Finger flexion is primarily achieved through the coordinated action of two main muscles located in the forearm: the flexor digitorum profundus and the flexor digitorum superficialis.
The Primary Finger Flexors:
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Flexor Digitorum Profundus (FDP): This muscle is located deep within the forearm and is the only muscle that flexes the Distal Interphalangeal (DIP) joints of the fingers (the joint closest to the fingertip). It originates from the anterior and medial surfaces of the ulna and the interosseous membrane (the tissue between the ulna and radius). Its tendons extend all the way to the distal phalanges of the fingers.
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Flexor Digitorum Superficialis (FDS): Situated more superficially in the forearm, the FDS flexes the Proximal Interphalangeal (PIP) joints (the middle joint of the finger) and also assists in flexing the Metacarpophalangeal (MCP) joints (the joint where the fingers meet the hand). It originates from the humerus, ulna, and radius and its tendons split before inserting onto the middle phalanges of the fingers.
Summary Table
| Muscle | Location | Primary Action | Joints Flexed |
|---|---|---|---|
| Flexor Digitorum Profundus | Deep Forearm | Flexes distal interphalangeal (DIP) joints | DIP, PIP, MCP (assists) |
| Flexor Digitorum Superficialis | Superficial Forearm | Flexes proximal interphalangeal (PIP) joints | PIP, MCP (primary), DIP (assists indirectly) |
Other Contributing Muscles
While the FDP and FDS are the primary flexors, other muscles contribute to finger flexion to a lesser extent:
- Lumbricals: These small, intrinsic hand muscles assist in MCP joint flexion and PIP and DIP joint extension.
- Interossei: Also intrinsic hand muscles, these assist in MCP joint flexion as well as finger abduction and adduction.
In essence, finger flexion involves a complex interplay of muscles, with the flexor digitorum profundus and flexor digitorum superficialis playing the most crucial roles in bending the fingers.
