Levers, Movement, and Biomechanics - Anatomy and Physiology

1. What is the role of a prime mover (agonist) in a movement?

   It provides the major force that produces a specific movement.

2. A muscle inserts 5 cm from the elbow, and the load is held 35 cm from the elbow. What is the mechanical advantage?

   $\frac{5}{35} = \frac{1}{7}$, about 0.14. The muscle must pull with about 7 times the force of the load.

3. Posterior neck muscles tilt the head back at the atlanto-occipital joint. Which class of lever is this?

   First class. The joint (fulcrum) lies between the weight of the face (load) and the pull of the neck muscles (effort).

4. What is a fixator?

   A synergist that holds a bone or a muscle's origin steady so the prime mover can act efficiently.

5. The biceps brachii flexes the elbow while the hand holds a weight. Which class of lever is this?

   Third class. The effort at the radial tuberosity lies between the elbow (fulcrum) and the weight in the hand (load).

6. How is the mechanical advantage of a lever calculated?

   Effort arm length divided by load arm length. A value greater than 1 favors force; less than 1 favors speed and range of motion.

7. What is the trade-off of a third-class lever in the body?

   It works at a mechanical disadvantage, needing more effort than the load, but it moves the load faster and through a greater range of motion.

8. Which class of lever has the fulcrum located between the effort and the load?

   A first-class lever.

9. What is the role of an antagonist muscle in a movement?

   It opposes or reverses the movement; it relaxes and stretches as the prime mover contracts and helps control the motion.

10. Which class of lever is most common in the human body?

    The third-class lever, in which the effort is applied between the fulcrum and the load.

11. A muscle exerts 600 N at a perpendicular distance of 0.05 m from the joint axis. What torque does it produce?

    30 N m. Torque equals force times moment arm: $600 \times 0.05 = 30$.

12. In a body lever system, what acts as the fulcrum, the effort and the load?

    The joint is the fulcrum, the muscle contraction applied at its insertion is the effort, and the body part plus anything it carries is the load.

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