Demonstration that classical teaching of force of effort for 2nd and 3rd class levers is erroneous

Demonstration that classical teaching of force of effort for 2nd and 3rd class levers is erroneous.

In our first manual, we demonstrated that there was functionally no such thing as a 2nd class lever. We have since learned that there are teachers who still want to use classic instruction because they think that this methodology is still valid for teaching force of effort. We will now demonstrate that this is erroneous teaching.

Note! The secret to the proof will be the application of Benedetti's discovery of the effective lever arm in 1599 as it applies to the principle of rotation (torque).

The difference between 2nd and 3rd class lever systems begin by teaching that levers are structurally defined relative to the placement of the fulcrum (F), force of effort (E) and force of resistance (R).

Classical teaching next moves to emphasizing functional importance by first demonstrating that force of effort is determined by the ratio of the effective effort arm to the effective resistance arm (the Principle of Equilibrium of Rotation).
The first formula taught leading to proof of the above statement is:
Force of effort x Effective effort arm (measured length) = Force of resistance x Effective resistance arm (measured length).
The second formula derived from the above formula is:

Force of effort = Force of resistance x Effective resistance arm Effective effort arm

When the ERA is greater than the EEA, the force of effort will be greater than the force of resistance.

When the ERA is less than the EEA, the force of effort will be less than the force of resistance.

Once the above formulas have been demonstrated, then classical teaching moves back to demonstrating their effectiveness as proof of knowing the difference between a 2nd and 3rd class lever system.

The 2nd class lever system has a shorter ERA than EEA, therefore requires less effort to lift the resistance.

The 3rd class lever system has a shorter EEA than ERA, therefore requires more effort to left the resistance.

Now that you have been shown the functional difference, classical teaching next moves to the human body by illustrating the arm as a 3rd class lever system. This seems to be a safe example because the position of the insertion of the bicep is quite a distance from the site of any resistance (typically the in the hand lifting something).

This seems to be a safe demonstration of proof that 3rd class levers require more effort than a 2nd class lever because anywhere you position the arm, the effective effort arm is shorter than the effective resistance arm

But, no matter how safe this example appears to demonstrate itself in human anatomy, it is still wrong. Let us examine the classic 2nd and 3rd class levers though movement to see just why it’s erroneous.

Note! This illustration applies Benedetti's discovery of the effective lever arms (effort and resistance). Those are defined as the perpendicular distance from their respective line of pull back to the fulcrum.

We put this illustration in as an example to demonstrate from a functional standpoint in the real world of forces, why classic teaching of lever systems is erroneous.

The movement of a 1st class lever system is termed extension when the force of effort (muscle tension) is moving the true resistance arm into 180° alignment with the true effort arm.

The movement of a 1st class lever system is termed flexion when the force of effort (muscle tension) is moving the true resistance arm out of 180° alignment with the true effort arm and shear is increasing in the direction of the force of effort (line of pull).

Note, when we state shear is increasing, we want you to understand that is a general rule, but that every movement must be examined (due to changes in lever systems such as ERA) to actually determine if, in fact, it is.

The movement of a 3rd class lever system is termed flexion when the force of effort (muscle tension) is moving the true resistance arm away from 180° alignment with the true effort arm. This is characterized by shear forces increasing as ion forces decrease.

The movement of a 3rd class lever system is termed extension when the force of effort (muscle tension) is moving the true resistance arm into 180° alignment with the true effort arm. Shear forces are away from the force of effort and would be decreasing.

The definition of extension and flexion is an objective functional definition, not a subjective arbitrary decision.

Extension is when either a 1st or 3rd class lever system is moving the TRA into 180° alignment with the TEA. Compression forces increase and shear forces decrease.

Flexion is when either a 1st or 3rd class lever system is moving the TRA out of alignment with the TEA. Shear forces increase and ion forces decrease.