Many people prefer sleeping in the position that brings the least pain. For some people, this means lying on the back. However, this may interfere with your rib cage’s ability to open fully. For this reason, if you choose to sleep on your back, do not use pillows to prop yourself up...
The same applies with building your body. It makes no sense to focus on the details of your physique and doing specialization training when you weigh a buck fifty soaking wet. Focus first on putting some meat on your bones and building up your base strength. A good goal to strive for is...