“Run slower to get faster.”
“Follow the 80/20 rule for running.”
“Stay in Zone 2 for your easy runs.”
“You should be able to have a conversation with someone on your easy runs.”
“You have to do recovery runs.”
You have heard one or more of these before. The common theme is that running at a comfortable pace helps develop fitness in an endurance runner. And there is ample evidence to suggest that this is true. However, I still spend a lot of time fielding questions from run-coaching clients and injured patients about why it is important. Let’s try to simplify why it helps and whether or not there are rules that need to be followed.
There are 3 primary systems to produce the energy (in the form of ATP or adenosine triphosphate) our bodies need to function.
Immediate Energy – ATP-PCr System
If you were reading this and had to suddenly take off on a sprint, your body would do that by using the ATP-PCr system. It will give you short bursts of energy lasting 8-10 seconds. It uses the small amount of ATP that is stored in muscles to fuel movements. Endurance runners may need this to jump over big puddles, sprint across the street to beat a red light, or take on a very short climb.
The Fast Lane – Lactic Acid System
When we need energy quickly and for longer than the ATP-PCr system can sustain, we use the lactic acid system. It can also be called the anaerobic system. Because we need this energy quickly, this system is not as efficient as the aerobic system at breaking down the glucose. It ends up producing a host of metabolites that interfere with muscle contractions and lactic acid is one of them.
Lactic acid is not as sinister as we used to think – it is likely not the cause of your fatigue, cramping, and soreness. It just happens to be present in high concentrations in the blood when you feel those things. With better fitness comes the ability to use that lactic acid for energy or to re-make glucose in the liver. As you improve your fitness, you can go longer before the feelings of fatigue set in. Lactate threshold refers to the point of exercise at which your body is producing more lactic acid than it can get rid of or use as fuel.
For The Long Haul – Aerobic System
The aerobic system is how we produce the majority of our energy day-to-day and for exercise. It is efficient and leaves very little behind when breaking down carbohydrates for energy. Your muscle uses oxygen and carbohydrates (or fat) to create this energy, so getting oxygen to the muscle is also important. Training at low intensities for a “long” time trains your aerobic system to transport oxygen from your lungs to your muscles, and to use this oxygen in the muscle to produce energy more efficiently. Endurance runners need this system/pathway to be robust and powerful!
There are no on/off switches and these systems don’t operate independently. But, if you are looking to run longer and faster, you will have to spend a lot of time using your aerobic system. This is done by keeping your intensity relatively low to avoid using the other systems as heavily.
That doesn’t mean we don’t train with intensity for the anaerobic system to improve. It serves the larger purpose of building your ability to go longer and faster. But training at a high intensity must be managed and can’t be done too often for someone’s level of fitness.
So, easy days should be done easy to build your aerobic engine and overall fitness. It is not just to get miles on your legs or to recover. If you skip this part of your training or aim too high, you are missing the chance to fine-tune your running engine.
