Estimating Training Stress Scores for Mountain Biking

One moment I’m barely working, the next flat out. Mountain biking is a sharp contrast to the road. The need for short bursts of power and greater technical skills test my weaknesses. I’m enjoying the challenge and will improve. A handful of rides is enough for me to consider an Enduro race in two weeks.

This is supplemental training. It may provide benefits to my triathlon performance, but whatever those gains it will add to fatigue. As with any training I want to account for its contribution to my performance. I want to incorporate mountain biking into WKO’s Performance Management Chart (PMC). Any form of riding will affect bike fitness, I can’t ignore the time spent on trails.

Tracking performance and measuring training is easy with power. WKO takes Powertap data, calculates a Training Stress Score (TSS) and adds it into the PMC. The rare occasions I ride without power I’ve enough experience to estimate TSS well. I know the feel of a 200 TSS ride and whether today was one. Mountain biking is a new world without steady state riding. Constant fluctuations in effort and intensity are unfamiliar; I’ve no frame of reference for estimating TSS.

My budget was spent on the new bike, another power meter is out of the question. Hard to justify the cost when you’re exploring a new area of training. A few trail rides already makes me question their use. Power seems to be a function of necessity – whatever it takes to keep moving forward. I’m too busy concentrating on where I’m going to look at a display. If it takes 500 watts to get up a rise what am I going to do? Get off and push or pedal harder? Post ride analysis would be difficult, but TSS could be calculated.

I want to record the impact of mountain biking on training, but not at the price of a power meter. I lack the experience to estimate TSS from perceived exertion; ninety minutes on the road feels very different to ninety minutes in the woods. I have heart rate data from the rides, this could provide the basis for scoring each session. TRIMP – a measure of training load based on heart rate – could be calculated.

TRIMP and TSS factor training intensity over time, the former based around heart rate and the later power. The results are different, TRIMP values can’t be mixed with TSS in the PMC. I can easily calculate TRIMP in Excel, but need to convert it to an equivalent TSS before adding to WKO. I’m not the first to attempt this, Google provided a few pointers to get me started. I calculated TRIMP for twenty-two recent rides with known TSS; then plotted the values to find a relationship.

TRIMP to TSS Linear Regression

A simple linear regression gives a means to estimate TSS from TRIMP. Applied to this week’s mountain bike rides the numbers seemed high, but ranked them identically to my perception. It appears to reflect the effort involved though I’ve no way to judge the ‘accuracy‘ of TRIMP derived TSS. I’m willing to incorporate the values into WKO and see how they develop.

The approach is flawed. It would be remiss of me not to mention some of the many issues. Not least the assumption that my heart rate is consistent with power output. I expect harder training would tend to have higher power and heart rate, but am well aware the relationship is not linear. Of course it’s as flawed to assume there’s a linear relationship between TRIMP and TSS. The sample set of rides compared steady state TRIMP-TSS, but mountain biking isn’t steady state; the TRIMP-TSS relationship may be entirely different.

There is plenty of opportunity to critique the approach. Without an alternative way to measure impact of mountain biking this provides a means to estimate TSS. With time I’ll be able to judge my training and score TSS myself. Mixing estimated TSS with calculated TSS will widen the margin of error in the PMC, but it would be worse to exclude a portion of training. This rough approach may be the better of two evils.

I bought a mountain bike to add variety to my training and potentially to take my racing off road. It’s about having fun. Even so I want to incorporate the training into my PMC; I want a means of judging its overall role in my performance. TRIMP-TSS conversion may not be the best solution, but it gives me something to work with.

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  • I will be going MTBing tomorrow – for the first time since I started paying attention to my TRIMP. It’ll be interesting to see what it is as it seems that most of the riding around Cannock doesn’t involve pedalling much!

  • I’m really enjoying it. Not particularly good yet, but I view it as a work in progress. Found some nice training loops at Swinley Forest now. Very different to road riding, constant changes in intensity. A lot of easy riding (which TRIMP puts too much value in) and a bit of very hard riding (which TRIMP doesn’t value enough). Potentially something to tweak with a zone based TRIMP calculation.

    Most of the trails I ride on are either physically easy because it’s all down hill and I spend more time to fall off or just a pain to get up. I did wonder what the affect on TRIMP of the occasional adrenaline rush and boost to HR from close calls is!

    The question is do I give the Enduro a shot? I’m informed the trails won’t be that technical so even my skills should cope.


  • Dave

    Hey Russ, nice blog. I got directed here from TrainingPeaks on Twitter. I’m based in Reading and have just acquired a MTB PowerTap with disc rotor, though I’m having it built into a 29er wheel, which is a bit unusual. If that works for you, I’d be happy to lend it to you for a few rides so you can compare power/TSS with your calcs.

  • Hey Dave,

    Very kind of you to offer, my bike is 26″ wheels unfortunately otherwise I’d love to take you up on it. I had considered the way to go would be borrowing a MTB power meter for a while, training on it and doing a TRIMP-TSS relationship for these rides. Likely to better account for the variable nature of power on the MTB.

