Three Surprising Revelations About the Energetic Costs of Weight Training | Effects of Intensity, Rest, Speed & More

The patterns that emerge in Figure 1-3 already suggest that Crossfit gyms are the places where the average user will "leave the most calories" on the floor. Even regular circuit training programs burn ~10kcal/min (Gettman 1981). Graduate research from 2013 suggests that exhausting Crossfit workouts can burn as much as 20kcal/min (Babiash 2013).

From previous SuppVerity articles, you will remember that the number of calories the classic cardio equipment will tell you you'd burned during your last session often is completely off. You have also learned about a year ago that even the best fitness trackers have a margin of error of ~10% - and that's for walking, jogging or running, where it's really easy to estimate the energy requirements. And, assuming that you've been around for quite some time, you may remember that there's credible evidence that we systematically underestimate the energy expenditure of push-ups, pull-ups, burpees and other bodyweight exercises.

If you know all that, you're a smart alec, but do you also know how many calories you've really left at the gym during your last resistance training workout? I can guarantee, it's not what your fitness tracker is telling you.
Now, I would be lying if I told you that I knew the correct value, what I can and will tell you, however, is how you can get much closer to a realistic estimate by using the data from Rodrigo Ferro Magosso, et al.'s recent review "of the relationship between aerobic and anaerobic metabolism and the main factors that affect EE during RT exercises" (Magosso 2017). As the Brazilian researchers point out, it is common for to determine the exercise-induced energy expenditure (EE) of resistance exercise (RE) using oxygen uptake (VO2) measurements. In that, scientists will usually sum up the aerobic component, the amount of energy that's spent on excess post-exercise oxygen consumption (EPOC), and the anaerobic component to arrive at an estimate of the total energy requirements of the workout. An estimate that may hold up to three surprises.

Surprise #1: Most of the energy you spend during your resistance training sessions feeds into the aerobic (="cardio"), not the anaerobic (=sprint & lift) energy cycle

The first thing that may seem counter-intuitive is that the existing scientific evidence clearly indicates that, despite the predominance of anaerobic metabolism during the actual lifts, the largest part of the energy you spend during and after your gym sessions is spent aerobically.

Figure 1: Rel. (%) contribution of the aerobic (intraset + EPOC) and anaerobic component of energy expenditure to total energy requirements of different resistance training protocols (figure is based on a tabular overview of the data from the studies by Scott and Campanholi in Magosso 2017); LP = leg press, BP = bench press, BC = biceps curls.

Let's take look at the sample data from four studies I plotted for you in Figure 1. It shows quite conclusively that even in the most anaerobic workout, the 50% 1RM 21 rep bench press trial in Scott 2009, most of the spent energy fed into the aerobic, not the anaerobic energy pathway (the average over all four studies is 72% to 28% for aerobic to anaerobic, respectively).

Surprise #2: In response to strength training you burn hardly more energy than you would if you took your dog for the literally walk in the park.

As a SuppVersity reader this shouldn't be that surprising, but for a handful of bros who've always wondered why they're getting fat despite spending 2h in the gym on a daily basis, this may come as a revelation: the highest hitherto recorded energy expenditure during a resistance training workout is 8.3kcal/min that's significantly less than what you'd burn during a decently paced jog.

Figure 2: Plot of the influence of rest intervals, sequential vs. super-set training and using a slow 4s vs. fast 2s rep tempo on the energy expenditure (kcal/min) of healthy subjects in selected studies (based on tabular overview in Magosso 2017).

As you can see in Figure 2 this was observed in Kelleher's 2010 study comparing a regular sequential multi-set to a super-set resistance training pattern - otherwise the workouts were identical with 4 sets to failure at 70%1RM and 1 min rest between sets and supersets of bench presses and bent over rows, biceps curls and lying triceps extensions and leg extensions and leg curls. This is in line with the observation that shorter rest times will increase the energy expenditure per time unit significantly - you got to keep in mind, though, that the reduced workout time with shorter inter-set rest will also reduce the total metabolic cost of the workout... after all, surprise #1 was that most of the energy is spent in-between the sets and after your workout. The simple assumption "The less I rest, the more I burn" is thus not accurate.

Surprise #3: A single rep burns hardly more extra calories than what your body requires to maintain its basal metabolic function

Likewise inaccurate is the assumption that you'd spend a lot of energy on the actual act of lifting a weight or, as it was the case for two out of three studies the results of which I've plotted for you in Figure 3, pushing it up on the bench.

Figure 3: Effect of exercise intensity expressed in % of 1RM on energy expenditure per repetition (kcal/rep) in studies w/ trained individuals by Scott et al. (2006, 2009 & 2011 | based on tabular overview in Magosso 2017).