The effects of a cold on athletic performance

Indoor trainers provide a really great way to analyze the effects of a given environmental parameter without all of the fuss of the environment itself changing. Last Saturday I had an indoor trainer workout.  The exact same workout I did just 3 days prior on Wednesday.  Given my bike was on a trainer in my living room, it was easy to compare workouts.  Heck – I even watched CSI (Miami) on the TiVo (well…technically on the Microsoft Windows Vista Media Center) for both of them – thereby also reducing differences (if I had watched Americas Next Top Model and then CSI, they clearly wouldn’t be valid comparisons).  Anyway…the only difference was one I had a very slight cold and the other I didn’t.

You may remember the last time I did some quick and dirty comparison around the effects of alcohol – this time however I was able to remove virtually every potential test changing variable except time of day (noon vs 5PM) – however as you’ll see, that isn’t anywhere near enough to account for the massive differences.

But back to being sick…So late last week I started to get a very slight cold – just a stuffy nose, but otherwise no issues.  It didn’t appear to affect my track workout on Thursday, however my swim on Friday was a touch bit weird as it was if I had a slightly reduced breathing capacity.  Just to point out though – my life carried on as normal, I wasn’t bed ridden or anything else.  Just an annoying stuffed up head.  I don’t get sick a lot – perhaps once a year, and this would be about ‘as bad as it gets’.

So what’d things look like on the trainer?

October 1st – “Healthy” (snippet):

Avg Power for section: 242 watts (upper yellow line on chart)
Avg Heart Rate: 138bpm (not even Z2 for me – upper red line on chart)


October 4th – “Not Healthy”:

Avg Power for section: 191 watts
Avg Heart Rate: 145bpm (middle Z2 for me)


So what does this mean?  Well, it means what we all probably already knew – being sick makes your performance suck.  How much suckage?  In my case – 21% loss in power while also increasing HR by 7BPM.

If you look at the entire 59 minute workout, then my overall power is reduced by almost 25%.  Yes, basically a quarter of my cycling power is lost.  Doh!

Oct1Summary Oct4Summary

(October 1st on left, October 4th on right)

So anyway, we’ll keep this short.  I just found it interesting, that’s all.  Of course, because I generally ignore semi-logical advice – I still ended up running 20 miles the next day.

Have a good weekend everyone!  And yup, I have the Garmin Forerunner 405 write-up planned for Sunday night sometime, based on the results of the vote last week.  And then I’ll do the rest of the products at one per week in the order they were voted on (see the little results gadget on the right hand column).  Oh, and I’m off to Dallas next week too – and I was just starting to enjoy the nice cool weather up here!


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  1. So how come you ended up going 5 miles farther on the bad day?

  2. Ahh yes…the fun of the Computrainer. It all just depends on the gearing I’m in. The CompuTrainer will provide event resistant regardless of gear (in this particular mode) – so it doesn’t matter which gear I choose from a cadence and/or HR standpoint as it will simply adjust to add pain accordingly. However, from a wheel rotation standpoint the gearing I choose will affect my speed/distance. So effectively, speed/distance are meaningless with this particular mode.

  3. it confirms our suspicions…

    feeling especially when you train or race and the suckage is more pronounced….

    btw…your post sucked.

  4. Hope to see you in Dallas sometime next week!

  5. Dang it! I wish you were headed to H-town rather than the Big D.

  6. SLB

    Just goes to show if you feel like crap you are! Relatively speaking of course!

  7. Phil Mahony


    I picked up a slight cold and cough on Boxing Day (26th Dec.) Although the symptoms have disappeared, I am still struggling to complete sessions on Trainer road that I would normally find hard but easily achievable. Its now 20/1/17 and this morning again was another bail out on Trainer road. My heart rate is sky high on intervals that normally have it at least 15 beats less. My legs feel totally drained. Is this normal?

    • Adrian Bennett

      I’ve had a similar issue due to having a cold / cough for 2 weeks. Just before I became aware of being ill I had to abort a FTP test as couldn’t maintain power during the first significant ramp in the warm-up. And yesterday did a half marathon yesterday about 8 minutes slower than hoped for. First 3 miles were OK then struggled badly for 10 miles. Pace dropped but HR didn’t! After my legs feel worse than if I did a 20 mile run.

  8. Salim

    I’ve had a cold for 4 days but have a boxing match in 7 days now and this is slightly concerning, I’ve been training whilst ill but do you think ill be okay for my fight next friday after 10 days post illness?


  9. Bruce Hill

    This is very comparable to my results after a cold. I did a Zwift race on Jan 2 and my 20 minute power was 216 watts which is 1 watt less than my best ever 20 minute FTP test. Nine days later on Jan 11 (after a cold a few days earlier), I did a 20 minute FTP test and my avg was 176 watts – 40 watts and 18.5% less.