My question is purely hypothetical, but it's something I need to know for a book I'm writing and so far no one has been able to answer my question. So I would like to ask you if someone would not consciously use their body from a very young age, how would the muscles develop? meaning that an average human would stop living life as we know it within the ages of 1 - 3 (the years barely anyone  remember correctly). they would start to live inside technology. Some devices are connected to their brains and body so they find themselves in a digital world, much like ours. Their senses still work, they recieve artificial images, smells etc. These individuals do not know they're living a so called artificial live. Their brain receives the same kind of information as it would get in our lifes, so it develops normally. The individuals know what running and jumping is, but while their brain is connected to different devices, they do not use their bodies. Their bodies however get the same kind of hormones and nutrition as they would need. So my question is how the muscles would develop and if they would be able to use them after not doing so for let's say 20 years. any other details as: what could be done to make sure their bodies and muscles develop as good as possible during the time the individual is in that kind of slumber, how they can recover and how long it would take to recover are very welcome. Thanks in advance!

sounds like the Matrix!

In the situation you describe (and leaving aside the almost certain impossibility of someone consciously not moving for years)  I would think the muscles would develop but be very atrophied, similar to someone with a severe spinal cord injury at a young age.