HRMs cannot count calories during strength training

Since this keeps coming up, I decided I am going to write this once and then just repost it.

I know that we are all concerned with counting calories, and we all want to know our exercise calories. Unfortunately, there is no consistent way to calculate actual calories burned during strength training. I also understand that, human nature being what it is, we tend to place great faith in fixed numbers--there is something very authoritative and definitive about that number on our wrists that makes it hard to resist--even in the face of all scientific facts to the contrary. (Remember--for a long time people had trouble accepting that the earth was round).

It's just something you are going to have to live with. Heart rate monitors CANNOT count calories burned during strength training.

The most commonly accepted method for measuring the calories burned for a particular activity is to measure oxygen uptake (VO2).

During *steady-state*, *aerobic* exercise, the TCA cycle is the primary means of producing energy, and oxygen is utilized at a relatively consistent rate depending on the intensity of the exercise. There is an observable and reproducible relationship between heart rate and oxygen uptake. If we have some individual data--resting heart rate, maximum heart rate, VO2 max, weight--it is possible to make reasonably accurate estimates of caloric expenditure based on the percentage of HRmax or percentage of HRreserve at which someone is working.

From the other perspective, basic exercise activities that have a common movement--walking, running, cycling, stairclimbing--have been extensively studied and equations to predict energy cost have been developed that are applicable to most of the general population. Cross trainers/ellipticals are the exception since they do not have a common movement design.

It is under these conditions and with these types of activities that calorie estimating equations and heart rate monitor estimations are the most accurate--exercises and exercise movements that are aerobic in nature and that are performed at intensities between 40% of VO2 max and the lactate threshold.

If an activity does not meet these criteria, then prediction equations and heart rate monitors become less accurate.

When it comes to strength training, they are not accurate at all.

There is a mistaken belief among many people--repeated even by many "experts" on bodybuilding websites--that ANY increase in heart rate reflects aerobic conditioning and an increase in caloric expenditure. This is not true. The primary reason is that the increase in heart rate that occurs with strength training results from a different physiologic mechanism than it does during aerobic exercise.

The increased heart rate that occurs with aerobic exercise is the result of the need for increased cardiac output--the heart must pump more blood to meet the energy demand of the activity. Heart rate increases because of a VOLUME load.

The increased heart rate that occurs with strength training is the result of changes in intrathoracic pressure and an increase in afterload stress. There is no corresponding increase in cardiac output, and thus only a modest increase in oxygen uptake. Heart rate increases because of a PRESSURE load.

So, unlike aerobic exercise, the increased heart rate during strength training DOES NOT reflect either an increase in oxygen uptake or a significant increase in caloric expenditure. Moving quickly from machine to machine to keep the heart rate elevated does not change this fact. It is still a pressure load, not a volume load.

Does this mean that strength training is a less useful activity for weight loss, or that it does not contribute to maintaining a calorie deficit? Of course not. Strength training is a critical part of a weight loss program. Strength training may only have modest immediate calorie burn--actually it's more like a simmer--but that simmer can continue for many hours after the exercise session. But the effects of strength training are not general in nature--they are very specific to the individual, and they are affected by so many different variables, it is impossible to formulate an equation or prediction table that is applicable to the general population. Since the focus of this article is strength training and heart rate monitors, I will not go into detail about the many benefits of strength training and weight loss.

So far I have been discussing "traditional" strength training programs--i.e. structured routines consisting of "sets" and "reps" at relatively heavy intensities--up to 10RM-12RM.

What about "circuit training"? There are two basic types of circuit training--One features alternating cardio and strength stations; the exerciser performs one set at a strength station, followed by a 1-3 min cardio interval, and alternates. The second type features a circuit of strength machines only. Traditionally, circuit training routines feature higher-reps, higher speed of movement and lower resistance levels--often 40% of 1 RM. Because of the lower resistance (or in the case of the first type, the inclusion of cardio intervals), these types of circuit training will involve more of a dynamic volume load, and thus a higher caloric burn. Using HRMs is still problematic, however, because the inclusion of upper-body lifting movements and the higher resistance (compared to aerobic exercise) means that HRMs will most likely OVERESTIMATE caloric expenditure--by as much as 30%-35%. For example, a heart rate of 85% of max that would normally reflect a VO2 of 70% of max might reflect a VO2 of only 51% of VO2 max during circuit training.

Once again, a primary benefit of high-intensity circuit training, cross fit, etc is not only the calories consumed during the workout, but the longer "afterburn" or EPOC (Elevated Post Oxygen Consumption). And, as with strength training, this "afterburn" is more variable and harder to quantify.

Does this mean that heart rate monitors are not useful? Not at all. For a number of aerobic activities--most ellipticals, spin classes, running outdoors, other aerobic-style classes--they are still the best option for estimating calories. And they can be used for circuit training and some mixed classes or cross fit workouts--both as a more vague estimate of calories burned, but also for workout-to-workout comparisons. And for many people, by the time you get to the point where you can and need to start doing more intense lifting and circuit workouts (e.g. tabata, crossfit), calories burned during a workout is less relevant anyhow.