I first heard / read about muscle testing 50 years ago.  Below are a few links I just found, but have only scanned.  In most cases, the site/writer is trying to sell something.  You could also find many YouTube videos, but realize there are different methods, so viewing a few may be helpful.

Muscle testing is a way to tap into the acupuncture meridians of the body, to reveal what product, supplement, course of action is in tune with your body, and/or thoughts. 

These days, many chiropractors, some health / allergy practitioners tend to use it, for limited purposes -- in addition to some holistic MDs.

Obvious case,  one could quickly physically show that sugar (placed under tongue, or even held in hand,) is not "good" for you, while a piece of egg, or steak, or even water, is good

Over the years, I've been patient of holistic MDs whom extensively use muscle testing, and keep 1,000+ vials of various substances for testing purposes.

One trick I learned, is to place a container of say 20+  individual vials on ones chest, while MD tests various organs for weakness.  If a weakness appears, then each of the individual 20 vials would be tested to find the culprit. If not, on to the next container of 20.

I - or more accurately my dogs -- have seen veterinarians whom use muscle testing to diagnose possible illness in one of my Airedale Terriers (same breed for 40+ years.)  Also to test which supplement - and what dosage - would help them.

One can also test on oneself, but working with someone probably more useful.

For senolytics, or any vitamin, supplement, or food, adding muscle testing may be a useful addition as a feedback mechanism.

Simple: hold a fisetin cap in right hand; say "This is good for me," and result will be positive or negative.

More:  hold 3,4,5 or more caps, to query for correct dosage.  Etc.

Another caveat: Everybody (almost every cell) is always changing, and responses will likely change, even one day to the next.  However, it is one of the best mechanisms - and virtually free - to provide useful way to monitor ones health.

Some possible questions to muscle test:

Fisetin is good for me

500 mg of fisetin, 3 times daily is good for me.

I should take fisetin with quercetin

I should take F + Q every 2 (3,4) weeks

My legs feel stronger because of fisetin.

This vitamin -- or group of vits -- (held in my hand) is good to take 2 hours after F + Q

The questions can be almost endless, and very specific.  In some cases, it could  approach just a waste of time, such as:  I should take F + Q at 1:15 pm...etc

Some info -- I have no financial connection with any below:

https://physicaenergetics.com/muscle-testing/

https://amybscher.com/getting-answers-from-the-subsconscious-mind/

https://thefoodcodes.com/muscle-testing-foods/

https://www.naturalbioenergetics.com/articles/muscle-testing-allergies/

https://luvlolaonline.com/muscle-testing-for-food-sensitivities-and-compatibilities/

https://catherinecarrigan.com/5-easy-ways-to-muscle-test-yourself/

Muscle testing for you and your pets:
https://www.animalbliss.com/muscle-testing/