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