Cooking papaya or pineapple together with meat makes it less chewy?

Not exactly. The fruits contain enzymes, papain (papaya) and bromelein, (pineapple) that break down protein. However, enzymes, like other proteins, are denatured by meat. What you might do is either marinate the meat in the fruit juices or place the fruit on top of pieces of meat. Then you would want some sort of strain gauge to determine how much force is required for a knife to cut the meat.

This is not a "myth", it is well known that there is an enzyme in pineapple that tenderizes meat, by attacking the collagen fibers that hold the meat together. This could have evolved as a defence against predators - if an insect eats too much pineapple, it falls apart and dies.

Poking a hole in meat with a sharp object like a fork is not the same action as chewing it. You would be better measuring the force needed to tear a piece of meat into two pieces. Of course getting consistent sized samples of meat to test will be difficult.

It would be easier to just put meat in contact with a slice of pineapple, or soak it in pineapple juice, without actually cooking it. The cooking process introduces a lot more variables into the situation and makes it harder to control the experiment.

I would suggest attaching a force gauge to a fork, so you can measure how hard you need to press to prenetrate a piece of meat of a specific thickness. Tender meat should be easy to penetrate, and require less force.

You may also want to consider what variables you want to investigate. How long do you cook the meat for, what is the ratio of meat to fruit, are there other fruits/foodstuffs/chemicals you want to look at as well (coke or vinegar should give a similar effect), does marinating have an influence, different cuts or species of meat?