Physiological changes

take place when we simply just get up and move.

The Metabolic Chemistry of the Pre-Exercise Warmup
In order for muscles to contract, they require energy in the form of adenosine triphosphate or ATP. There's just one problem: you only have enough available ATP to last for about 10 seconds. Then you need to start manufacturing it again. The body uses several energy pathways, including anaerobic glycolysis and aerobic glycolysis, to provide fuel to the working muscles.

The anaerobic system offers about two to three minutes worth of ATP, but soon the body will need to switch to aerobic glycolysis to continue making fuel. "Aerobic" means "with oxygen." This process uses lots of oxygen to convert stored glycogen, fat and even lactic acid into energy. Once you tap into aerobic glycolysis, you can essentially keep the muscles contracting for hours on end.

But switching from one energy system to the other can create an oxygen debt until you are able to move oxygen to the working muscles fast enough to facilitate aerobic glycolysis. You may feel a burning sensation and discomfort in the muscles, and start breathing heavily as you increase the delivery of oxygen to the muscles. Your heart rate will also increase to pump more blood, along with oxygen and nutrients, throughout the body. Once the transition occurs, you can move along comfortably at a steady pace and your breathing rate and heart rate will settle down as well.

While some people may find this initial burning and fatigue a major deterrent to exercise, keep in mind that if you make it through this challenging phase, your body will adapt to a steady state in which the aerobic energy system is able to keep up with the oxygen demand more readily. Yes, you may need to adjust your pace to an appropriate level based upon your physical conditioning, but whether you are running a marathon or out for a brisk walk, you will find a steady pace where you will be able to continue moving for quite some time before you run out of energy.