Energy Requirements of Resistance Training: Training Legs Burns 2x More Energy Than Biceps, Squatting 35kcal+/min

Squats feel like and are energy hungry.

I don't know if you own a fitness tracker. If you do, however, you will know that the number it's going to give you when you've been working out (as in lifting weight) is random and usually completely off what you'd estimate you burned in the gym... speaking of which: What's your estimate? How much are you going to burn on that biceps curls and during those squats?

Today's SuppVersity article is going to help you estimate how much energy your workouts really require by providing you with a concise, commented summary of the latest study from the University of Trás-os-Montes & Alto Douro in Vila Real, Portugal (Reis 2017). A summary that will also address how accurate and reliable the results of the study are.
The scientists were interested in filling a gap in contemporary research, which lack studies with thorough investigations of the energy cost during isolated resistance exercise performed at various intensities. All we have are...

• Robergs et al. (2007) based their estimation on a different hypothesis, unlike Reis et al. they used an exponential model to approximate the energy expenditure during squat and bench press (11-18 kcal/min and 8-16 kcal/min for high rep 40% vs. lower rep 70% 1-RM training),
• Scott et al. (2011, 2014) presented a series of studies on isolated exercises, in which they combine aerobic estimates from gas exchange with anaerobic estimates from blood lactate (results 3-16 kcal/min for bench presses, 3-7 kcal/min for biceps curls; and 6-9 kcal/min for leg presses; all from 50-90% 1RM respectively)

With the exception of these studies, there's, as Reis et al. point out, virtually no reliable data on the rate-based energy cost measurements in isolated resistance training - especially, at low-intensity loads, which are of great relevance for mainstream exercise interventions targeted, among others, at the elderly. Accordingly, ...

"The aim of the present study was to estimate the energy cost across various low-intensities at eight popular resistance exercises: half squat, 45° inclined leg press, seated leg extension, horizontal bench press, 45° inclined bench press, wide grip front lat pull down, standing triceps extension on high-pulley and seated arm curl in Scott bench with Z bar. This was achieved by combining measurements of oxygen uptake and anaerobic estimates by the accumulated oxygen deficit method. It was hypothesized that energy cost would be higher in lower body exercises and that it would rise linearly with intensity" (Reis 2017).

Luckily, the scientists did not recruit grandmas and grandpas, but a total of 58 young men (27.5 ± 4.9 years, 1.78 ± 0.06 m height, 78.67 ± 10.7 kg body mass and 11.4 ± 4.1% estimated body fat), who, and that's another plus, engaged in RE training for at least one year with three or more training sessions per week. During the study period, however, no other resistance training was allowed.

What did the scientists do?

Briefly, the subjects were randomly assigned to two exercises. That's "bad" because not everyone did every exercise. Rather than that, "only" 15 subjects performed each exercise. Obviously, that's more practical and will still allow for statistically significant results, but having all subjects do all exercises would obviously have produced a more significant data set.

On the first day, anthropometric measurements were conducted and 1-RM tests were performed twice, on day one and day two.

"On the third to the sixth visit (with 48-hour intervals), the subjects performed (on each visit) two bouts of 4-min constant-intensity exercise -one bout for each of the two assigned exercises [no warm up]. Exercise order for each individual was random and so was the intensity. At each and every RE four intensities were used: 12%, 16%, 20% and 24% 1-RM, amounting a total of four bouts for each exercise All exercises [namely half squat, 45° inclined leg press, seated leg extension, horizontal bench press, 45° inclined bench press, wide grip front lat pull down, standing triceps extension on high-pulley and seated arm curl in Scott bench with Z bar] were performed with trademark standardized machines (Panatta Sport, Apiro, Italy)" (Reis 2017).

What you will probably consider more important, though is that on their last visit (48 hours later) the subjects performed exhaustive bouts at 80% 1-RM (2x in the same two exercises (in random order and with 1-hour recovery between them | this time with 2x15 reps at 24% 1-RM as a warm-up 20 and 10-min before the experiment).

