i went for an easy run this morning and was thinking about the purpose of training and zone 2. a cornerstone of pretty much any aerobic training program is the easy (zone 2, 60-70% of max heart rate or 5-6 RPE) run. there’s usually the long easy run combined with shorter easy runs throughout the week. when i first started training for longer races (15k), i thought the sole purpose of these longer runs was to progressively overload until i’m comfortable running the race distance. so if i’m training for a 15k, i’m increasing my ability to sustain a comfortable aerobic effort little by little until i’m able to do it for my desired distance.
if i’m training for a 5k, there must not really be a purpose of doing these longer runs. right? there’s a principle in training called specificity – basically it means you tailor your training to the specific energy system and skills that you are trying to improve. so if you’re trying to become a better long distance runner, run long distances. if you’re trying to become a better sprinter, sprint! this seems pretty intuitive, except what’s not obvious is that if you want to become a better runner at any distance, you also want to incorporate long runs!
base endurance
i’m not really an expert on physiology and there’s a ton of resources covering the benefits of long runs, but my layman understanding of this so far is that doing easy runs at roughly 60% of MHR is what allows you to
- build your heart muscle (increasing stroke volume or how much blood can be pumped per beat) with minimal effort
- these improvements are primarily a function of duration. so, generally speaking, the longer you are working your heart at that intensity the more of the benefits (up to a point, we can’t run forever without risking injury).
- allow your body (muscles, bones, ligaments, joints, etc) to gradually adapt to higher volume
- by doing easy runs at higher volume without injury, you unlock higher volume of more intense workouts into your schedule. someone who is comfortably running 30 miles a week can introduce a couple of 5k intense threshold runs into the week to build even more speed and endurance. if you’re doing 5 miles a week, there’s just no room for that. nothing wrong with running 5 miles a week, but my point here is to illustrate the relationship between steady state volume and training opportunity
the minimal effort point here is pretty key. you can train a far higher intensities to build your heart muscle, but turns out your hearts current maximum stroke volume is reached at 60% of MHR. so if you do a full out run, your stroke volume is still the same – you’re just expending more energy for the same heart muscle building benefits. also since doing high intensity runs all the time means you likely sacrifice on volume aka less time overall in this zone. people are also all different – in some situations there may be runners that can do very high volume and intensity and that works for them. i know that’s not me 😀
there are also numerous other related responses that support this gradual volume buildup of the heart muscle, a couple that i notice come up often are:
- increase mitochondrial density (mitochondria generate energy in a cell using oxygen and glucose) so higher numbers of mitochondria means being able to use more of the available oxygen and glucose during aerobic activity
- increase in ability to use fat stores as fuel instead of glycolysis, using glucose and oxygen (able to run longer)
so overtime, spending a lot of time in easy runs builds the heart muscle and its ability to pump out blood and increases your capacity to make use of that higher volume of blood per beat thanks to cellular level changes like mitochondrial density (more efficient). how this translates to races is that you’re able to do them at any distance without getting as tired because your aerobic system is more efficient. and because of the gradual buildup in your overall muscular strength you can run at higher volumes at a comfortable pace per week. this higher mileage then unlocks higher quality / higher volume intensity training.
jack daniels, a well known running coach, often says that you should know the purpose of your training. why are you running today? what is the purpose of this long run? well there’s the purpose of long runs. you do long easy runs because it builds the very foundation of your aerobic performance.