IS IT SIMPLE TO LIFT?
A lever can be defined as "a rigid bar resting on a pivot, used to help move a heavy or firmly fixed load with one end when pressure is applied to the other." or a "simple tool to overcome resistance". In other words, levers are tools used to lift weights and loads.
Items we use on a daily basis most likely function while based on a lever of sort. Some levers promote faster movement speed at the expense of energetic efficiency, while others promote energetic efficiency at the expense of faster movement speed. Some levers do not promote speed nor energetic efficiency.
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In order to identify the type of lever being used, we use letters to represent key elements and components of the lever. While there is more than one set of letters used to represent levers, we will use F to represent the application point of the force; A to represent the axis that is allowing movement to occur; and W to represent the point of application of the resistance/load needed to be overcome in order for movement to occur.
Naturally, three possible levers exist represented by the letter sequences AFW, AWF, and FAW. The same levers can be written in the opposite direction as WFA, FWA, and WAF. The order in which the letters appear within the name of the lever, represent the relative locations compared to each other in reality. Class I levers are organized where the axis is located between the application of force and the application of resistance/load (FAW or WAF). Class II levers are organized where the resistance is located between the force application and the axis (AWF or FWA). Class III levers are organized where the force application is located between the resistance application and the axis (AFW or WFA).
The closer the force application is to the axis, the more the lever promotes movement speed over energetic efficiency. The closer the resistance application is to the axis, the more the lever promotes energetic efficiency over movement speed. If the distance between the force application and the axis is the same as the distance between the resistance application and the axis, the lever promotes movement speed and energetic efficiency to the same extent.
Applying what we know so far to the body, we will use F to represent where the skeletal muscle's tendon pulls on the bone it connects to. We will use A to represent the joint within the body that allows the movement being analyzed. Finally, we will use W to represent where the resistance to the skeletal muscle's work is applied to the body.
A lever's mechanical and energetic efficiency can be calculated or estimated using the knowledge of two "arms" that are created. The effort arm is created by multiplying the force (F) by the distance between the force application point and the axis (D1). The resistance arm is created by multiplying the resistance (W) by the distance between the resistance application point and the axis (D2). Thus, FD1 represents the effort arm of the lever, while WD2 represents the resistance arm of the lever.
The mechanical advantage of a lever represents it mechanical efficiency, as it related to how much energy does the lever save or waste. Levers promoting speed waste more energy than others. The mechanical advantage (MA), also termed ideal mechanical advantage (IMA), equals the effort arm divided by the resistance arm. Accordingly, IMA of a lever = FD1 / WD2. An IMA of less than 1.0 (0.0 - 0.99) represents a lever that promoted speed over energetic efficiency (AFW or WFA). An IMA of 1.0 exactly represents a lever that does not promote speed nor energetic efficiency (WAF or FAW where the distance to the axis is the same for F and W alike). An IMA greater than 1.0 (1.01 and greater) represents a lever that promotes energetic efficiency over movement speed.
Human evolution teaches us that over time, the vast majority of skeletal muscles in the human body are class III levers, meant to prefer faster movement over energetic efficiency. Knowing the rules of nature as described in one of my earlier introductory blog posts, movement is crucial to survival as the number one goal in nature. Faster movement translate to better hunting and/or better chances of avoiding being hunted, explaining the evolutionary "choice" to have the majority of lever be class III.
Let's give an example for each type of lever that people might use, and then another example for each lever type as represented in the human body.
The seesaw used in playgrounds is a classic class I lever (FAW or WAF).
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The wheelbarrow used to carry loads is a classic class II lever (AWF or FWA).
The use of a baseball bat creates a class III lever.
Now, let's give examples of the three levers as they apply to the human body.
The biceps muscle creates a class III lever (AFW), promoting movement speed.
The gastrocnemius and soleus muscles create a class II lever (AWF), promoting energetic efficiency.
Multiple extensors of the back of the neck (skull) use a class I lever (WAF) to lift the head upwards by tilting it backwards.
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