Physics of Fitness Fridays - Body Levers
Did you know that your musculoskeletal system is composed of levers? Today I’ll talk about this simple machine, how they work in your body, and why certain joints and muscles are the type of lever they are…
A lever is a simple machine that consists of a fulcrum, or pivot, a weight that you are trying to move, and the force or effort that you apply to move that weight.
As we learned last week, when you apply a force around a pivot point, you are producing a torque. We’ll assume that we are in equilibrium and all of the forces are balanced like so. If the fulcrum is the pivot, the balance of torques requires that the Weight times its lever arm r is equal to the Effort applied times its lever arm R or:
W*r = E*R —>
E = W*(r/R)
There are actually three classes of lever; they are determined by which of the three elements - fulcrum, weight or effort - is in the middle. For a class I lever, as we saw above, the fulcrum is in the middle. A see-saw is an example of a class I lever. For class II levers, the weight is in the middle; wheelbarrows come to mind. Finally, for a class III lever, the effort or force applied is in the middle. A baseball bat is a common example.
Your body has all three types of levers in it. Class I levers are the least common; an example is the muscles that attach at the neck and pull your head to nod up and down. A common class II lever example is your calf muscles; they raise the weight of your body while pivoting on your toes. Your tricep is another Class II lever example.
Class III levers are the most common in the body. Some examples include your bicep and hamstring muscles.
When discussing torques in a lever system, we often talk about mechanical advantage. Mechanical advantage refers to how much effort is applied relative to the weight that you are trying to move. For a class I lever, the advantage depends on the location of the fulcrum relative to the weight and effort. If the fulcrum is equidistant from both, the effort required is equal to the weight. If the fulcrum is closer to the effort, then the effort required is more than the weight. If the fulcrum is closer to the weight, the opposite occurs.
For a class II lever, the weight is ALWAYS closer to the fulcrum. Therefore, the effort applied is always LESS than the weight of the object and the lever is said to have a mechanical advantage.
For a class III lever, the opposite occurs; the weight is ALWAYS farther from the fulcrum; therefore, the effort required is greater than the weight and the lever is at a mechanical disadvantage.
Since class III levers are the most common in the body, let’s look at the example of the bicep muscle. In this example, the fulcrum is your elbow joint. The weight lifted is the weight of the dumbbell plus the weight of the arm itself; we’ll set that to 20 lbs total. The distance of this total mass —ie, the center of mass— we’ll set to 35cm from the fulcrum.
The distance from the bicep to the fulcrum is determined by the insertion point, or attachment of the bicep muscle to the bone; a typical value is around 5cm.
So if we balance the torques and solve for the effort, we find that for a 20 lb weight, the bicep needs to apply 140 lbs of effort, a factor of seven higher! Yowza! That’s basically the weight of a human by itself!
You may be asking yourself…”wait! You said that there are more class III levers in the body than any other type. Why would the body evolve to be so…inefficient?”
The answer is that although these muscles aren’t efficient in effort, they do provide an advantage in speed and range of motion. In the bicep example, a relatively small bicep contraction results in the moving the weight and your arm through 90 degrees of motion.
So class III levers are pretty useful, and are what makes the human body so powerful.
Your body is a machine. An inefficient one, but a powerful one. And if anyone says you’re not strong, just remember that the balance of torques means you’re lifting significant portions of your body weight, even when the weight you’re lifting is relatively light. I hope that makes you feel like the superhero you are!