Very short (15–20s) Smart Intervals for VO₂ work with high HIE — how should this be interpreted?

Training
1

Hi all,

I’ve been using Xert for a while and really appreciate the model and the concept of strain-based training. However, I’m struggling to understand how to properly use Magic Buckets together with the Smart Interval recommendations for high-intensity (VO₂-type) work.

My fitness signature

  • TP: 361 W

  • HIE: 24.7 kJ

  • Peak Power: 1182 W

So I appear to have relatively high anaerobic capacity.

The issue

When riding with Magic Buckets and following the real-time interval recommendations on my head unit, I consistently get suggestions like:

“30 intervals @ 546 W for 17 seconds”

In practice, most of my suggested “intervals” are:

  • 15–20 seconds

  • sometimes even shorter

  • almost never longer (e.g. 60–90+ seconds)

What’s happening (as I understand it)

It seems that:

  • I can generate very high power

  • HIE depletes quickly

  • MPA drops fast

  • strain accumulates very quickly

So the system reaches the intended training load almost immediately.

My concern

From a physiological standpoint, these efforts feel like:

  • repeated short anaerobic bursts

…rather than anything resembling sustained VO₂max work.

Coming from a more traditional background, VO₂max training usually involves:

  • longer efforts (2–5+ minutes)

  • sustained high oxygen uptake

  • accumulating meaningful time at intensity

With the current recommendations:

  • I reach high strain very quickly

  • but spend almost no time at that intensity

Key question

For athletes with relatively high HIE:

:backhand_index_pointing_right: Is Xert effectively allowing us to “overuse” anaerobic contribution to hit strain targets?

:backhand_index_pointing_right: And if so, could this lead to insufficient time at VO₂max, despite technically hitting the intended training load?

Buckets vs execution

I understand that:

  • Buckets accumulate over the entire ride

  • They are not tied to individual intervals

So the real issue is not the bucket concept itself, but rather:

:backhand_index_pointing_right: how the recommended efforts are executed and metabolically achieved

Right now, it seems very easy to:

  • fill high-intensity buckets via short spikes

  • rather than sustained work

What I’ve tried

I’ve experimented with:

  • Ignoring the short interval recommendations

  • Riding longer, steadier efforts closer to MPA

  • Adding more traditional VO₂ intervals

This feels much more effective physiologically — but also like I’m deviating from Xert’s intended use.

What I’m trying to understand

  1. Are these very short interval recommendations expected for high-HIE athletes?

  2. Should these be interpreted more as “intensity signals” rather than literal interval prescriptions?

  3. Does Xert assume that proximity to MPA is sufficient regardless of duration?

  4. Are there recommended ways to avoid ultra-short, spike-driven bucket accumulation?

Final thought

I really like the Xert model, but in this case I’m struggling to reconcile:

  • very short (15–20s) recommended efforts
    vs

  • the need for sustained high-intensity work

Especially for VO₂max development.

Would be very interested to hear how others with similar profiles approach this.

Thanks!

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MIcrointervals & Workout descriptions
2

I am also a bit confused by that. Most MB workouts seem like a Z2 ride with many very short “needle” intervals. When you create a MB workout in the generator, I think you can determine the interval duration. But I am not sure it transfers in any way to “dynamic” outside MB rides.

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3

This behavior of the algorhithm reminds me of this old story - when the military started using the first computers, they asked what would be the cheapest way to feed a soldier, in a way that provides all the necessary nutrients. The computer calculated that each soldier should just eat 25kg of potatoes every day. Similarly, Xert says you shold just do 100 short, 15s bursts during your workout.

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4

I think I read somewhere in the Knowledge Base that MB calculates “the most efficient” way to fill the 3 XSS buckets for the day. My hunch is that it does not necessarily take into account the traditional idea of what a VO2Max interval should look like. I don’t have enough knowledge or experience to critisize the Xert approach, but I do wonder how grounded it is in actual sport science. I’m afraid we might be just getting “25kg of potatoes”.

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5

I really love Magic Buckets, it makes training fun. But unfortunately, I have to ignore its recommendations. It suggests a 30-second recovery, then a 2–3 minute one . From a training efficiency standpoint, recovery is one of the most critical factors, so you need to clearly understand what exactly you are training and how

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6

So you use MB basically to monitor your XSS across the buckets rather than an interval guide, correct?