    If you don’t mind it’s be great to see some MTB power data when you’ve had a play with it? Interesting to look at these entirely different files and to see what you get from it. I’ve been shown a cyclocross file the other day and it was mad! Oh and as you’re a local and know your way around a MTB, any good tips for places to ride other than Swinley/Lookout? I’m enjoying it down there, but a bit of variety would be good. Some of the trails are pretty testing for someone new to the game.



  • Dave

    Hi Russ,

    No probs. If you can get hold of a 29er, the offer is still there. My bike’s a 21″ frame so maybe a bit big, but if not you’re welcome to borrow the whole shebang!

    For MTB rides, I have just been estimating an IF for the ride and letting WKO+ take it from there. Will be interesting to see how my typical rides compare.

    As far as routes, I’ve only really ridden around Swinners as well – I tend to do most of my riding on the road. I also tend to do more MTB riding over in the Surrey Hills, with a mate who lives near there. I’d recommend that if you can get over there, but again it is a bit of a maze and I tend to just follow my mate, who knows his way around. We usually start around Holmbury St Mary, and from there you have access to Leith Hill, Holmbury Hill, Pitch Hill and some others, which you can link up into a pretty challenging loop. I think there’s a good bike shop in Peaselake and you could probably hook up with rides out from there if you fancy exploring the area.

    All the best


  • Thanks again.

    Think I’d need a step to get on a 21″ frame! I’m on an 18″ one, I figure those 3 inches would make quite a difference!

    I’ve heard about Surrey hills, but not yet figured them out. Trails are very new to me as I say so part of the learning experience at the moment is about not falling off! Getting a better sense of how to keep traction and momentum going, but takes time. Good fun though and a nice variation to hours on the road.

    I considered the estimating IF route, but it’s just so different to me, particularly coming from a long course triathlon perspective I find it hard to say. Moments are IF 1.1, others barely 0.5.


  • Douglas

    Same problem I’ve come across, though it’s easier to say MTBing is like training for a crit for me because of the endless short/hard bursts.

    I always just guess at my normalized power / intensity factor based on how hard I was hitting it out of corners and how much non-pedaling downhill there was. I figure if I’m not riding hard, an hour on the mountain bike is more like 55-60TSS compared to the 45-50TSS I’d generate on the road. If I’m going hard, it creeps up to 70-80 TSS/hr, but it’s all subjective! On a related note, it’s the same approach I use to calculating TSS/hr for cross country skiing in the winter. I want the stress in my training log because it’s largely the same muscles but have to figure out a way to compare it to actual riding.

    For data gathering purposes, it would be good to find a good group ride on the road with a lot of accelerations and attacks (rather than just a flat-out paceline) and use the TRIMP/TSS data from that to set your trendline. The effort is a lot similar. That or race a crit or road race and use those. The key is to see how different the TRIMP/TSS ratio is for non-steady state riding. Just a thought from a fellow training geek.


  • Douglas,

    Thanks for the comment and thoughts. Coming from a triathlon background most of my training history is about steady state efforts. My power at short durations is terrible. So the use of my 22 recent rides where long periods are spent holding even power aren’t very comparable with my aren’t very comparable with my few MTB rides.

    Might be worth me joining up with the local cycling club and trying to hang in with the fastest roadies. Towing along local triathletes isn’t quite the same! Clearly a bigger data set with more relevant riding would be useful to get a better picture of TRIMP/TSS. I am wondering about sampling a few other periods of my training history and comparing the relationship. Changes in fitness are going to change that.

    Thanks again for the thoughts


  • Jaime

    Hey Russ

    Good to see you are getting the MTB bug! I will be fully dedicating myself to the sport right after Lanza. I am also doing a 4hr enduro race in a few weeks somewhere in Devon, but also planning to do some X-terra/Triathlon off road races.


  • Jaime

    It’s a lot of fun and I will be looking to race events in a few months time. Decided the Wiggle Enduro event was a bit too close to be testing myself, but down the line definitely up for some off road racing. Will be taking the MTB out again tomorrow, more practice on the trails, trying to master the basics still! Getting better, but terrible at really technical terrain and still bottle too many steep descents! Enjoy climbing though.

    Did you see that X-Man Tri next year? Off road Ironman?


  • Jaime

    I can understand, some descents can be very frightening, but just like everything, the more you do it the better you’ll get at it!Back in Spain people are crazy with mtb, my brother is one of those nutters that throws themselves into almost vertical walls. I learnt quite a bit last year. Also been training with a retired pro from Brazil that finished on the podium on several occassions at Cape Epic. We should def catch up after I finished Lanza.


  • Practice makes perfect after all.

    Will see you out in Lanza and cheer you on. Looking forward to spectating an IM and it’ll be good to catch up. Definitely should meet up for some riding after you’ve recovered.