Flexor digitorum superficialis

What it is

A large muscle in the front of the forearm, lying between the superficial wrist flexors and the deep layer. It sends four tendons through the carpal tunnel into the hand, one to each finger.

Its tendons do something remarkable. As each approaches its finger it splits in two, allowing the tendon of flexor digitorum profundus to pass through the gap, and then reunites behind it to attach to the middle bone of the finger. The deep tendon continues to the fingertip.

That arrangement is why the muscles are named superficial and deep despite the deep one ending further along the finger.

What it does

Flexes the middle joint of each finger — the middle knuckle — and assists in flexing the knuckle at the hand and the wrist.

Because it has some independent muscle slips, it gives the fingers a degree of individual control that the deep flexor does not.

What loads it

Gripping anything. Hanging, carrying, pulling, and climbing all load the finger flexors heavily — in most people, grip is the limiting factor in hanging long before the back and arms are.

What is commonly got wrong about it

That grip is a hand muscle. The muscles that close your fingers with force are in your forearm; the ones in the hand are for fine positioning and spreading. This is why forearm size tracks grip work.

Carpal tunnel syndrome as an overuse injury of these tendons. It is compression of the median nerve where it passes through the same tunnel as these nine tendons. The relationship with keyboard use, long assumed, is much weaker in the evidence than the popular account suggests, and persistent hand numbness needs assessment rather than a wrist brace bought online.

That it and the deep flexor can be trained separately. They are loaded together in every grip.