Lever Systems
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By definition, lever systems are rigid bars that, when force is applied, move resistance about a fulcrum or axis of rotation. In most cases, particularly in the arms and legs, lever systems are how we move; your muscles create the applied force to move your bones (rigid bar) by way of your joints (fulcrum). There are three possible types of lever systems that are simply named class 1, class 2, and class 3.
Class 1 lever systems
In class 1 lever systems, the fulcrum sits between the force and the resistance. Because of this setup, the movement direction of the resistance is opposite the movement direction of the force. A commonly recognized example of this lever system is a seesaw on a children’s playground.
The most prominent exercise-specific example is for the hamstrings on the hip-hinge as performed in deadlift variations. During the hip hinge, the hamstring muscles pull downwards on the posterior, inferior aspect of the pelvis; the hip joint is the fulcrum, and your trunk and the weight you are holding (which are anterior to your hip joint) move upwards.
Other examples of anatomical class 1 lever systems include extending your neck (e.g., looking up), side-bending your trunk, and abducting your hip via the gluteus medius and gluteus minimus. In each of these cases, the agonist muscle is generating a force in one direction, and the target body part moves in the opposite direction.
Advantages of class 1 lever systems
The primary benefit of a class 1 lever system is changing the direction of the force required to move the resistance. In the children’s seesaw example, only one side can be on the ground at any given time. To lift one side off the ground using the lever system, you would need to pull downwards on the elevated side.
In most anatomical examples (assuming anatomical position), the muscles are pulling downwards on their bones to pull another part of the body upwards,* and thus the primary benefit for human functioning is actually in maintaining upright posture. In a unique example, the gluteus medius and gluteus minimus pull inwards on the top of the thigh, above the hip joint; as a result, the thigh (below the hip joint) moves outwards in a movement called abduction.
Is there a mechanical advantage?
In physics, the mechanical advantage of a given machine is the ratio of the force produced by the machine to the force applied to it, and this is dictated by the length of the force lever compared to the length of the resistance lever (see image below).
A positive mechanical advantage means the force applied to the system is amplified to overcome the resistance; in this example, if an object weighed 20lbs, it would take fewer than 20lbs of force to move it. A negative mechanical advantage (represented as a decimal) means the force applied to the system is reduced; the same 20lbs object would require greater than 20lbs of force to move.
In human anatomy, a class 1 lever system mechanical advantage does not exist.
In terms of the basic physics of a class 1 lever system (not specific to anatomy), it is possible for a mechanical advantage to exist. If the force lever measures longer than the resistance lever, this would be a mechanical advantage. Picture using a hammer to remove a nail. This is another example of a class 1 lever system, and the handle of the hammer is longer than the back end of the hammer head providing a notable mechanical advantage.
Class 2 lever systems
In class 2 lever systems, the resistance is between the fulcrum and the force. The most common example of this lever system is a wheelbarrow.
In the human body, the only common exercise example of a class 2 lever system is the calf muscles acting on the ankle joint. In this example, the forefoot becomes the fulcrum, the resistance is your body weight plus any added resistance, and the force is the Achilles tendon via the calf muscles pulling the heel upwards.
Because the class 2 lever system provides a guaranteed mechanical advantage for overcoming resistance, the calf muscles are only muscles in the human body that produce exceptional force on their own. No other single muscle in any position has such a high ceiling for force production.
Advantage of class 2 lever systems
The benefit of a class 2 lever system is that there will always be a mechanical advantage; in other words, less force will be required to move a given resistance (the resistance is always perceived to be lighter than it really is).
Class 3 lever systems
In class 3 lever systems, the force is between the fulcrum and the resistance.
A real-world example of the class 3 lever system is pulling broom. Your hand on the top of the broom is the fulcrum, your hand pulling the shaft of the broom is the force, and the broom head dragging on the floor is the resistance.
In the human body, the majority of your extremity (arms and legs) muscles and joints are configured in a class 3 design.
Advantage of class 3 lever systems
This advantage of a class 3 lever system is achieving distance and/or generating velocity. With the force between the fulcrum and resistance, both the distance traveled and velocity of the end-point (often where resistance is added) are greater than the distance traveled and velocity of the applied force.
In anatomy, this allows for large ranges of motion of the arms and legs and/or the ability to generate high velocity (e.g., a professional baseball pitcher throwing a ball 100+ mph).
Unfortunately, as opposed to the class 2 lever system noted in the ankle joint, the class 3 lever systems of the human body exhibit a negative mechanical advantage. Using the example in the image above, if you held a 10lbs dumbbell in your hand, your muscles would need to generate significantly greater than 10lbs of force to overcome and move the weight.
This means that the majority of your muscles are not particularly strong. This isn’t an insult to you or anyone else but rather a result of the laws of physics. If you cannot accept this fact, then Art of Anatomy is not the right program for you.