REGARDING ROCKY'S MOVEMENT AND HEALING EXERCISES (buckle up my beloveds)
All of this is speculation based on Andy Weir's informational document on Eridians.
I've seen some tags and comments and yes, the Eridian system is largely steam-hydraulic, AKA water state change, so Rocky's regimen isn't exactly due to 'muscle atrophy!'
It's kind of worse, it's 'heart atrophy.'
Now, only major blood circulatory issues and injuries are healed during dormancy (sleep), since it's the only time the Hot Circulatory System is cool enough for the worker cells to survive and work in. This is actually what we saw in the book/movie in Rocky's case, during the Tauboeba Incident. That was a special case of healing. For the average eridian, regular sickness and superficial injuries are healed by exercising to move the blood flow along, as opposed to humans resting/ sleeping to recover!
Eridians have two separate blood systems (both containing tens of thousands of blood vessels) :
HCS (hot circulatory system): Thermal insulator, 310 C
ACS (ambient circulatory system.): Contains all the regular worker cells.
And two types of muscles:
Skeletal muscles: The eridian muscles, spongey, full of expandable vesicles that each have a small amount of liquid water in them. Surrounded and interspersed with HCS and ACS capillaries. Performs the main macroscopic movement.
Micromuscles: Valves in the blood vessels that control the blood flow, exists inside the bloodstream. They are powered by microelectric charge from the bloodstream.
How does the hydraulic system work?
The HCS vessels flows and heats up the miniscule amounts of water contained in the structures of the skeletal muscles into a gas, which expands the vesicles. This state change results in ACTION and MOVEMENT.
Flexing: HCS heats up to expand (via steam) and ACS vessels constrict
Water has a different boiling point under different atmosphereic pressure, so due to long-term exposure to the much lower atmospheric pressure of the hail mary (and 0 pressure for space in general), it's the circulatory systems that go out of whack.
What Eridian organs are weakened in space?
Eridians have 5 hearts, each near the major joint of the leg. They not only pump blood along the limbs, but metals that alternate through the body to a steady electrical charge (energy) for the micromuscles.
Rocky's hearts are the actual organs that are atrophying. they're used to pumping the heavy mercury under Erid's intense atmospheric pressure, so they are definitely affected by the gravitational difference in space.
When the hearts weaken, so does the blood flow that maintains the energy required to power the micromuscles, which control the blood flow, which is required for heating water for expanding vescicles.
TLDR: Eridian muscles require state change, which is affected by different atmospheric pressure. Eridians exercise to heal in general, but Rocky's regimen is mainly cardiovascular exercises, reintroducing the circulatory system to the atmospheric and gravitational pressure on Erid.
Grace's regimen is focused on his muscular system!
Perhaps the most important point of all, Rocky's exercises sort of look like human exercises simply because I think it looks cute! 'tis all :))