7

There was a very interesting discussion on the Xert podcast, where Armando and Scott from Xert had a kind of disagreement with their guest coach Steve Neal:

Listen to the bit from 36:10 to 44:00. It’s not exactly the same problem you are describing, but it does seem to highlight the difference in thinking about training stimulus between Xert team and a more mainstream-minded coach.

8

I found this topic, where the issue of microintervals was discussed:

My takeaway is that it is up to me to decide if MB microintervals is something that will be beneficial for my training, because the algorithm seems to care more about time-efficiency and filling the buckets than about giving me the best workout in a traditional sense. Which is a pity for me because I don’t know anything about training and was hoping Xert would do all the thinking for me.

9

Same here. My TP is similar to yours but my hie is considerably smaller 14.7 kj and my pp also lower 945w. So I don’t think that those short intervals recommended by XMB is for anaerobically gifted athletes since it happens also in my diesel case.

I heard Scott saying somewhere or at least that was my interpretation that XMB adapted to the duration of intervals you decided freely to do disregarding initial XMB targets. So if you prolong your intervals to 60s it will increase the duration of the recommended target. Am I right @ManofSteele ?

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10

The challenge with longer, sustained efforts is that they’re almost exclusively low strain, so they’re not taxing on your HIE or PP systems. In order to effectively put strain on those systems, you need to be doing high-power efforts. But of course you cannot hold very high power efforts for long, which leads to the micro interval-like behaviour of Magic Buckets, specifically when you have larger High & Peak XSS targets.

If you feel that you’re missing out on some longer, sustained efforts and that those are beneficial for your training/performance, then include them in your training!! Do some 5x5 or 4x8 efforts first, then switch to XMB and “top up” your buckets with the shorter intervals afterwards.

I’ve done that a few times and it worked well! For example, this ride was done outside and was something like 6x4min around ~5 min power (ish). Then some tempo to fill up the low bucket, & finished with XMB to complete the training for the day. It was a fun ride/workout, too!

Vo2max into XMB
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3 Likes
11

Thanks Scott, that makes sense and I really appreciate the flexibility of XMB here. I can see how it can work very well if the rider already has a clear idea of the workout structure they want to do — for example doing 5x5 or 4x8 first, and then using XMB to top up the remaining buckets afterwards.

I think my concern is more about how this is communicated to less experienced users.

As someone who is still learning about training structure, when I see my Garmin telling me to do something like 180% TP for 15–20 seconds, my natural assumption is: “OK, this must be the recommended/best way to do today’s workout.” I don’t necessarily have the background to know that I might be better served by doing longer sustained efforts first and treating XMB more as a bucket-management tool afterwards.

That’s where I think the current experience can be a bit misleading. XMB is flexible, which is great, but the real-time prompts feel prescriptive. A more experienced rider can bend the system around their training goals. A newer rider may simply follow the prompts literally, and end up doing lots of micro-intervals even when a more traditional workout structure might have been more appropriate.

Also, Xert’s own Smart Workouts are not made only of micro-intervals, so it seems Xert itself recognizes that short spike-driven work is only one possible approach, not the only good one. That is why I think the issue is not whether XMB can be used well, but whether the app makes it clear enough how it should be interpreted.

Maybe XMB needs a little more guidance around this. For example, something like:

  • “These recommendations are an efficient way to fill your buckets, not necessarily a complete interval prescription.”

I like the XMB concept, and I’m not trying to criticize the model itself. I just think that for users who want to be guided by Xert rather than use it as a flexible expert tool, this distinction could be made clearer.

1 Like
12

I can tell that I almost exclusively did XMB this season and still have my all time best signature (and even some real world PBs) this year. TP peaked at about 320 W, PP at about 1250 W. LTP at about 255 W.

Where I do fall off though are steady efforts above threshold pushing into my maxHR. Firstly I did not do them, secondly I did not need them (training for hilly brevets/24h races with 1000m+ climbing per 100km riding), thirdly I did not train for them.
But where I shine a lot more than last year are repeated short surges to stay in the group on small climbs (as a heavy 80+ rider) and on long flats where I can push decent watts in low/mid zone. Both of those have been my main training goals. I can push my NP a lot more than last year meaning I am more flexible rider for my needs.

