What happens during physical inactivity and systematic physical exertion?

With hypodynamia, the thickness of muscle fibers in skeletal muscles and heart muscle decreases, as a result, physical weakness and low tolerance to physical exertion develops. The depth of breathing decreases, the breathing process itself becomes more energy-consuming. Areas of muscle tissue are replaced by fat. During training, the opposite phenomena occur, muscle mass increases, along with this, oxygen utilization processes improve, and breathing efficiency increases.