doing high intensity exercise,u burn glycogen in ur muscles quickly,once u have depleted ur glycogen stores ur muscles has the ability to carry,running on empty is the worst feeling ever,ur muscles feel weak like u are unable to stand let alone walk,ur entire body breaks out in cold sweats and u feel the weirdest hunger pangs.iv felt this a couple times before during NS and cycling.normally ur body reaches this point after 45 mins to an hour of exercise,but eating gels or a handful of gummy bears during ur workout which are a instant source of simple carbohydrates that can be digested easily and converted into glycogen and help elevate ur blood sugar.it helps to delay the outcome for sometime.