Why does cycling feel harder than running at the same heart rate?

Triathletes often make the observation that cycling at the Maximum Aerobic Function Heart Rate (MAF HR) feels a lot harder than running at the same heart rate. Due to a common perception that exercising at the MAF HR should feel “easy,” people often ask whether they should lower their cycling MAF HR by ten or twenty beats in order to bring down the perception of effort for cycling and match it to what they feel when running.

The assumption is that if exercising at the MAF HR corresponds with a certain perception of effort—or as it is formally called, perceived exertion (PE)—a higher PE must indicate the presence of anaerobic function even though the heart rate is the same. If it feels harder, it must be due to anaerobic function (or more generally, that the body as a whole is working harder).

However, this isn’t necessarily the case: As far as the body is concerned, “working harder” and “increased effort” are NOT the same thing.

PE measures the power of a particular muscle contraction relative to the muscle’s maximum contractile capacity (a.k.a. its full power). Every voluntary contraction starts as a signal that the brain sends down the nerves and into the muscle. In order to produce a more powerful contraction, the brain must send a more powerful signal. PE is the intensity of this signal relative to the signal intensity required to produce the most powerful muscle contraction. A contraction that takes up a greater percentage of a muscle’s total capacity produces a more intense PE.

In other words, PE is your brain telling you how close you’re getting to the muscle’s redline.

There’s two things that need to happen for a muscle to contract at a given percentage of its full power:

  1. The requisite signal power coming from the brain.
  2. The necessary oxygen and metabolic fuel availability.

If a particular movement involves a large portion of the musculature, the body will have to distribute its metabolic fuel out across a wide range of muscles. But if a certain movement involves fewer muscles, the same metabolic fuel can be focused to a much greater degree.

When a movement is focused, there is plenty available fuel for each muscle—allowing each muscle to contract at a greater percentage of its full power. But when a movement is distributed, there is less fuel available to power each muscle. Even if the brain sent out a very powerful signal, the muscle wouldn’t contract as hard as expected because the fuel simply isn’t there.

This means that if the body uses the same amount of fuel to contract more muscles, causing each brain signal (and the muscle contraction it provokes) to become less powerful, the PE will be lower. Why? Because PE fundamentally isn’t about how much energy the body (or the brain) is using. PE is the brain telling you what’s happening in the muscle.

A good illustration of this discrepancy is the effort needed to pry open a stuck jar lid. Only a few small muscles in the arm and upper body are involved in this effort. The big muscles in the legs and hips are essentially dormant. Because of this, the metabolic involvement (or total brain involvement) is very low—much lower than cycling or running. And yet the PE experienced in opening a stuck jar lid is extremely high. Why? Even though arm muscles are much weaker than leg muscles, they are contracting as hard as they can.

The reason this matters for the triathlete is because running and cycling are very different: Running is very distributed, while cycling is very focused. This is largely because running has much higher stability requirements than cycling. A cyclist almost always has 5 points of support: handlebars, seat, and pedals. A cyclist is able to keep the upper body relatively still (merely gesturing to maintain balance) while the lower body does almost all of the work. A runner, on the other hand, has at most 1 point of support: the foot they get to place on the ground each step. For a runner, the upper body has to rotate powerfully in order to achieve and maintain balance throughout every step they run.

A cyclist can focus much more fuel into a few leg muscles, while a runner has to make it available across the body. This means that a cyclist’s leg muscles can contract very powerfully in comparison to a runner’s leg muscles—even though as a whole, both bodies are using the same amount of fuel. Therefore, the runner’s PE will be much lower.

While a higher PE in a similar activity typically means more work (which takes the body toward anaerobic function), it is by itself not a surefire indicator of anaerobic activity. As long as the aerobic muscle fibers in a cyclist’s leg muscles are powerful enough that they can accommodate and utilize all the fuel and oxygen that the body can focus into them, that cyclist will be able to work at a much higher PE than a runner without ever going anaerobic.

In my next post, I’ll answer the question of why a person crosses the threshold from fully aerobic to anaerobic at very similar heart rates even when perceived effort, number of muscles involved, or even fuel utilization changes dramatically.

9 thoughts on “Why does cycling feel harder than running at the same heart rate?”

  1. Ever used an elliptical machine in the gym ? I feel it ranks somewhere in the middle of running and cycling in relation to PE at a given heart rate.


  2. I’ve found that I can more easily (far more easily) maintain my MAF on a bike. What I mean is I can stay right at or just below MAF on a bike and keep it up for a long time while maintaining a consistent cadence. I only ride a couple times a week just to add a bit of cross training to running – and I find it just about as enjoyable as running. When I run I have to either shuffle a lot or walk a lot to stay at or under MAF. I also have to look at my HRM a lot more. On my bike I seem to have a better sense of my HRM and when it’s over ‘MAF and when i look, I’m usually right. Running at MAF requires a ton of patience and I usually find I’m 10 bpm over MAF all of a sudden and didn’t realize it.

    Does any of that sound typical?



    1. Brian: It sounds very typical. In terms of feeling like you’re better at knowing your HRM on the bike, it’s probably simply because your cycling perceived exertion (RPE) matches your metabolic output better: that a cycling RPE of say 82% is 86% of your metabolic output, whereas the same metabolic output is a running RPE of 57% (because of the mismatch described in thr article). Does this make sense?


      1. Ivan

        Thanks for the quick reply! Yes, that does make sense to me. Very helpful. I suppose that slowing my pedal cadence a bit to regulate my HRM is really about the same as when I slow down to almost walking while running to stay at MAF – but it just feels more constant on a bike. Struggling to keep a 180 running cadence at MAF.

        Thanks again! A friend introduced me to your site and it’s immensely helpful.


        Sent from my iPhone



  3. Ivan
    Two questions
    1-why is my perceived exertion walking up a steep trail greater than the perceived exertion of running a smooth flat surface when the heartrate is the same for both. My breathing on the uphill is labored but the heartrate is not as high as I would imagine it to be based on the labor, like about at the aneraerobic threshold level or 80 percent max.
    2- you once commented to me that a good hack for ultramarathon racing was to walk the uphills as fast as you can go. Did you say that because at your level of fitness you rarely get into the high anaerobic range so your not blowing out your sugar reserves, or is it something odd like even if your high anaerobically on a climb since you’re walking you’re not blowing up your muscles and it’s not as hard a thing to recover from. I’m trying to figure out racing for an ultra while keeping the priniciple of running it around aerobic threshold, but the big climbs are a problem unless I am walking them very slowly to stay below anaerobic levels.


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