 ruptures
Extension is a net, not a rope — one break shifts the neighbouring joints
Six compartments at the back of the wrist, each with its own condition
Tendons need movement to heal, and movement is also what threatens the repair

Roughly thirty-odd tendons, none of which you have ever consciously thought about, all cooperating every time you pick up a cup.