Peak power on a mountain bike on a trainer

(apologies for the double post)

I’m having trouble reaching my peak 5-second power on my mountain bike on the trainer. My smart trainer specs are suppose to reach 1200W, but with with the gearing on my MTB, I simply spin out on the hardest gear at maximum effort. The best I can do is grind at high cadence, and fatigue after 30 second or so. Whatever I hit during that time seems to more like a 30-second peak power, rather than the max peak power Xert needs to establish for my training profile…

This is the only bike I got, so looking for any ideas on how to get around this.

You don’t need to worry about hitting your actual PP during a breakthrough workout.
Xert is looking for patterns in how well you perform while fatigued.

You can actually boost your PP number during a breakthrough event without ever getting close to it. Math-wise that is because Xert is estimating your hypothetical 1 sec power based on how much you can generate for 5-7 secs while fatigued.
That’s why the breakthrough workouts in the library are designed to draw down MPA then ask you to sprint right before you collapse completely. :smiley: When that time comes stand up, spin up, and go all out for 5-7 secs or as long as you can.

Also, it’s impractical reach the same PP indoors as you can outdoors. Mounting a bike on a trainer limits the amount of torque you can apply compared to thrashing your bike outdoors. :grimacing:
PP doesn’t have to match what you can do outdoors for your fitness signature to work.
If you feel it’s too far off the mark you can adjust peak power using the Advanced MPA tab when viewing an activity. Change the PP value, click Extract to view what the change will do to your current signature, then click Save if you want those numbers to apply to future activities.

If you know your power curve numbers you can alternately use the Power Curve & Calculator option on bottom left menu of XO (Xert online) to generate your fitness signature.

Thanks for the informative answer. So what are breakthrough events exactly? I have been nothing but Xert workouts for the past 8 weeks through a pre-base and current in base phase… I’m not sure I have yet come across any breakthrough specific events (although it occasionally tells me I achieved breakthrough in something).

As long as you are achieving breakthroughs periodically you should be fine. :slight_smile:
Breakthroughs validate your fitness signature and update it if any change occurs.
The Advisor doesn’t specifically recommend a breakthrough workout but under your name on the home page you will notice “Last Breakthrough:”. Under that is a warning if your signature has not changed in more than 3 weeks. When that happens you should search the workout library for “breakthrough” and perform one of those workouts.
Other ways to generate a breakthrough include Zwift races or a group ride with wind-up sprint or any hard effort that draws MPA down and you force you way past it generating a breakthrough event.

Check out this post and you’ll find many answers and tips which may generate more questions. :smiley:
Beginner questions - Support - Xert Community Forum (

Examples: Let's See Some Recent BT's

Reference: Breaking Through the Xert Way! – Xert (

Just checking but do you actually have a smart trainer as you say (with ability to control resistance via app, or manually for that matter) or is it a simpler trainer where resistance is purely based on trainer speed (eg a fluid trainer like kinetic road machine)?

I assume the latter from your question, otherwise you should be able to just put the trainer in slope or resistance mode to increase the resistance for any gear.

Assuming you can’t change resistance somehow, you could experiment with accelerations (e.g. from standing starts or slow speeds to max as quick as possible) to generate short term power. That said, depends on the trainer as it may not pick that up accurately if it’s a fluid trainer e.g. due to tyre slippage.

Yes, it is a Tacx Flux S. This is what I’m experiencing with resistance modes: To be able to reach anything above 300 watts in ERG mode for less than 110 rpm, it has to be somewhere in the mid cogs of the rear cassette.

Yesterday I tried setting the resistance to 900 watts. At the hardest gear I have on my bike, I can push up to 600 watts with cadence around 100 rpm, and it is just spinning out. I feel like I need shift down at least a few more to get up to 1000 watts (which my bike doesn’t have).

Will try setting the slope instead…

There’s your problem - ERG mode adjusts resistance to maintain power at the same level. The faster you spin, the less the resistance, so it’s no good for max efforts. Try either slope or resistance mode, and use the + sign to increase resistance or slope (assuming you use the app). You will for sure be able to find a resistance you can’t overcome :slightly_smiling_face:


Cool, thx! I’m assuming the Xert EBC app will let me do this. Will give it a try.

That clears up a bunch of things, thanks!