Lecture Notes: Internal Force in Human Movement

Overview of Human Movement

• Human body segments move in a circular pathway.
• Combined joint movements create a linear path for carrying objects from point A to B.

Features of Human Movement

• Body segment movement is circular, changing orientation along the path.
• Linear movement keeps orientation constant.
• Distal segments have more displacement than proximal segments in rotary movement.
• Displacement stays equal in linear movement.

Mechanical Advantage

• Mechanical advantage: tendency of a force to cause rotation relative to its application point.
• Example: seesaw, where position impacts leverage.
• Torque (tendency to rotate) is influenced by the distance (moment arm) where force acts.
• Torque = Force × Moment Arm

Example: Torque Force

• Brachialis muscle example: torque varies with elbow angle.
• Different elbow angles change the moment arm, affecting required muscle force for constant torque.
• Isometric, concentric, and eccentric contractions influence muscle force and torque.

Lever Systems in the Human Body

• First-class lever: pivot point between load and resistance (e.g., seesaw, doorknob, car steering wheel).
  • Human body example: spine acts as pivot point, head weight is the load, neck extensors provide resistance.
• Second-class lever: moment arm for force is longer than for load (e.g., bottle opener, standing on toes).
  • Human body example: standing on toes, pivot point is the toe, muscle force acts farther out.
• Third-class lever: mechanical advantage favors load over applied force (common in human body).
  • Human body example: biceps insertion close to the elbow, requires more force for movement.

Importance of Lever Systems

• Third-class lever systems allow for greater range of motion but require more muscle force.
• Example: biceps insertion close to the movement axis enables more joint displacement.
• Patella increases mechanical advantage for quadriceps.
• Pulley systems in the hand maintain tendon position for functional range of motion.

Conclusion

• Majority of body movement is rotary to increase joint displacement and range of motion.
• Understanding leverage and torque is crucial for explaining human movement and muscle function.
• Lever systems in the body optimize mechanical advantage for various tasks.