The flexor digitorum superficialis (FDS), formerly known as the flexor digitorum sublimis, is the largest of the extrinsic flexors of the forearm. It forms the intermediate muscle layer between the superficial and deep muscle groups of the forearm. It contains four digital components, with a tendon that inserts onto each corresponding finger. The flexor digitorum superficialis is the primary flexor of the proximal interphalangeal (PIP) joints of the middle phalanges, but it also assists in flexion of the metacarpophalangeal (MCP) joints.
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