This is a hot topic that I have been wanting to tackle for a bit, as many people I see use HRMs for tracking calorie burn for strength training in addition to cardio, and question their accuracy.

First, let's get an understanding of something: Aerobic activity versus Anaerobic activity.

Aerobic Exercise

Aerobic exercise is what everyone here considers low intensity cardiovascular activity that can be supported by aerobic metabolism aka the body's ability to generate energy using primarily oxygen. Light to moderate cardio activities like short to medium distance running, dancing, swimming, walking, cycling etc can all be supported primarily by aerobic metabolism. As the body moves oxygen through the blood using the heart, a heart rate monitor can be used to gain a better picture of this than simply using equations based off body mass, height, age, sex etc. It is still an estimation, but in the case of an HRM during Aerobic exercise we go from ball park guesswork to a much tighter picture. This is where you see your body primarily use carbohydrates as an energy source before turning to fat (*kitten* your "fat burning zone", it's not that simple), but basically your body will be fueled by glycogen reserves, fat reserves, or some combination of both depending on the intensity of the activity. This is a slow and steady chemical reaction.

So let's be clear, running a distance over a long period of time at a moderate pace is Aerobic, but sprinting is not. This is where we start to get into the anaerobic flavor of things. Cardio based exercises are something that you can quickly become more efficient at however; run the same distance over periods of times and you will notice your heart rate will be lower each time, and thus the energy (calorie) demands can quickly diminish. This is where cardio kind of sucks for fat loss.


Anaerobic Exercise

Anaerobic exercise is high intensity exercise that begins to demand more than simple oxygen/glucose metabolism can quickly supply in short term bursts; this is where you get into training strength, power, speed, and the building of muscle (lean) mass over fat mass. This is the quick and intense activity (Strength training, HIIT, Sprinting, Jumping on top of things like a psycho), not the slow and steady. Not to try and get to deep into the specifics; there are two types of anaerobic energy systems: 1) the high energy phosphates, (ATP adenosine triphosphate) and CP creatine phosphate; and 2) anaerobic glycolysis. The translation of this is that anaerobic activity needs more ATP (cellular energy) in a short term than aerobic metabolism can quickly provide. This is why ( to put it simply) carbs are AWESOME for strength training as this is what provides all that sexy glycogen storage that can be quickly called upon for energy. This is also why I advocate creatine supplementation for anyone who strength trains because this is essentially a way to bolster those quick energy stores (see energy system number one above).

Fun fact time! Weight lifting is not an activity that requires a huge amount of caloric energy. The energy required to move 200 lbs one foot, say, in a bench press, is less than the amount of energy it takes to stand up. Standing up fifteen times isn't going to make a significant impact on caloric balance. But, the activity of repairing your muscles after a weight-training workout with sufficient adaptive pressure to cause microtears, in other words, a serious strength training workout, is extremely calorically demanding. Protein synthesis required to repair and build muscle uses a lot of energy, and if you're working at an overall caloric deficit, this energy is going to come from your fat stores while you sleep. They key words here are adaptive pressure, as this ties into why I throw out the word "progressive" along with strength training; you need to increase the work load to see the benefits.

(To support the above if you are a math nerd):

200lbs = 890 newtons
Work = Force x Distance = 890 newtons x 0.3 meters = 267 joules
1 Calorie = 1000 calories = 4184 joules

Is there some aerobic activation and cardiovascular involvement with strength training? Absolutely, your heart should be pounding after doing 5x5 back squats with a sufficiently challenging weight. Even that one set of five deadlifts can leave you panting for breath. If the activity is not sufficiently intense (you are using those girly 5lb dumbbells all day every day), this doesn't require that quick burst of energy and becomes pretty much an aerobic activity.


So after I just smacked you all in the face with a bit of science, what about the million dollar question. Does a heart rate monitor accurately measure caloric burn from strength training with progressive loading?

No, not really.

As I mentioned above you can get an idea of the general aerobic demands (which occurs even resting between sets), which is what your HRM and even basic mass based caloric burn calculations will go off of. However even then an HRM is almost as bad as the calculations on MFP or some websites for figuring this out as it is still just a ballpark estimation. The magic is really happening when you sleep and your body has to repair and adapt to all that damage you did earlier in the day. Your body has to go through extremely complex processes to turn energy you have taken in (food), and utilize existing fat stores to not only rebuild those damaged muscle fibers but also (in the case of caloric surplus or 30+% body fat) build new muscle fibers in order to recruit these to perform the lift more effectively the next time. This is why as you need to continually add more load to keep that adaptive stress magic happening.

A fun comparison for this is martial artists who break bricks without wrecking their hands. Did you know bone works the same way as muscle in the above? They continuously break smaller bricks, and actually create micro fractures (breaks) in the bones of their hands that will be repaired over time and be stronger. Then they break more bricks, create more fractures; rinse repeat. This is how these dudes move to plowing through a stack of f*cking cinder blocks with their fist.

In summary, focus less on what a HRM says about caloric burn while strength training and just figure out your TDEE and get on it. Spending 30-45 minutes, 3-4 times a week, with serious weights doing compound movements will work wonders.