The benefits of a cycling class, whether it is in a boutique studio or a big-box gym, are plentiful. From aerobic development to muscular endurance, cycling offers a safe, non-impact, easily accessible option for individuals to affect their health. One of the most common questions we hear from instructors and participants alike is “How many calories did I burn in my cycling class?” This is a valid question; however, it is not always so easily answered.
In this article, we discuss the different ways calories are burned in the body, some of the myths around measuring calorie burn, and some of the best ways to estimate your caloric expenditure from your cycling session.
The different ways calories are burned
Unless you’ve been living under a rock for the past few decades, you will have at one point heard, “To lose weight, you need to burn more calories than you take in.” But what does that mean?
Metabolism is the sum of all chemical reactions in the body to either use or create energy. It is a process whereby the food and liquids you consume are broken down to fuel all functions of the body, including ventilation, blood circulation, controlling body temperature, cell growth, adjusting hormone levels, brain and nerve function, contraction of muscles, and maintaining a state of homeostasis. Metabolism means life. The body is constantly either in a state of anabolism (the buildup of substances) or catabolism (the breakdown of substances). Adenosine triphosphate (ATP) is the main molecule used to fuel all of these processes, and even the act itself of creating energy requires energy.
Basal metabolic rate (BMR) is the amount of energy that a person needs to keep the body functioning at rest. The basal metabolic rate typically accounts for about 60% to 75% of the daily calorie expenditure by individuals and is impacted by factors such as age, gender, body size, and body composition. The energy requirements of the body stay fairly consistent and are not easily changed.
Resting metabolic rate (RMR) is defined as the amount of energy required to keep the body functioning and in homeostasis while asleep or in a low-energy state. RMR includes calories burned while eating and doing light non-exercise activities (NEAT). RMR readings are typically higher than BMR readings. According to the US Department of Agriculture, RMR is 10%–20% higher than BMR. While there are things you can do to impact your RMR, a larger impact may come from calorie burn as the result of planned exercise.
Hence the question in question: “How many calories did I burn in my cycling class today?”
The different ways calorie burn is measured
When a person burns calories, the body uses oxygen and produces carbon dioxide. Due to the fact that not all body processes that use energy require oxygen, we are able to isolate and measure the ones that do. It means we can quantify the consumption of oxygen and the production of carbon dioxide as an indirect measure of calories used at rest and in exercise.
The only way to closely estimate calorie burn rates for an individual is in an environmentally controlled lab, using either a treadmill or an ergometer stationary bike, a heart rate strap, a face mask, and a metabolic cart. A metabolic cart uses indirect calorimetry, which determines the energy expenditure of a person during rest, steady-state, and progressive exercise by measuring expired air per unit of time and determining the amount of oxygen utilized.
As you can imagine, there are some roadblocks to utilizing this methodology for the masses. It can be costly, it is not readily available outside of a testing facility, a trained individual must run the test, and it captures data for an individual at a singular point in time. Knowing this, how can we help our instructors and our students understand how they can accurately predict their calorie burns in class? The next section will outline some popular methodologies currently in place.
Smartwatches. In 2016, I presented a session at a fitness conference on “Wearable technology—the number one trend in the fitness industry.” According to the American College of Sports Medicine, it was the leading fitness trend. In 2019 wearable tech again tops the list.
While there are many different forms that wearable tech is taking, leaders in the industry have designed and refined smartwatches to democratize health and fitness tracking for the masses. According to a 2017 study at Stanford University using over 60 test subjects, results showed that the most accurate of seven different monitors were off by an average of 27%, and the least accurate was off by 93%. The heart rate monitor was the most reliable function, but the researchers concluded that the calorie counter should not be counted on. Smartwatches also calculate the “gross energy expenditure,” which includes the calculated calories you would burn including your RMR, rather than the “net energy expenditure,” and it is not hard to understand that your own actual calorie burn will vary.
Bottom line, if you’re just walking and running, most smartwatches have good algorithms to get rid of light and movement artefacts for heart rate monitoring. However, if the intensity is a lot higher, above 160 beats, then the blood passing is so fast that beats may be missed or delayed. If you’re doing 10- to 20-second intervals and there’s a delay of 10 seconds, it’s beyond unacceptable. The more athletic you are the more likely you are to wear a chest strap.