4 Likes
13

To me, this makes a lot of sense. Following the XMB structure as it’s given by the system is not helpful for anyone who has event specificity in mind. Eg if someone’s target race has short climbs, it would be useful to do some intervals around 4-6 mins to mimic the event’s demands.

I’d appreciate if the system would give some even generic example targets for the day, eg if the goal is to hit HIE, “focus on 3-6 min intervals at XX power range”. Or, for PP, “do X sprints in 10-15s range, repeat until buckets are filled”.

2 Likes
14

I see a great value of XMB on completion of hie and pp XSSs after Xert workouts. 2 days ago I tried one of the hardest workouts in Xert (Let the Sparks Fly). I did it in Zwift using the slope mode. I couldn’t meet the power targets of the first micro intervals which are brutal so I ended up with a bit of frustration. I was cooling down with a robo pacer and at the same time was running XMB in my Garmin. So I decided to try some of the 15s intervals prescribed by XMB. I allowed myself to get dropped by the robo pacer and waited until the “catch” zwift message popped out the screen and the distance to the pacer was 60m. Then l started the interval and ended it when I caught the partner. Again got dropped and so on. I eventually changed to a faster partner to make the intervals a bit longer. It was so fun…I finished filling all buckets and very satisfied with the workout. Regardless of the physiological stimulus imo your subjective perception after a hard workout is an underrated aspect of training specially for amateurs. At the end it’s what fuels consistency.

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2 Likes
15

Sure, I can agree with that. But could you not simply do the hill repeats if those efforts are specific and important to your training?

For example, do a few hill repeats at 75-100% max effort, then look at how those efforts filled (or didn’t fill) your buckets. From there, you can use XMB to “top up” whatever remains. To me, that’s one of the biggest strengths of Magic Buckets… it complements the training you want to do rather than replacing it.

Likewise, if you feel longer threshold intervals are important, go do them! XMB isn’t saying you can’t - it simply provides one of many ways to achieve the desired training strain, then recalculates its guidance second by second as you ride. (It’s the smartest of SMART workouts. :grinning_face_with_smiling_eyes:)

If you prefer longer XMB intervals, you can also do that. Instead of riding one Challenge Level at a time, try to work your way from CL1 to CL5 (or higher), hold that effort, then let your Challenge Level recover all the way back to CL1 before repeating. That ends up feeling much more like traditional longer intervals. It’s mentally tougher, but there might certainly be value in practicing those longer, sustained efforts if it’s important for your events.

I think the bigger design challenge is balancing flexibility with simplicity. We could add options for short intervals, long intervals, hill mode, threshold mode, etc., but each new option adds another decision for the athlete. One of the nicest things about XMB today is that you can simply ride, fill the buckets, and know you’re accomplishing the intended training.

For athletes who want more specificity, though, I think your proposed approach works really well: do the race-specific efforts first, then let XMB adapt and guide the rest of the session. I see those two approaches as complementary rather than competing.

4 Likes
16

I think this is a really important point!

Most of us are amateurs. We all want to train effectively, but we also want to enjoy the process enough that we keep coming back day after day. A workout that’s engaging and leaves you feeling accomplished is incredibly valuable because, as you said, consistency is what ultimately drives improvement.

Personally, this was probably my best winter of training in years, and I did maybe 2 traditional high-intensity workouts over ~5 months. I replaced almost all of them with Zwift races + Magic Buckets, and I honestly had more fun than I have in a long time. The races were motivating, XMB made sure I still achieved the training I needed, and I never got that feeling of burning out from staring at another interval session.

That’s not to say structured workouts don’t have their place - they absolutely do. I like a lot of the unique & adaptive workouts that we have in our library! But I think XMB gives athletes another option that is both effective and enjoyable, and for many amateur cyclists, that’s a pretty powerful combination.

4 Likes
17

Shorter Focus Duration (FD) = higher target watts with shorter duration.
Longer Focus Duration = lower target watts with longer duration.

