FDP (flexor digitorum profundus) and FDS (flexor digitorum superficialis) are the two long tendons that run from the forearm, through the palm, into each finger, and bend it. FDS attaches at the middle knuckle. FDP runs further, to the fingertip — it’s the only tendon that can bend the very tip of the finger.
Both tendons are held tight against the finger bones by the pulley system, especially the A2 and A4 pulleys. That’s what lets tendon force bend the finger instead of lifting away from the bone.[1]
These tendons can also get irritated from heavy crimping (tenosynovitis) — a different problem from a torn pulley, even though both cause finger pain. The anatomy here is well mapped out. What is not settled is any peptide effect on flexor tendon healing — no human trial supports that claim for either tendon.