Heart Rate Monitors. The heart adapts to the needs of our body. It is important to understand that heart rate response is a measure of how hard the heart is working, not necessarily how hard you are working, how much power you are pushing, or how much energy you are expending. Understanding the various factors that impact heart rate will help to highlight that heart rate monitoring is a great tool but is not an accurate proxy of the actual work that we are doing.
Inaccuracies regarding heart rate monitoring start with the formula designed to determine your maximum heart rate (MHR). MHR is the highest number of beats per minute (bpm) that an individual can achieve in an all-out effort. Industry standard for this calculation is the Fox and Haskell formula: 220 – age = MHR, understanding that as we age we have less beta sensitivity, therefore heart rate response decreases with age. The problem with this formula is that it has no scientific basis. There are over 36 MHR equations and this particular one is the least accurate, with a 12 bpm standard deviation. There are other calculations, such as the Karvonen Method, Tanaka formula, and the Inbar calculation, which are much better suited for individuals looking to track their intensity. While these are all good for establishing “heart rate zones” for training intensity, the fact remains that doing an actual maximal heart rate test is not appropriate for the vast majority of exercisers that we see, given the intensity and strain the individual would be subjected to.
Heart rate response is as unique as your fingerprint. Here is a list of factors that can temporarily or permanently affect your heart rate response on any given day.
Age: The maximum heart rate usually decreases with aging because the activity of the sympathetic nervous system decreases. (But it’s not necessarily linear and being active may reduce the decline.)
Gender: Women usually have a higher heart rate compared to men because women have typically lower VO2 max and smaller heart size.
Fitness level: If your ﬁtness level is high, your heart rate decreases faster in recovery.
Resting heart rate: When your ﬁtness level improves, your resting heart rate decreases.
Maximum heart rate: The higher your maximum heart rate is, the higher your response during training.
EXERCISE FACTORS and ACTIVITY
Due to different muscle mass involved, level of experience, and technical proﬁciency the heart rate response across activities can change.
In group exercise, heart rates are higher in high-intensity cardio classes than muscle conditioning classes.
Running typically elicits the highest maximum heart rate during a stress test, whereas maximum heart rates while cycling and swimming can be 10–15 beats lower during a similar test.
Temperature: Hot or humid conditions elicit a higher heart rate given the strain the body is under to push blood to skin surfaces in an attempt to keep core temperature in check.
Altitude: Due to reduced oxygen content in the air, higher altitudes make oxygen extraction and delivery to working muscles more difficult.
Hydration and fuel: Poor nutrition or hydration mean the body has to expend more energy for the same or lower effort and output.
Excitement: Norepinephrine, epinephrine, and even raising your voice can cause changes in heart rate not attributable to effort levels or energy output.
Stress, low hydration levels, smoking, and alcohol will all raise your heart rate.
Nutrition: If you start to run low on carbohydrates, it will become difﬁcult to maintain your pace at a given heart rate. Your perceived exertion and subjective feeling will increase but your heart rate will be falling. This can be corrected by eating food high in carbohydrates.
Hydration: Insufﬁcient hydration increases your heart rate as the blood volume is reduced and becomes more viscous. The heart has to work harder to deliver the same volume of blood through the system.
Medication: Affects your heart rate in both ways, either raising (e.g., asthma medication) or lowering it (e.g., heart and blood pressure medication).
Smoking/drinking habits: Smoking and alcohol consumption raise your heart rate due to inefficiencies created within bodily processes.
Garmin, one of the leaders in heart rate–based activity tracking, believes that heart rate is a good source of information about the amount of energy the body spends performing any activity. “However, it is very difficult to correctly predict energy expenditure based on heart rate alone, because there are huge differences in heart rate behavior between individuals, such as different maximum heart rates, resting heart rates, and fitness levels.”
Bottom line, heart rate monitoring is a great tool but does not determine calorie burn. Don’t confuse an increase in heart rate caused from being excited, hyping up your participants, and cueing loudly in class to mean you are actually burning more calories.