Figure 1 Energy expenditure during half squat, 45° inclined leg press, seated leg extension, horizontal bench press, 45° inclined bench press, wide grip front lat pull down, standing triceps extension on high-pulley and seated arm curl in Scott bench with Z bar; with 80% 1RM (left) and 12-24% 1RM (right | plotted based on Reis 2017).

The scientists analyzed the expired gases by the means of an open air circuit analyzer (COSMED® K4b2, Rome, Italy) and calculated the corresponding energy expenditure as described in Reis et al. (2010 | if you're interested, the file is on ResearchGate).
The statistical analyses of the data revealed three key observations (Reis 2017), namely...

• energy cost increased steadily with exercise intensity in every exercise (see Figure 1, right | sign. intensity effect: F(4, 416) = 796.337; p < 0.001; ηp2 = 0.88);
• the lowest mean values were found in biceps curl and the highest in half squat exercise (see Figure 1, left | sign. exercise effect: F(7, 104) = 62.451; p < 0.001; ηp2 = 0.81);
• a significant interaction exercise x intensity was also found (F(28, 416) = 37.077; p < 0.001; ηp2 = 0.71). 

As you can see in Figure 1, the half squat (some people say parallel squat as you never go past parallel) showed statistically significantly (p<0.001) and practically relevantly higher values of energy cost in all intensities, when compared with the remaining exercises.

Be careful with interpretations: this doesn't mean that your 1h leg workout burns 2156.26 kcal!

One has to keep in mind, though, that the energy expenditure was calculated estimated based on last 30 s of exercise with 10 s averaging procedures. In other words: your energy expenditure during the rest times I am sure you will have during your leg workouts are not included.

Figure 2: Average time under tension (in s) until failure in the different exercise (Reis 2017).

If you do, e.g. half-squats, leg presses, and leg extensions for 4, 3 and 2 sets training to failure with 80% of your 1RM, the data in Figure 2 tells you that you will be actively lifting weights for only 348 seconds, i.e. 5.8 minutes. That's a bummer, right? After all, that's only 151kcal in total. Obviously, you'd have to account for the extra energy expenditure (on top of your basal metabolic rate) while you're resting for ~18 minutes (assuming you rest for 2 minutes).

Figure 3: Most exercises will indeed mostly burn glucose, biceps curls and pull-downs, however, are mostly aerobic (burn fat)

What the study at hand appears to confirm, though, is that intense strength training (80% 1RM) is mostly, but by no means exclusively anaerobic. However, aerobic energy was predominant in biceps curl and in front lat pull down.

The common assumption that it would necessarily fully deplete your glycogen stores, on the other hand, is - although not tested in the study at hand - another bro-scientific myth. You just have to take a look at what scientists have to do to actually deplete muscle glycogen to a decent extent (never fully) to know that your workouts will challenge, but not deplete your muscle glycogen stores. Because of the short time-under-tension, high-intensity endurance exercise is much more suitable to deplete glycogen (Gollnick 1974) than resistance training, anyway.
References:

• Melby, C., et al. "Effect of acute resistance exercise on postexercise energy expenditure and resting metabolic rate." Journal of Applied Physiology 75.4 (1993): 1847-1853.
• Reis, Victor Machado, et al. "Examining the accumulated oxygen deficit method in breaststroke swimming." European journal of applied physiology 109.6 (2010): 1129-1135.
• Robergs, Robert A., et al. "Energy expenditure during bench press and squat exercises." Journal of Strength and Conditioning Research 21.1 (2007): 123.
• Scott, Christopher B., et al. "Aerobic, anaerobic, and excess postexercise oxygen consumption energy expenditure of muscular endurance and strength: 1-set of bench press to muscular fatigue." The Journal of Strength & Conditioning Research 25.4 (2011): 903-908.
• Scott, Christopher B., and Victor M. Reis. "Steady state models provide an invalid estimate of intermittent resistance-exercise energy costs." European Journal of Human Movement 33 (2014): 70-78.