Lecture Notes: Internal Force and Human Movement

Human Movement Overview

• Movement Pathways: Body segments move in circular pathways
  • Combined joint movements create linear paths
• Rotary vs Linear Movement: Key differences
  • Rotary: Orientation changes (e.g., pointing up to pointing right)
  • Linear: Orientation remains constant

Concepts of Force and Movement

• Newton's Second Law: Displacement varies by segment
  • Distal segments have more displacement in rotary movement
  • Displacement remains equal in linear movement
• Mechanical Advantage: Key physics concept
  • Tendency of a force to cause rotation
  • Example: Seesaw
    • Equal weights on either side; position affects rotation
  • Torque: Force tendency to rotate
    • Torque = Force x Moment Arm (distance)

Example: Brachialis Muscle

• Elbow at 90°: Moment arm perpendicular to axis
  • Different elbow angles affect moment arm length and torque
• Isometric, Concentric, and Eccentric Contractions
  • Isometric: Holding a dumbbell steady, muscle force counteracts extension
  • Concentric: Bicep creates force for elbow flexion
  • Eccentric: Muscle force less than external load, allowing controlled lowering

Lever Systems in Human Body

• First-Class Lever
  • Pivot point between load and resistance (e.g., seesaw, doorknob)
  • Example: Spine as pivot point, head as load, neck extensors as resistance
• Second-Class Lever
  • Longer moment arm for force than for load
  • Example: Bottle opener, standing on toes
• Third-Class Lever
  • Load has mechanical advantage over force
  • Common in human body
  • Example: Bicep muscle insertion close to elbow joint
    • Requires more force due to shorter moment arm for muscle compared to load
• Functional Benefit: Allows greater range of motion despite requiring more force

Human Body Structural Examples

• Patella (Knee Cap)
  • Increases mechanical advantage for quadriceps
  • Without patella, more force required for same torque
• Pulley System in Hand
  • Keeps hand flexor tendons in place
  • Ruptured pulley system reduces range of motion

Key Takeaways

• Rotary Movement: Allows greater joint displacement and range of motion
• Lever Systems: Different classes affect force and movement efficiency
• Understanding mechanical advantage and torque is crucial for explaining human movement and designing assistive technologies