Fitness trackers and sports watches use build-in sensors to break a complex movement down to its more basic parts, e.g. linear acceleration and rotation along the single axis. Given this data, each company uses its own set of algorithms to identify the type of movement. The following description is based on the assumption that every fitness tracker or sports watch is able to identify certain movements as steps. This is important as steps are the basis for the calculation of calories burned and distance covered.
Calculating distance covered
If you use a fitness tracker or sports watch for the first time, it is usually required that you create a user account. One part of setting up your account is to provide personal data by means of
- Date of birth
- Body height
With this data, it is possible to lookup databases that provide the average step length for a person of your sex, body height, and age. Such databases result from screenings and are consolidated by statistical means.
If the step distance is known, it’s easy to calculate the distance covered by multiplying the total number of steps and the step length.
Distance too short or long?
If you consider this simple approach, it should be obvious that the results may be correct for some users, but may differ from reality for others. Even if they share the same height, their step length may differ depending on the length of their legs. To make things worse, people having the same leg length may have different step lengths. For some users, the wearable might show a distance that is too short compared to reality, for others, it might show a distance covered that seems to be too long.
More sophisticated devices allow for calibrating your step length. This is done by covering a fixed distance (let’s say 400 meters so that you can use your nearby sports stadium). Usually, the calibration is started and stopped by pressing a button on the fitness tracker or sports watch. Given the distance and the steps measured, the device will derive the step length.
Why are calibrated step lengths do not guarantee for exact results?
If you watch people doing their daily routine, you recognize that their movements are far from steady. People are giving way to bicycles, turn around the corner or slow down to watch the displayed goods in a shop-window. All these movements, if considered as steps, probably do not match the average step length.
Calculating calories burned
Calculating the calories burned is different from calculating the distance covered, but again it is based on mean values. For the distance covered the body height is the most influential value. Sex, age and weight gain more importance when it comes to calculating the calories burned. This data allows for deriving the basal metabolic rate.
Basal metabolic rate
The human body is burning calories all the time, providing energy to keep you alive, even when you are sleeping. The basal metabolic rate refers to the amount of energy per unit time that a person at rest, on empty stomach and in a thermally neutral environment needs to keep the body functioning. A precise measurement (in kcal/day) requires a strict set of criteria that must be met. In addition to these complex, clinical measurements there are some formulas that predict the number of calories required.
Metabolic equivalent of task
Most often, a person owning a fitness tracker or sports watch is not interested in the basal metabolic rate, but in the number of calories burned due to extra workouts, running, cycling and the like. To ease comparing different activities the metabolic equivalent of tasks (MET) is used. The MET is a simple number that describes how many calories are burned compared to the basal metabolic rate during the same interval. For example, a MET of 2 describes a task, when done burns twice as many calories as the body at complete rest.
MET values are available for a lot of tasks/activities. The following table is just an excerpt:
|Task / Activity
Greater precision if heart rate is considered?
Monitoring the heart rate might increase the calculation’s precision however, the relation between heart rate and energy consumption has to be known. This necessitates sport-medical examinations. As fitness trackers and sports watches again make use of mean values, the result’s precision will increase for many people. For others that differ from these mean values, it won’t.
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