Table of contents

1. Introduction to Anatomy & Physiology5h 43m
2. Cell Chemistry & Cell Components12h 36m
3. Energy & Cell Processes10h 6m
4. Tissues & Histology10h 3m
5. Integumentary System2h 20m
6. Bones & Skeletal Tissue2h 16m
7. The Skeletal System2h 35m
8. Joints2h 17m
9. Muscle Tissue2h 33m
10. Muscles1h 11m
11. Nervous Tissue and Nervous System1h 35m
12. The Central Nervous System1h 6m
- Introduction to the Central Nervous System
  18m
- The Cerebrum
  48m
13. The Peripheral Nervous System1h 26m
14. The Autonomic Nervous System1h 38m
15. The Special Senses2h 41m
16. The Endocrine System2h 48m
17. The Blood3h 22m
18. The Heart3h 42m
19. The Blood Vessels3h 35m
20. The Lymphatic System3h 16m
21. The Immune System14h 37m
22. The Respiratory System3h 20m
23. The Digestive System2h 5m
24. Metabolism and Nutrition4h 0m
25. The Urinary System2h 39m
26. Fluid and Electrolyte Balance, Acid Base Balance37m
27. The Reproductive System2h 5m
28. Human Development1h 21m
29. Heredity3h 32m

8. Joints

Joint Movements

Anatomy & Physiology

8. Joints

Joint Movements

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Problem 20

Textbook Question

Of the following actions, the one that illustrates that of a second-class lever is
(a) Knee extension
(b) Ankle extension (plantar flexion)
(c) Flexion at the elbow
(d) None of these

Verified step by step guidance

Understand the concept of a second-class lever: In a second-class lever, the load is positioned between the fulcrum and the effort. This arrangement allows the effort to move a larger load with less force, and it is commonly seen in actions where strength is prioritized over speed or range of motion.

Identify the anatomical components involved in lever systems: The fulcrum is the joint, the load is the body part being moved or resisted, and the effort is the muscle force applied to move the load.

Analyze each option: For (a) knee extension, the quadriceps muscle applies effort to extend the knee joint, but this does not fit the second-class lever arrangement. For (b) ankle extension (plantar flexion), the fulcrum is the ball of the foot, the load is the body weight, and the effort is applied by the calf muscles, which matches the second-class lever arrangement. For (c) flexion at the elbow, the biceps muscle applies effort to flex the elbow joint, which is a third-class lever, not second-class.

Determine which action fits the second-class lever arrangement: Based on the analysis, ankle extension (plantar flexion) is the action that illustrates a second-class lever because the load (body weight) is between the fulcrum (ball of the foot) and the effort (calf muscles).

Conclude the reasoning: The correct answer is (b) ankle extension (plantar flexion), as it demonstrates the characteristics of a second-class lever system in the human body.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Levers in Biomechanics

In biomechanics, levers are simple machines that amplify force or speed. They consist of a rigid bar (the lever arm) that pivots around a fixed point called the fulcrum. Levers are classified into three types based on the relative positions of the effort, load, and fulcrum, which affects their mechanical advantage and function in human movement.

Second-Class Lever

A second-class lever is characterized by the load being positioned between the effort and the fulcrum. This arrangement allows for a mechanical advantage, enabling the effort applied to lift a heavier load with less force. Common examples in the human body include actions like standing on tiptoes, where the ball of the foot acts as the fulcrum, the body weight is the load, and the effort is applied by the calf muscles.

Knee and Ankle Movements

Knee extension and ankle extension (plantar flexion) are movements that involve specific lever systems in the body. Knee extension primarily functions as a third-class lever, where the effort is applied between the load (body weight) and the fulcrum (knee joint). In contrast, ankle extension acts as a second-class lever, where the load (body weight) is between the effort (calf muscles) and the fulcrum (ball of the foot), illustrating the mechanics of second-class levers.