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.