The Cori Cycle Explained

High intensity exercise will mostly get it's energy or ATP from the pathway of the glycolitic system.  Less intense activity will receive its energy or ATP from the aerobic pathway utilizing the Krebs cycle.  When utilizing the glycolitic system, cycle after cycle, lactate will start to build up.  Lactate from the glycolitic system will diffuse from the muscles into the bloodstream.  It will then be transported into the liver.  In the liver it is converted from lactate back to pyruvate back to glucose, which is then available to the muscles again for energy, this is called gluconeogenesis.  The whole process is called the Cori Cycle.  The more you train with high intensity exercise, the more capable the enzymes and transporters become that are needed for the Cori Cycle.  Your liver gets better at using the lactate, not more efficient (it still needs the same amount of ATP to run the Cori Cycle)  but it will do the cycle faster.