Pacing & Energy Systems
Why an article on pacing and energy systems? Well, both are inextricablly linked to one another, I also think it’s important to provide a bit of evidence of why I’m always going on about why you should run your runs on feel. My aim is to keep it as simple as possible so here goes;
Even before we get out of bed in the morning, our energy systems require energy to simply keep us alive and healthy, we need food and water even if we didn’t do anything all day. This system is called the Aerobic energy system and creates energy with the presence and the consumption of oxygen for an indefinite period. This system is used if you are resting, walking or easy/moderate running, anything that is a non-intense form of movement, your body is so efficient that if you kept fueling, you could literally run all day, it would be your musculoskeletal system that would let you down or that annoying blister!
The problems occur in running when you switch from this aerobic system to the Anaerobic system.
The Anaerobic energy system can create energy without the presence of oxygen; however, this system has a finite and shorter duration during which it can create energy, the harder you work, the faster you run then the shorter the duration in which this system can provide energy.
As you pick up the pace you have trouble consuming enough oxygen to meet your needs and your body responds by relying less on fat for energy shifting instead to burning more glycogen as it switches to the anaerobic system. Fat cannot be mobilized or burned quickly enough to meet the energy demands of a more intense pace, because the burning of carbs for energy requires less oxygen so carbs become the preferred fuel whenever oxygen is a limiting factor.
But once you have moved into the anaerobic system is the body’s limited carb/glycogen stores, remember this system has a finite and shorter duration than the aerobic system. It doesn’t matter how ample your fat stores are, after you deplete your muscle glycogen stores you will experience fatigue and will be unable to sustain your current pace.
When you no longer have enough energy to power your body, you hit the wall and it doesn’t have to be the dreaded “20 mile” wall, it can happen at any mileage, for 5ks you can usually hang on but sometimes even a badly paced 10km can feel like a living hell if you switch the anaerobic system on too soon, especially if you haven’t eaten breakfast or fueled properly before the race. Once you get to the last of your available glucose in the bloodstream it becomes difficult to replenish it, even if you continue to ingest carbs, the gels and jelly babies will not make a lot of difference, you cannot just simply switch to using oxygen and your aerobic energy system when you start to get in trouble, this is because glucose is still necessary for the aerobic system to use oxygen to create energy.
Hitting the Wall
If you’ve ever gone out too fast in a race you know the feeling of “hitting the wall” and having to continue running when you feel like this just stops being fun!
Basically, if you don’t properly train your body to efficiently create and utilize energy and/or stick to running within the limits of your aerobic system you will sabotage your performance either in training or in your race. Even if you run too fast on an easy 5k training run you will be switching on the anaerobic energy system, be out of breath, and have to walk so you never train your body to run efficiently and burn the right fuel. There are also other factors that can affect your aerobic threshold/increase your HR including heat, humidity, elevation, hydration level, stress, and sleep quality. So you might be running your “easy” pace but it feel hard and that’s because there are other factors that effect which energy system you use, some that are out of your control.