Microsoft Band claims to measure VO2 max. Does it?

In a previous post, I described Microsoft's entry into the fitness wearables market, the Microsoft Band.  To distinguish itself from its competitors, Microsoft loaded the Band with extra sensors including "a UV sensor for sun exposure and a galvanic skin response measurement, which can help identify stress." Continuing this trend, Microsoft (as reported in re/code) recently updated its Band adding new features for sleep data, cycling data, and even purporting to measure VO2 max. The latter caught my attention because usually VO2 max monitoring requires specialized equipment.

VO2 max is the maximum rate of oxygen consumption measured during strenuous exercise (Wikipedia). Increasing your exercise load beyond this point causes one to become anaerobic resulting in lactic acid build-up. It is considered to be a good measure of cardiovascular fitness and aerobic endurance. Endurance athletes such as long-distance runners, cyclists, and cross-country skiers all have high VO2 max scores.

VO2 max is typically expressed as the volume (milliliters) of oxygen per kilogram of body mass per minute (e.g. ml/kg/min). The normalization to body weight is necessary because a bigger person is expected to consume more oxygen. A typical individual has a VO2 max of about 35 ml/kg/min; elite endurance athletes will have scores above 70 ml/kg/min.

VO2 max is measured by placing a mask over the nose and mouth of the subject which measures the volume of air inhaled and exhaled, and the concentration of oxygen in the inhaled and exhaled air (Figure 2). By multiplying volume by concentration one can obtain the total amount of oxygen inhaled and exhaled. From the difference, one can determine the quantity of oxygen being consumed per unit time. Next, the person wearing the mask is subjected to increasing exercise load from either a treadmill or a cycling machine. The exercise intensity is slowly raised until a maximum oxygen consumption value is reached (VO2 max). Increasing the exercise load beyond this point does not increase the consumption.

Factors affecting VO2 max include lung capacity, heart stroke volume, blood volume, red blood cell count, and capillary density of the skeletal muscle. All of these factors relate to the supply of oxygen to the muscles which is limiting.

There is an inverse relationship between resting heart rate and VO2 max. Highly-trained endurance athletes tend to have low resting heart rates along with high VO2 max values. One consequence of cardiovascular fitness is that oxygen supply to the body has been enhanced (see the factors listed in the previous paragraph) so that the heart does not have to beat as much (i.e. lower heart rate). Similarly, one would expect that high maximum heart rates during intense exercise would correlate with a higher VO2 max because more blood would be pumped to the muscles.

Uth, Sørensen, Overgaard, and Pedersen (2004) took advantage of these two relationships between resting (HRrest) and maximum (HRmax) heart rate and VO2 max to derive the following estimate for VO2max termed the Heart Rate Ratio Method:

VO2max = 15 * (HRmax/HRrest) ml/kg/min

The only free parameter is the proportionality constant (15). After fitting this parameter, the authors tested their expression on 36 well-trained men (aged 21-51), comparing the estimates from the heart rate ratio equation to the measured values. The agreement was good with a correlation coefficient r = 0.87 (Figure 1, right). More importantly, the standard error of the estimate (SEE) was approximately 5% with nearly all of the VO2 max estimates having an error of less than 10%.

Figure 1. Measured (x-axis) versus estimated (y-axis) VO2 max values using the heart rate ratio method. Left is absolute VO2 max, and right is relative (normalized by weight) VO2 max. Correlation between measured and estimated values is shown. From Uth et al. [link].

One important caveat to these results is the relative homogeneity of the test subjects, who were all well-trained men of middle age. It is important to check how well this equation (perhaps with a different proportionality constant) works for a more heterogeneous group of individuals.

A second method for estimating VO2 max is called the Rockport Fitness Walking Test. It depends on your one-mile walk time, gender, age, body weight, as well as your heart rate after the one-mile walk:

Because walking is non-strenuous exercise, good cardiovascular health should produce a lower walking heart rate. The advantages of the Rockport method is that you don't need to determine the maximum heart rate (which can be demanding), and that it takes into account other variables like age, weight, and gender. The disadvantage is that it may not be as accurate as a carefully calibrated heart rate ratio procedure.

I would speculate that the Microsoft Band uses the Rockport Fitness Walking Test. This guess would be confirmed if you need to perform a one-hour walk to calculate your VO2 max and then enter information about your age, weight, and gender. On the other hand, the Microsoft Band could easily measure your resting heart rate and then calculate your maximum heart rate by identifying the maximum rate after strenuous exercise that you have ever recorded. The challenge would be picking the correct proportionality constant although 15 would be a good start for middle-aged men. Perhaps, the Microsoft Band uses both methods and then somehow combines the results.

In summary, the Microsoft Band does not directly measure VO2 max. Instead it measures your heart rate (resting, walking, maximum) to estimate this quantity. These estimates (especially using the heart rate ratio method) are likely to be reasonably accurate.

Figure 2. An athlete having her VO2 max measured on a treadmill.