The Physics of Muscles

All of us biology people (mostly the pre-med and pre-vet) like to complain about the fact that we have to take physics. “We’re never going to use this!”, we say. Sure, physics teaches us good problem-solving skills, but so do many other classes. However, much to our dismay, many phenomena in our body are in fact controlled by processes that we have already learned about with Professor Metzler.

One day while I was sitting in vertebrate physiology, which I happen to have right after physics, I realized that what I was learning would be perfect for a blog post! Our muscles are a great example of physics at work in the human body. In order to move loads, our muscles must undergo isotonic contractions. In this contraction, the muscle contracts, shortens, and creates enough force to move the load. This force is a tension force created by the contracting of the muscle.

There are also contractions in which the muscle applies force, but there is not movement. This is an isometric contraction. The muscle also contracts in this situation, but it does not shorten, therefore the force created cannot move the load. The tension force in this situation is not great enough to move the 30 kg load, so the load does not move.

Another interesting thing about muscles is the fact that they act like a lever. A lever is a rigid bar that pivots around a point called the fulcrum. Our bones form the lever, and flexible joints form the fulcrum. My textbook compares the human forearm to a fishing pole. At one end of the lever is the fulcrum (the elbow joint for the arm, the part where you grip the rod for the fishing rod), and at the other end is the load (gravity for the forearm, the fish for the fishing rod). Force is applied between the fulcrum and the load, therefore allowing the lever to lift the load.

And, we can even calculate the force that the bicep needs to apply in order to remain in a certain position. Here is the example from my textbook:

So, next time I complain about having to learn physics as a molecular biology pre-veterinary student, I’ll think back to muscles, and know I’ll have to relearn all of this for many different animals in veterinary school.

Source: Human Physiology by D.U. Silverthorn