Science Buddies: "Ask an Expert"

Hi! Okay so I have chosen this project as the one I will be completing for my Independent Research Project (IRP) for my chemistry class this year. I've already done a bunch a research on it all and what it will take to accomplish this and I am fairly confident that it will be a success. The only thing that is worrying me is a question the my teacher asked me about this project during the interview where I presented the project idea to her. It sort of stunned me when she asked and I'm still not sure how to answer it, so I decided to seek help from the experts on this site. The question is how does the guitar string relate to the actual tendon in the knee? Does the string act exactly as the tendon would in the body? If not, how does it differ? As I thought over this, it made me realize how much this question affects the accuracy of the project itself. Which sorta made me panic a little inside. Please try to explain this to me if you can. Thanks!

Hello Superwholock12,

You've raised some excellent questions! Models (mechanical, mathematical, conceptual, etc.) are often used in experiments to investigate or demonstrate a basic principle. To be successful a model should demonstrate the principle you are investigating. But the model does not have to be a perfect representation of reality. It is a good idea to think through and understand how the model demonstrates the principle.

In your project, you're investigating how the angle of a “knee model” affects the force applied to the quadriceps (represented by the spring) and patellar tendon (represented by the guitar string). A carefully constructed “knee model” like the one described in the Deep Knee Bends project is adequate for this task. That is, you can use this model to adjust the knee angle and measure the resulting spring length to determine the relative force.

The guitar string clearly does not act exactly as the tendon would. For example, the guitar string is not as elastic as a tendon. And the tuning peg tension adjustment used to stabilize the knee model is actually accomplished by actions of muscles present in a real knee that are not represented in the model.

I'm sure that your background research to answer your teacher's question will reveal other differences between the model knee and a real one. My suggestion is to think through how these differences might affect the outcome of your experiment, if at all.

I hope this helps and good luck with your project. Please post again if you have more questions.

A. Norman

A. Norman,
Thank you so much for your help! I think what I'll do is explain those differences in my project and how although it doesn't capture the exact force exerted on the knee, it captures the basic principle and the approximate amount of force exerted. Once again, thank you so much! And I hope you had a very merry Christmas!