Lecture: Extensive Code Mechanism and Hand Movement

Interaction Between Extrinsic and Intrinsic Muscles

Key Concepts

• Extensor Tendon: Passes the MCP joint, joins the extensor hood/expansion, splits into central slip and two lateral bands.
• Intrinsic Muscles (Lumbricals, Interossei): Originate from the FDP tendon, help flex MCP joint and extend IP joints.
• Extensor Hood: Structure where the extensor tendon integrates and collaborates with intrinsic muscles.

Mechanics

• MCP Joint Flexion and IP Joint Extension: Intrinsic and Extrinsic Muscle Collaboration
  • Intrinsic Muscles: Pull volarly to assist in MCP flexion and IP extension.
  • Extensor Tendon: Stronger at MCP joint due to splitting into central slip and lateral bands.
  • Resting Position: Fingers slightly flexed due to stronger flexor tendons compared to extensor tendons.

Muscle Actions Without Intrinsic Muscles

• Extensor Tendon vs. Flexor Tendon: Flexor tendons are stronger, causing fingers to remain in slight flexion at rest.
• Intrinsic Muscle Contribution: Helps counteract the pull from flexor tendons to extend the IP joints.

Intrinsic Plus and Minus Positions

• Intrinsic Plus Position: MCP in flexion, IP in extension due to concentric contraction of intrinsic muscles.
• Intrinsic Minus Position: Opposite of intrinsic plus; MCP in extension, IP in flexion.
  • Stretching Intrinsic Muscles: Hand placed in intrinsic minus position to stretch intrinsic muscles.
  • Intrinsic Muscle Failure: Hand defaults to intrinsic minus position, dominated by stronger flexor tendons.