I'm trying to pinpoint why on some days I can cycle on the same stationary bike, with the same settings (saddle @ "E", handlebars height at "7"), for 20 minutes, while other days I can only cycle for 7 minutes before I have to stop due to muscle aches in my thighs.

I've been tracking variables like,

  • sleep - same amount, including deep sleep; sometimes I can cycle for 20 minutes on only 4 hours of sleep, other days I have to quit after 7 minutes despite 7 hours of sleep (in both cases the deep sleep was about 50 - 60 minutes)
  • food intake - both types of days, after dinner, which includes some carbs (though I haven't tracked the amount of carbs very precises). I could also do 20 minutes on an empty stomach in the morning
  • time of day - early afternoon, early evening or late evening don't seem to make a difference
  • hydration - did ~20 minutes while pretty dehydrated

What else could I track more precisely?

Note that I don't have sore legs afterwards, and on the days I can cycle for 20 minutes, I don't really stop due to muscle pain, but rather due to running out of breath or just beating my PR by 1 minute.

I cycle every other day, so I don't think there's risk of overtraining.

  • You make no mention of the schedule of your training. That would be helpful since you may be over-training.
    – rrirower
    Commented Oct 5, 2017 at 19:07
  • @rrirower - updated the question Commented Oct 6, 2017 at 22:59
  • Do you ensure you use the same saddle height each time? Too low or too high would affect performance compared to just right.
    – zeFrenchy
    Commented Oct 9, 2017 at 9:22
  • @zeFrenchy: yes, same saddle and hendlebars height, but not sure about the saddle dept (never touched it). Commented Oct 11, 2017 at 8:37

2 Answers 2


Dan you need to see your doctor ASAP to rule this out.

Intermittent Claudication

"If left untreated, claudication and peripheral artery disease can reduce the quality of your life and lead to potentially life-threatening complications."

This is partial obstruction of the arteries. If the obstruction dislodges this can cause a heart attack or pulmonary embolism. Your symptoms sound similar to early stages but is very treatable.


  • Hopefully it's not that... I have none of the risk factors and my cholesterol levels were just fine (albeit two years ago, I haven't screwed up my diet). Also, I only get the symptoms occasionally... still worth seeing a doctor; thanks for the advice! Commented Oct 14, 2017 at 7:24
  • @DanDascalescu - What you're describing can be associated with a thrombosis. A quick test (blood / ultrasound etc) can rule it out and at this point, can be easily treated. Glad to hear you're being seen! Let me know how it goes.
    – Mike-DHSc
    Commented Oct 14, 2017 at 12:00
  • 1
    My doctor and PA nurse said it's not claudication. Commented Feb 16, 2018 at 21:34
  • -1: Alarmism not useful. Burning/fatigue in thigh muscles is completely normal when cycling at a high power level for several minutes.
    – J. Heller
    Commented Nov 6, 2018 at 19:19

I've had something like that happen once when I did my weekly stationary bike ride. I usually ride for between 8-20 minutes at a constant power level (working up to 20 minutes as I progress, and then moving the power level up and going back to 8 minutes) and then switch down to a lower power level until I've gone for a total of 10 miles.

This one time I could only go for about 7 minutes instead of close to 20 and then had to finish the 10 miles at a lower than usual power level; this was due to fatigue/burning in my thigh muscles, rather than having a high heart rate or being out of breath. I think this was caused by forgetting to take creatine the previous day (at the time, I was using creatine once per day, in the evening). If you don't take creatine, your creatine levels could still be raised by eating a lot of meat or lowered by more intense exercise than normal or reduced meat consumption.

In case it is not clear from the above, my guess for one thing that could be causing your legs to tire out quicker on some days than others is varying levels of creatine in your muscles. On days when your muscles have lower creatine levels, they will tire out faster. Ever since that one time my legs tired much quicker than normal on my exercise bike ride, I have been taking 10 grams of creatine a day (5 in the morning and 5 in the evening), ensuring that my muscles are always full of creatine. I have always been able to ride close to my expected time since increasing my creatine intake.

Another possibility is varying vitamin B6 intake. I normally eat at least two bananas a day. A while back, there were a few days where I didn't eat any bananas for a day or more and then had reduced stamina when lifting weights or cycling.

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