Pacing ultra distance races is a balancing act. You have to manage fatigue (both mental and physical) Glycogen stores, and physical capability to ensure you cross the finish line in your most efficient way without any major detriment to performance or overall pace. To understand the intensity a runner operates at during an ultra we must first look at the energy systems of the body and their duration's.
To complete an Ultra marathon you need energy over a long duration, this means utilising the aerobic energy system that breaks down Carbohydrate, Fats and to a certain degree Protein to produce ATP (Adenosine Tri Phosphate) in the presence of oxygen. Figure 1 illustrates the energy stems of the body and their duration's, the shorter duration systems produce a high percentage of readily available energy but deplete rapidly therefore for endurance events and especially Ultra Marathons the body primarily uses the Oxidative aerobic system. Our bodies break down glucose from stores in the muscles or liver, or ingested during exercise, to be used firstly in the Krebs cycle and later in the Electron Transport Chain (ETC) to produce ATP which is then used by the muscles to contract producing movement. In terms of energy storage we have around 2000 kcal of Glycogen, and between 60,000-100,000 kcal of Triacylglycerol fatty acids stored in Adipocytes plus 3000 kcal from intramuscular Triacylglycerol stores (2). As you can see Glycogen (carbohydrate) stores are limited in comparison to the potential energy available from fat.
When it comes to catabolising fat stores Triacylglycerol must undergo a process called Beta Oxidation before it enters the Krebs cycle (citric acid cycle) as Acetyl-CoA and then ETC to produce ATP. When compared with one glucose molecule which yields 32 net ATP's, one fat molecule will yield 460 ATP's. To allow Beta Oxidation to take place you need glucose to start the process of Glycolysis which leads to the Krebs cycle, Beta oxidation takes place at between 45-65% vo2 max so at low intensities which brings us back to intensity and pacing for Ultra Marathons (4). Firstly your primary goal building into an ultra should be increasing fitness and endurance, this will increase your efficiency and the fuel source you can rely on, by raising your Vo2 max you can therefore operate at a relative faster pace using fat as a fuel source, with increased aerobic fitness you will also be more efficient at catabolising fats at higher intensity levels so closer to the 65% than 45%.
What does this mean for ultra pacing? To successfully complete an ultra distance event you need to run at a low intensity (relative to you), taking into account elevation gain/loss, you would look to complete the distance with an even energy distribution/intensity through the race. In terms of average intensity this would look like 55-65% of threshold pace, RPE 1-3 (will rise with fatigue), HR 60-75% threshold (will also increase with fatigue and dehydration). This strategic pacing may mean fast hiking or walking steep climbs and keeping intensity levels low throughout to ensure you can keep going for the distance. Fueling and hydration
becomes very important to enable consistent energy levels and prevent dehydration. Ingesting carbohydrates also allows beta oxidation to take place as well as directly fueling muscle contractions and blood glucose levels (see our blog on long distance nutrition). To summarise, in order to pace your ultra marathon you need to run at a very low intensity to utilise the long distance energy systems, alongside a planned and tested nutrition strategy whilst going into the race with the highest level of fitness you can achieve in the time you have available.
(2) Exercise Physiology McArdle, Katch and Katch Eighth Edition