Extensive Code Mechanism: Interaction of Extrinsic and Intrinsic Muscles for Hand Movement

Overview of Extensor Tendon Mechanism

• Extensor tendon passes the MCP (Metacarpophalangeal) joint and joins the extensor hood or expansion.
• Splits into:
  • Central slip
  • Two lateral bands
• Radial side interactions:
  • Lumbrical muscle
  • Palmar and dorsal interossei muscles

Role of Intrinsic Muscles

• Intrinsic muscles (lumbricals and interossei) help flex the MCP joint.
• Originates from the FDP (flexor digitorum profundus) tendon.
• When contracting:
  • Extends the IP (Interphalangeal) joints
  • Flexes the MCP joint
• Extensor tendon is stronger at the MCP joint due to splitting into central slip and lateral bands.

Interaction of Extensor and Intrinsic Muscles

• MCP flexion leads to the line of pull from lumbricals and interossei moving to volar (palm) side.
• Helps in extending the IP joints.
• Without intrinsic muscles:
  • Extensor tendon vs. flexor tendons
  • Flexor tendons are stronger, leading to fingers being slightly flexed at rest.

Detailed Mechanisms

• Lumbricals and interossei help extend IP joints, countering flexor tendons.
• Extensor tendons mainly work on MCP joints, while lumbricals and interossei act on IP joints.

Intrinsic Plus and Intrinsic Minus Positions

• Intrinsic Plus Position: MCP in flexion, IP in extension.
  • Intrinsic muscles are working/concentric contraction.
• Intrinsic Minus Position: MCP in extension, IP in flexion.
  • Used to stretch intrinsic muscles.
  • When intrinsic muscles are weak, hand defaults to this position due to stronger flexor tendons.

Practical Implications

• Understanding these mechanisms is crucial in therapies and exercises aimed at improving hand functions.
• Proper stretching and strengthening of intrinsic muscles can prevent hand deformities.