so. When people take testosterone, like anabolic steroids or hormone replacement therapy for

instance, it has an effect on their muscle mass and strength. But I was curious as to

why this is a thing. How does this hormone influence muscle growth? Well, like with everything

in the body, the answer is much more complex than a simple cause and effect. In today's

video, we'll look at the interaction between hormones and hypertrophy.

The big, bulky skeletal muscles that we see with the naked eye are made of bundles of muscle fibers. These things

are living cells with nuclei, mitochondria, and active metabolisms. Because of that, they're

dynamic structures — they can respond to stresses placed on them. That might mean growing

in size, or undergoing hypertrophy to meet the demands of weight lifting or your favorite

exercise. That might also mean shrinking, or undergoing atrophy, if the muscle isn't

being used heavily.  Now, to understand how a muscle grows, we need some basic skeletal

muscle anatomy. Within each muscle cell are strands of stiff proteins called myofibrils,

which contain long proteins called filaments arranged into a parallel pattern that lets

them slide back and forth past each other. This sliding mechanism is ultimately what allows

our muscles to produce tension. And if we zoom out to the muscle fiber's surface,

we see tiny cells called satellite cells that hang out on the edge of muscle fibers underneath

the connective tissue. These things are essentially dormant muscle cells, kind of like muscle

stem cells. When our muscles produce a high amount of tension like during resistance exercise,

those myofibrils incur a little bit of damage. This produces an inflammatory response at

the muscle which then promotes healing. That inflammation also wakes up the satellite cells

which prompts them to fuse into the muscle fiber they're attached to. And each satellite

cell has a nucleus with the instructions for making more of those contractile proteins.

And increases in muscle size mostly comes from adding those proteins to muscle. I say

that hypertrophy comes mostly from more myofibrils because we can also grow the connective tissue

within muscles, or increase the volume of water in the muscles, and a few other things. So

then where does testosterone come in? Well, each muscle cell has receptors for androgen

hormones which lets them receive messages from hormones like testosterone. Once testosterone

is in the cell and binds to the receptor, it moves over to the nucleus where we keep

our DNA, then it encourages transcription of certain genes that make more proteins.

If the DNA is a recipe for protein, then testosterone gets the cell to read the recipe and make

more of that protein. Testosterone can also stimulate the release of other growth factors,

and promote the replication and activation of those important satellite cells. So all

in all, testosterone promotes muscle hypertrophy by stimulating the creation of proteins, or

anabolic processes. But it doesn't just build up proteins, it also inhibits protein

breakdown, or catabolic processes. Now, that answered my question from the top of the episode,

but as always, learning one thing prompts ten more questions. Like, if testosterone

helps my muscles grow after training, how can I make testosterone more available for

my muscles? The first thing you can do is have testes. For those wondering, yes, the

name for the hormone comes from testes plus stero - since it's a steroid hormone. If

you do, the vast majority of your testosterone comes from the Leydig Cells in the testes.

If you don't, you still make testosterone in the ovaries and adrenal cortices, albeit,

much less. It doesn't matter what kind of hardware you have. You have a way to make

testosterone. It's hard to say exactly how much less, though. In my research, I saw figures anywhere

from one fifth to one fortieth. So take some of those normative values with a grain of

salt. The second thing you can do is be a young adult. The testes make much more testosterone

after puberty than before but around ages 35 to 40 years old, they start to create 1

to 3 percent less testosterone every year. If your primary way of making testosterone

is through the ovaries and adrenals, the same rules apply. You make more after puberty and

make less in your late twenties, early thirties. What about a weight lifting routine though?

Right after resistance training, the testes do secrete more testosterone, but only for

about 30 minutes. The exercise you do might influence that. According to a review from

2010 and another from 2017, high volume and short rest periods usually induced the largest

acute testosterone release. But we really don't know if a long term weight lifting

regimen will cause your body to produce more testosterone all the time. Different studies

have come up with different results, with some concluding that long term weightlifting

does increase a person's base level of testosterone while some research didn't

see any differences. Many of the participants in the trials still get stronger and bigger

muscles, they just didn't increase base level testosterone. In reality, our hormones

interact with multiple feedback loops from other hormones after exercise. Reviews of

resistance exercise and hormones usually touch on growth hormone and insulin-like growth

factor in addition to testosterone. So while this one hormone gets a lot of attention,

we need to remember that it's just one piece of a much larger system. Thanks for watching

this episode of Seeker Human. If you want to learn more about hormones, we did an explainer

video in the last season of Human that you can find here. Otherwise, make sure you're

subscribed to us on Youtube so you know when we drop new episodes. Thanks for watching.