What is a "Base"?

Hi All,

Much like everyone else on this forum I seem to be constantly readjusting plan to events and have just had my last race option for the year cancelled.

So have decided to plan for next season. I am planning to hit up IM Marbella 70.3 in April next year and then follow up with a full IM in around August/July (possibly Tallinn, Estonia - looks interesting and heard very positive reviews).

Making this plan using a combination of plan builder, standalone plans and my own additions I have been trying to get a better understanding of what is meant by a “base”: what are we trying to achieve? and how do we measure it?


  1. What happens physiologically during base training (using either traditional or sweet spot)?

  2. How do I measure my “Base”?

Reading online, I see people writing about Efficiency indexes (HR vs Power). I don’t think the usefulness of this metric only applies to Base and wouldn’t know how to use it to see my base developing.

I’ve also seen people suggest that your resting Heart rate is a good measure along with your VO2 max (kg/l/hr).

Would be great to get some insight into this plus does anyone know of any Chad deep dives into this topic. I really need to demystify this concept!



[edit: new graph w/ SS incl]

Look at your long 3+hr Z2 rides, if your Pw:Hr gets down to 5% or lower (even negative), then you probably have an ok base.


Can you clarify this? Are you saying power to HR ratio?

On the “HR Decoupling” page of my sheet, you can enter the 4 bits of data and get the calculation.

Intervals.icu also does the same calculation for all rides. But keep in mind, per the references, that there are specific ride duration types that are appropriate to review, while others are not. So be aware of when to use that info vs when to ignore it.


Coach Joe Friel stated that base is the time of year when you “train to train, not train to race” which is I think is a good perspective.

Coach Chad viewpoint:

One big picture way to measure and compare base training across seasons is CTL (chronic training load) on the Performance Management Chart. There are other ways, for example this past season I had some level of detraining and only have a handful of seasons developing base. For that reason there was some value tracking absolute vo2max (estimates from WKO5) over the season as I went from 3.3 L/min to over 4 L/min. Personally I don’t find much value in tracking aerobic decoupling or EF, except very early in base, but if you haven’t done a lot of long aerobic rides then it might be helpful.

The adaptations you are looking for are fostered by spending time in the weight room, working on basic cycling skills, doing progressively longer aerobic endurance rides (basic aerobic development), and working on-bike strength endurance and advanced aerobic development (tempo and sweet spot).

Good luck at half and full IMs next year!

1 Like

Since you will go into triathlons and you will run. I would suggest reading
https://www.uphillathlete.com/shop/ if you have time. Author is in the school of thoughts of pushing volume and base over anything. You will become fat adapted and essentially tune your engine to go long.
A bigger base allows you to do more back to back days (which will pay dividend in tri imo). They say bike for show & run for dough, as a non-triathlete you should work your sweetspot to a power you’d be happy with (don’t be too eager) and then focus on running a fast 13.1.
I’m beating around the bush a little bit but essentially I’d use bike base as fillers on days you aren’t looking to build a power gain on the bike.

1 Like

Thanks for posting, Aerobic decoupling is something I hadn’t thought of calculating before but reading that it makes sense to me

1 Like

Aerobic decoupling is calculated automatically in TrainingPeaks (free). At least for me personally, just doing 3 weeks of aerobic endurance work will drive it down to 3% or less. Thats why I said its only something I look at during early base, and only after taking an off-season break.

What is “aerobic decoupling”? I mean, I get how is calculated, but what does it mean physiologically? In particular, how - or I guess, really why - can it tell you when your base is complete?

Per Joe Friel, in a more traditional LSD plan it merely tells you that a solid aerobic endurance foundation has been established and its time to either do longer zone 2 rides, or time to start doing tempo and sweet spot. Definitely not time to break out the gold stars and congratulate yourself on having completed base work.

Seems like splitting hairs, but regardless: how does it tell you what wrote above? What’s the mechanism? What’s the evidence? Or are we all just taking Friel’s word for it?

I don’t have any science to feed you, but Alan Couzens blogged about it. And sometimes you just need to take a coaches word for it.

Setting aside confounding factors like heat, hydration, etc., the only explanation for it that i think makes sense is about motor units. That when your HR starts going up for the same power it means that for whatever reason you’re starting to recruit bigger motor units with less efficient muscle fibers. So like, if you can ride longer at the same power before that happens, all else equal, it means your comparatively weaker but more efficient slow-twitch fibers are more fatigue resistant than they were before, have stored more glycogen, use more fat, whatever it is.

I think I can get behind the explanation but i don’t really use it as a metric because of the ‘all else equal’ part. All else is so rarely ever equal that in practice i’m never sure whether what i’m seeing is a result or just noise.

1 Like

That makes sense…until you realize that the data linking increased type 2 motor unit recruitment to increases in VO2 is equivocal at best. So, why would recruiting more type 2 fibers cause your HR to go up?

So i’m coming in here to weigh in because this is something that i’ve also struggled with for a long time. The terminology is confusing and while the concept seems simple at first, when you dive in it really is not that simple.

Sometimes people will be like “oh you build the base before you move onto intensity” and that is one way to do it but not the only way. Bottom line, all sorts of training will build what people refer to as your aerobic “base” including intense intervals. So many different trainings (intensity included) will be building your capacity and fatigue resistance and all that and taht also builds your base. So in a way, everything is base. But of course, that’s not helpful for organizing your season now is it?

Some people approach it based on specificity, and that’s also one way to do it but not the only way. It’s not my preference because for me anyway, specificity is mostly overrated.

I like the general rule of, base is when you train to train vs. training to race.

But for me personally, the most specific principle that i have been able to come up with is this: I can slam intervals and I get fast very quickly, but there is a clock that starts ticking and eventually it runs out and I fall back to earth. This might be two months, three months, sometimes I can maintain it for longer but no matter what I do, there is a limit.

So for me, “base” means doing whatever I can to raise my level, get stronger, get fitter, whatever, without starting that clock. i’ll do long rides, low cadence grinding, high cadence, sprints, weight lifting, cross training, whatever, with general goal of building capacity, repeatability, getting stronger. I do hard intervals sometimes too but i am very careful with the frequency and the length of the relevant mesocycles. But that’s the goal: raise the level without starting the clock, until such time as i’m ready to start it up. It took me 20 years (since high school track and field) to figure that out.

That said, eventually you have to hit it hard and let the clock start. Like right now with no racing and no events, doesn’t mean just run SSB back to back to back to back because eventually you’ll top out and your body will need different stimulus to adapt. Just like Olympic athletes with their four-year macrocycles, sometimes multiple years for them will be devoted to mostly training to train, but they still will be competing and still will be doing intensity during thsoe cycles.


interesting. But O2 and HR are not strictly linked, right? Is there anything else that could raise HR besides O2 usage?

Lots of things. Nothing that I can think of that would tell you your “base” is adequate, though.

haha well what else! I’m not trying to use this to decide if base is adequate. I’m not even sure what that would mean. I’d just like to know what yo uthink explains it, given that what i thought may be either wrong or incomplete or both.

What comes out your subwoofers