The default duration will range from <10 secs to about a minute depending on FD target.
Regardless of the default you can double, triple, or even quadruple the default duration as you execute the intervals. Try that next time and watch what happens.
You’ll drive up the Challenge Level and shorten estimated interval count required.
You can also shorten recovery times and keep MPA drawn down. That will raise the activity’s Difficulty Score even higher.
You might even perform Ronnestad-style sets (30/15s) or traditional steady blocks or hill repeats to fill your “VO2max” buckets.
Whether that’s practical to do depends on terrain, traffic, and your desire to endure pain. :smiley:
Some days you may prefer to simply free ride and only check XMB towards the end of your ride to see what’s left to do.

From main XMB article –

Key things to know when doing Magic Intervals:

  1. It takes 5 seconds for the gauge to switch from one interval type to another. When you start your high intensity interval, hold it for 5 seconds for the INTERVAL state to show. Similarly, after completing your interval, hold the recovery power for 5 seconds to see the gauge switch to the RECOVERY interval.
  2. Once your recovery is complete, you can continue to recover and collect more low intensity XSS or can start a new interval to stay at the same Challenge Level.
  3. Recovery intervals are calculated to give you the precise recovery and low intensity strain and maintain your current Challenge Level (Challenge Level on the left of the bar). If you sustain the RECOVERY power for the RECOVERY duration, you’ll start the next INTERVAL at the start of the Challenge Level.
  4. Let Challenge Level be a workout objective. The higher the level, the more efficient each second of the workout becomes.
  5. Reaching higher challenge levels requires you to sustain the target interval power longer.
  6. *Stop or start high intensity or recovery intervals at any time. You don’t need to be precise to complete your training since this will continuously adjust for you. However, the less training that remains, the more precise you’ll need to become to finish the targets without spending unnecessary training time.
  7. Start your intervals at any time. Because things continuously adjust, you can do the intervals at any time during a ride. For example, you could start your magic intervals after a competitive group ride to finish off your training. You’ll likely notice that the target interval power will change over the course of the riding you do beforehand. This is because Magic Buckets is continuously recalculating what you’ll need to do to complete the remainder of your training based on what you’ve done so far.
  8. It’s ok to be a bit off target or stop/start at random times if you need to / would like to. It will recalculate what you need to do to complete the remainder of the training.
  9. Use the integrated cadence optimizer. Your cadence is shown as a triangle on the gauge. Aim the triangle along the thin bar to have your cadence within a range that is suitable to your usual cadence pattern [at current torque].

XMB is both a carrot and a stick.
When the opportunity arises that includes chasing BTs.
That’s what makes it fun and challenging on HIT days.
Or XMB is a constant reminder to ease up when you should be doing LIT. :wink:

2 Likes
18

This is very interesting, but there is an opinion that mixing different workout types within a single session (like VO2 Max, Threshold, and Sweet Spot etc) isn’t the best approach, as the body gets “confused” about what adaptation it’s supposed to target.

To add to this, last season I did a lot of rides exactly in this mixed style. As a result, my HIE kept increasing (along with accumulation of fatigue), but my Threshold actually declined. During my main race, I blew up/faded very quickly.

Conversely, during the winter, I focused heavily on Zone 2 and Tempo work, with very few high-intensity sessions. Yet, for the first time ever, I experienced breakthrough during a Zwift race, pushing my Threshold up to 320 Watts.

I’m also curious if anyone here has experience transitioning from working with a real coach to training with Xert, and how much of a difference in results these two approaches can produce? A friend of mine trains with a coach, and even all of his outdoor rides are strictly prescribed by watts.

Is it true that the less training time you have, the more meticulous and disciplined you need to be with your workouts?

1 Like
19

My experience is similar to yours. micro- intervals are indeed effective, there is scientific evidence but it cannot be all one would do. You need longer efforts as well, if just to development the mental toughness and simulate race efforts (if you race of course). Shameless plug, I developed a prototype to deal with this. it simulate a race, I got 3 configured like Paris Roubaix, Tour of Flanders and Strade Bianche. It is based on xert and fitness signature. Try it if you wish. Call it magic buckets on steroid simulating race efforts. https://watt-lap-web.vercel.app/

6 Likes
20

Thank you guys for all the feedback. I think I get all the points and agree with the point saying that the Garmin XMB instructions can be a bit misleading. Oftentimes I use XMB as @ManofSteele suggests. I do my outdoor group ride with the club (intervals, small crits or just an easy ride) and then typically continue afterwards filling the buckets. And IT IS fun compared to the more traditional ways of training :grinning_face:

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