Agree with TbombZ 100%. All are related to the CNS, and the degree of recovery it is allowed to manage between training sessions.
Recovery abilities of the human body has been around since the dawn of time. All related to the basic core of survival and how humans adapt to stress, either physical, emotional or mental. All related to the peripheral nervous system, controlling most of the functions of the body. If the CNS is overtaxed, without proper rest or healing, any expected gains from workouts will be extra slow and zero. Which, in some cases, a person may be losing muscle mass & strength from workouts. Good Luck.