Today’s Friday Fun Fact is a question I’ve wanted to find an answer to for a while: just how many calories do we burn when weight training?
Heart rate monitors and fitness trackers can provide us with some basic numbers, and are great at determining how many calories we’re burning during some light to moderate cardio work, but when we do any strength training or perform high intensity interval work, they’re pretty useless!
You see, heart rate monitors estimate our energy expenditure through the linear relationship between power output (how much work you're doing per unit of time) and the amount of oxygen consumed (which is equivalent to energy expenditure during aerobic exercise). Because the relationship between heart rate and oxygen consumption is linear, you can use your heart rate to estimate total energy expenditure.
However, this linear relationship crumbles under very high intensities, like when you’re sprinting or doing some resistance training, which is why they’re not reliable.
When it comes to measuring these kinds of activity, fitness trackers aren’t much better. Although they may sense movement, they have no way of determining how much load (weight) you’re lifting. For example, a Fitbit worn on the wrist will likely show the same number of calories burnt whether you perform a squat with ten kilos or a hundred kilos – not very rewarding for you.
So how can we estimate energy expenditure during resistance training?
Before answering the question it’s important we highlight the word “estimate,” or rather, replace it with the word “guesstimate.”
When it comes to weight training, variables such as gender, age, weight, loading parameters, programme design (traditional sets vs. supersets) load on the bar, range of motion etc. will all have an impact on the amount of energy used during a session. So, when we look at this we can only really guesstimate the number of calories based on the evidence that’s been documented in the literature.
When we look at the literature the number of repetitions performed is our best predictor at guesstimating energy expenditure in each session.
• A study by Kelleher et al, comparing supersets to traditional sets, found the total energy expenditure ranged from 260 to 279 calories across 240 reps - that's about 1.1 calories per rep.
• A study published in the Journal of Strength and Conditioning Research looking at eight single set exercises found energy expenditure to range between 70 - 135 calories across 120 reps – that’s about 0.6 calories per rep.
• Hunter et al, took subjects through 8-10 exercises for two sets each (total of 160 reps), and found their average energy expenditure to be 113 calories - that’s 0.7 calories for each rep.
• Lastly, a study that looked at the relationship between rest intervals and total number of calories burnt on a leg press over five sets of 10 reps (50 reps) found an approximate energy expenditure of 90 calories – 1.8 calories per rep. However, when the same subjects then performed dumbbell chest flys for the same number of sets and reps, only 50 calories were burned – 1 calorie per rep (Farinatti, 2011).
As you can see there’s quite a range, with majority of the literature suggesting you burn between 0.5 and 1 calories per rep.
How do you know whether it’s really 0.5 or 1 calorie per rep?
To guesstimate the total number of calories you’ve used during a session I would hazard a guess using the following guidelines:
1 rep = 1 calorie when:
• Favouring super sets over tradition single sets
• As part of a circuit
• Compound exercises in favour of isolation exercises
• Larger muscles over smaller muscles
• Sufficient load is used i.e. nearing repetition maximums
• Rest intervals are incomplete
• Using stimulants pre-workout, such as caffeine before training
1 rep = 0.5 calories when:
• Single sets are performed
• Rest intervals are longer (+1 min)
• Smaller muscle groups are utilised
• Lighter loads are used
• The majority of your session is built around isolation work
The next time you’ve finished a workout and slumped over in a corner of the gym waiting to catch your breath, take a moment to tally up your reps, and match them against the points above to see how many calories you’ve burnt – it may just motivate you crank out a bonus 5 sets of 20 reps on the leg press as a finisher – after all, that’ll be 100 extra calories you’ve just used!