Indoor bikes. Stationary bikes are in their own class of cardio machines found in gyms because they support your body weight. Some less sophisticated indoor bikes calculate calorie burn through the use of algorithms that are based on testing and research on volunteers using the machines, however this varies from one manufacturer to another and testing protocol may be insufficient. Many older bikes base the caloric burn formula on what a 175 lb male would burn at the equivalent speed and duration. Regardless, researchers at the University of California at San Francisco’s Human Performance Center found stationary bikes to be the most accurate of all gym cardio machines when compared to metabolic cart testing.
Newer bike models calculate power expenditure of the rider in the form of watts, which can then be used to determine energy output. If the bike is calculating calories based on technical data such as watts, the calorie readout can be quite reliable.
Bottom line, out of all cardio equipment available in a fitness facility, the indoor bike is going to be the most accurate when it comes to calculating caloric burn; however, bikes that do not measure power output of the rider will be less accurate than bikes that do.
Power Meters. Many indoor cycling classes now use higher-quality bikes that measure performance metrics such as cadence, power, and caloric output based on watts. Watts are a measurement of the rider’s power output and are a result of cadence (rpm) X force (gear). Watts are typically measured through a strain gauge located either in one pedal, both pedals, or the crank arm on indoor bikes. A strain gauge has thin flexible strips with a metal pattern. When force is applied to the pedal, the strain gauge is stretched. This causes the metal pattern on the strip to alter. The bigger the stretch, the greater the force that is being applied to the pedal. If you have an opportunity to attend a cycling class with bikes that measure power, be sure to take notice of your wattage output throughout class—it is a great training tool.
The average wattage output throughout class will allow for the most accurate reflection of your calorie burn from exercise once watts are converted to kilojoules and then kilojoules are converted to calories. Sound complicated? Don’t worry, the power meter will do the calculations for you so that at the end of your class you will be given a highly accurate idea of how many calories you burned based on your own power production. If you are interested in taking a deep dive into the conversion of kilojoules to kilocalories you can read about it in this article. Essentially, the end formula for figuring this out is quite simple given that the conversion from kJs to kcal is essentially canceled out by the average efficiency rate the human body exhibits on the bike. The final formula is energy (kcal) = avg power (W) X duration (hours) X 3.6.
Bottom line, outside of getting tested in a performance laboratory, cycling on a bike that accurately measure watts is the best way of determining how many calories you burned during exercise. Keep in mind, different training protocols such as HIIT or VO2 max repeats on the bike will typically show a lower caloric burn during class given the low-intensity (i.e., low wattage) recovery required in between efforts. But they do not take into account the effects of EPOC, which allows for some additional caloric burn as the body eventually returns to a state of homeostasis following the workout.
The good news is that with consistent training and attention to power output, you have the ability to determine how many calories you can burn in a class.
Why Does Your Heart Rate Monitor Say You Burned More Calories than Your Power Meter?
In short, heart rate gives you an indication of what’s happening in your body, while power is the result of that effort. In a perfect world, training with watts and a heart rate monitor is going to give you the best insight into what is happening with your fitness levels. However, given that heart rate is subjective to many factors that may occur in a class—heat, dehydration, effort, hormonal responses, and other physiological responses—it is not a reflection of caloric burn. Additional confusion arises due to the fact that some heart rate monitoring systems include the body’s estimated basal metabolic rate in the “total burn” amount during exercise. So, not only do these systems estimate caloric expenditure based on an inaccurate parameter, they also include the calories you would burn anyway, just sitting there, which in some cases can over-report energy expenditure by 50–100 calories per hour.
Power meters, on the other hand, only report the actual calories burned as a result of the work performed (kilojoules) during that training session. The number may appear to be a lower number than on your heart rate monitor, but not only is it more accurate, it is also more relevant. This is the only way you can compare one workout to the next.
No method of determining your caloric intake or your caloric burn outside of a laboratory is going to be 100% accurate, but with indoor cycles the power meters are the most accurate. The different methodologies of measuring your intensity provide approximations that can be useful for setting general guidelines for what you want to accomplish, but if you really want to see fitness improvements, the key lies in being able to measure your power output. As we become fitter, we become more efficient and the body starts to find shortcuts for doing the work that we do. The best thing we can do is vary our workout regime, stick to a training schedule that provides progressive overload to encourage those physiological adaptations, and stay consistent in being active, whatever the format or tracking methodology we choose.