High Heart Rate ≠ Muscle Fatigue

You read that right. Having your entrails coming out of your mouth with exhaustion and deadly pain in your muscles doesn’t mean that you’re burning the calories of your life (while exercising, that is), although it may seem to you that you’re dying 🙂 In fitness terms we call this RPE (rate of perceived exertion) vs. HR (heart rate, i.e. how fast your heart is beating). Specially in isolated strength training you may feel exhausted and unable to do another extra triceps extension because of muscle fatigue, and however your HR be quite low. It makes you question: “then where is the energy coming from, if I’m feeling like dying?” The death feeling :p probably comes from the accumulation of lactate in your muscles, because for short periods of strength we don’t require O2, we only use carbs (glucose, specifically) as fuel. Even if your HR is low.

Below are the graphs of my HR as measured with the MyZone Belt (99.4% accuracy). In isolation muscle exercises (specially small muscles), the rate of perceived exertion was painful as hell, however the heart rate wasn’t high at all. In continuous not-so-exhaustingly-perceived exercise, the heart rate increases smoothly and is easy to sustain:

Anaerobic exercise includes strengthening activities and short, intense (80-90% MHR) workouts. For example, lifting free weights, using weight machines, resistance bands or bodyweight. Short sprints and plyometric exercises that last a short time (up to 2 min) also count as anaerobic. Anaerobic means we don’t require oxygen, but we do require immediate energy, that the body takes from breaking down glucose stored in the muscles. So we don’t rely on breathing here for energy supply. In the absence of oxygen it is the fast-twitch (type II) muscle fibers that can grab that glucose as fuel, which enhances muscle size and tone. A by-product of anaerobic exercise is the build-up of lactate in the muscles (muscle pain). During aerobic exercise, on the other hand, (which is any type of cardiovascular conditioning), we are never breathless because the body is continuously consuming enough oxygen for us to power through. If you ask me, I am all for a combination of both, also called metabolic resistance training (MRT). I lean towards it because you will continue to burn calories up to 36 hours after doing exercise. Needless saying, timing here is crucial.

But. Let’s get to the point. In healthy individuals, a high heart rate would correspond with intense exercise activity, and it would also correspond to a higher oxygen intake. Maximum heart rate (MHR), however, is not really the ideal way to measure the body’s response to exercise. The ideal way is blood lactate levels. Research demonstrates that there is no predictable relationship between heart rate and lactate threshold. This means that lactate borders tend to occur at around 90% of MHR in well trained people, and at 50% MHR in beginners. So, while knowing your heart rate is a great tool to measure your pace, level of fatigue and reference to different types of your own individual training (to track your personal progress), it doesn’t mean much in terms of indicating for sure whether you’re burning fat (using oxygen) or carbs (glucose). Both variables (heart rate and type of fuel) are not coherent or indicative of any definite conclusions. In the same way, high intensity interval exercise seems to recruit more fast-twitch muscle fibres (because of the slow component of oxygen absorption) but research also supports the hypothesis of an increase of proportional number of active slow motor units, which is, at best, not complementary.

This is also interesting:

• HR, Lactate levels and RPE during high intensity alternate exercises are higher than those during constant-intensity exercises.

• Leg exercises have more capacity for anaerobic exercise than arms, because of muscle size and their ability to store more glycogen.

• Aerobic exercises with the large muscle groups increase the breathing rate and HR rapidly, as well as temperature.

• Anaerobic exercise influences positively the cardiovascular system. CNP (a type of protein) is synthesized by the endothelium (the inner ‘skin’ of vessels) during high intensity exercise, which helps the vascular tone of blood vessels, blood pressure and vascular homeostasis.

So yeah, a high heart rate doesn’t necessarily mean you’ll be working out to gain muscle or get leaner. Something to consider 🙂


Links of interest:

Relationship between HR and RPE

Anaerobic and Aerobic training effects on the CV system

Don’t be a slave to heart rate training zones!

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