Triathlon & Nutrition
Hitting the wall" or "running out of gas" is how other endurance athletes describe the overly fatigued muscles and disoriented sensation that leads to a dramatic drop in performance. A runner's stride becomes a walk; and any blip of a hill looks like Mount Everest. What exactly is going on when your muscles and brain scream "no more"? Is this a situation you can avoid?
Extreme fatigue that severely impairs performance is likely a result of depleting muscle glycogen stores, liver glycogen stores, dehydration, or a combination of these. Muscle glycogen is the primary fuel for endurance athletes. Although you also burn small amounts of fatty acids and amino acids, glycogen is the major player.
You have a limited supply of stored glycogen in your muscles, and once you've used it all up, exhaustion sets in quickly. Without glycogen, your muscle cells have difficulty contracting, so at first you may compensate by using other muscle groups. You may have observed some of these glycogen-depleted individuals out on a race course or crossing the finish line with less than perfect form.
Your liver releases its stored glycogen as glucose into the bloodstream to fuel your brain and nervous system. When exercise depletes your muscle glycogen, your hungry muscles begin to draw more glucose from your blood, which leads to low blood glucose levels. At this point, your liver may not be able to maintain adequate blood glucose levels, making you feel tired, lightheaded, uncoordinated, and unable to concentrate. This low blood sugar induced mental fatigue can lead to a perception of muscular fatigue, even when muscle glycogen is not fully depleted. Your brain processes all kinds of information during an intense athletic event - your blood pressure, hydration status, stress hormones, etc. If enough of these are out of whack, it may just decide to slow or even shut your body down as a protective mechanism.
In addition to muscle and liver glycogen depletion, dehydration can impair your performance. During exercise, you are constantly losing water through sweating and evaporation. What some athletes interpret at low blood sugar levels or low energy stores may actually be fatigue from dehydration.