The most common lever systems in the body are those that have the applied force between the fulcrum and the resistance. These are called __________ levers.

• A. third-class
• B. first-class
• C. second-class
• D. fifth-class
• E. fourth-class

Answer: (A)

Verified step by step guidance

1. Understand the concept of levers in the human body: Levers are simple machines that help in movement and are classified based on the relative positions of the fulcrum, effort (applied force), and load (resistance).
2. Identify the three classes of levers: First-class levers have the fulcrum between the effort and the load, second-class levers have the load between the fulcrum and the effort, and third-class levers have the effort between the fulcrum and the load.
3. Recognize that in the human body, third-class levers are the most common. In these levers, the applied force is between the fulcrum and the resistance, which allows for greater speed and range of motion.
4. Consider examples of third-class levers in the body: The biceps brachii muscle acting on the forearm is a classic example, where the elbow joint acts as the fulcrum, the biceps apply the force, and the weight of the forearm and hand is the resistance.
5. Conclude that the correct answer to the problem is 'third-class levers' as they are the most common lever systems in the body with the applied force between the fulcrum and the resistance.