I will try to find some references, but the basic concept is that your heart will atrophy when it doesn't have to work against gravity, as in pumping blood from foot to head, while vertical.

This is why standing desks are a thing, why an astronaut will lose 30% of his heart-mass after one year on the ISS, and why patients after a long coma have a hard time doing cardio.

edit:

"Cardiac atrophy after bed rest and spaceflight"

https://www.researchgate.net/publication/11883764_Cardiac_at...

"Cardiac atrophy in women following bed rest"

https://pubmed.ncbi.nlm.nih.gov/17379748/