Basics and Beyond ® Anatomy of the Upper Extremity

Action of Lumbricals:

• May contribute to or help with MCP joint flexion
• Ensures MCP joint flexes prior to the IP joint and allows for grabbing large objects
• Lumbricals are being pulled by the FDP
• Assist with MCP joint flexion; interossei are the primary MCP joint flexors
• Flexor digitorum profundus also contributes to MPJ flexion 
• When MCP joints are stabilized, lumbricals may assist with abduction of the finger
• Contraction of lumbricals will relax the flexor digitorum profundus as it draws the lateral band proximally and pulls the FDP distally
• Lumbricals are primary extensors of the IP joints, irrespective of MCP joint position 
• It is well accepted in the literature that when the lumbrical is contracted, the FDP is relaxed; when the FDP is contracted, the lumbrical is relaxed
• Lumbricals originate from the FDP and insert into the radial lateral band of each finger
• First two lumbricals originate from the FDP of the index and middle fingers (median nerve innervated)
• 3rd lumbrical arises from the FDP of the middle and ring fingers (ulnar nerve innervated)
• 4th lumbrical from FDP of ring and little (ulnar nerve innervated)
• According to electromyographic studies, the lumbricals contribute little to MCPJ flexion.  However, if the interossei are paralyzed the lumbricals can initiate MCP joint flexion.  MCP joint flexion is achieved through the FDS and FDP.  NOTE: the FDS and FDP (extrinsic muscles) flex the IP joints first.  When the PIP/DIP are fully flexed the MCP joint will flex.    

Synergists:

• Dorsal interosseous
• Palmar interosseous
• FDP
• FDS