As an excon I will answe this question and it has NOTHING to do with drugs.

Imagine being locked in a cage with no access to ANYTHING enjoyable. With no responsibilities and NO human contact.

After the initial shock, crying, seizures you begin to learn ways to cope.
Your ways are this:
Pull ups
Push ups
Sit ups
Dips

Reading.

EVERYDAY ALL DAY.

People don't understand even at this level how it works.
I'll put it this way.

No matter WHAT you're locked up for, you are fvcking angry and mad as hell everyday you roll off your bed.
You essentially have a mentally induced steroid working in your favor everyday.
Plus exists the real threat of getting fvcked up by other inmates, a real threat to daily survival. YOU LEARN TO BE STRONG no matter WHAT.

This is how it works.
And you'll never understand how extreme it is untill or if you're ever in that situation.