Muscle Minute TUESDAY – Meet Flexor Digitorum Profundus

The flexor digitorum profundus (FDP) is a muscle we don’t think about a lot, but it gets used more than we know and it lies at the heart of a very common problem, carpal tunnel syndrome. Traversing through the carpal tunnel (resected in picture above), are the four tendons of FDP, flexor digitorum superficialis (FDS) and the median nerve. Irritation of any of these structures may cause swelling and limit the gliding potential of the tendons in this space, causing greater resistance and requiring more effort with finger flexion. Let’s take a closer look.
Origin | upper 3/4 medial surface of ulna, interosseous membrane |
Insertion | base of distal phalanges |
Action | wrist flexion and finger flexion |
Innervation | median n. (C6-T1) and ulnar n. (C8-T1) |
Antagonists | extensor digitorum |
The Real World Flexor Digitorum Profundus (FDP)
Wait. This muscle is innervated by two different nerves? We don’t often talk about the innervation of muscles, but we should. The median nerve innervates the 2nd and 3rd digits and the ulnar nerve innervates the tendons of the 4th and 5th digits. The nerve root derivation is from the lower cervical and upper thoracic area, which means compression in this region may compromise the signal and strength to FDP.
Another interesting thing to note is that strength of FDP decreases from the 2nd to the 4th digits. Optimal length tension states that a muscle activates best when placed at 1.2x its normal resting length. If you place a muscle in a slight stretch it will activate most efficiently. In order for FDP to reach its optimal length tension, the wrist must be slightly extended and the tendons of the 4th and 5th digits must be stretched more.
This is considered in the design of hand and power tools. Notice how the grip of a drill or a hammer is wider at the bottom.

This is to place the 4th and 5th FDP tendons on a slightly greater stretch to improve grip strength.
Here’s a short list of why this matters:
- FDP functions best with wrist extension and 4th and 5th digits slightly stretched
- If wrist extension is limited, it will require more effort for FDP to activate, causing potential overuse scenario
- If FDP is overused, swelling occurs and gliding in the carpal tunnel will be compromised
- If gliding is compromised, increased force is required to grip leading to a cycle of more overuse. Median nerve compression will occur and cause further compromise to FDP
Conventional thinking says: Strengthen your grip by working FDP with gripping exercises
Real World Thinking says: Adequate wrist extension and placing 4th and 5th tendons on a greater stretch will ensure efficient function of FDP and prevent overuse injuries. Considering alignment of C6-T1 may also help improve gripping.
Consider FDP and get a grip.
Because nobody has time to be in pain.
Until next time…

Kind Regards,
MoveWell Academy
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