Numbers can be a great thing. They strip away the hazy vision of wishful thinking and tone down the tint of rose colored glasses. Alternatively, they can shed light on what appears to be only shadow and reveal the rainbow behind dark clouds. Perhaps the best thing about numbers is their objectivity. That’s what makes them such a useful tool when it comes to tracking progress towards fitness. While the numbers themselves may be completely objective, our interpretation of them may not be. Each of us has a level of tolerance for “numbers truth”, and it is important to take that into account when using fitness data.
I think we can all agree that the elephant in the room when it comes to fitness numbers is weight. Most people who undertake a diet and/or exercise program are at least initially motivated to do so in an effort to confront the scale. Weight is quite likely the most useful and accessible fitness number available. It is also a prime example of the tolerance just mentioned. Regardless of the data being tracked, the more data points there are, the more accurate the tracking picture will be. That being the case, it would seem optimal to weigh yourself every day. It’s an easy habit to form if it is done consistently. I weigh myself every morning as soon as I get up and before I dress. That takes recent meals, water, and clothing out of the equation and makes the measurements as comparable as they can be. I was initially a little surprised by how much variance there was day to day. Most of the time, the difference is a fraction of a pound, but there have been times when it has been as much as a pound and a half. Temporary gains are normal, and can generally be attributed to differences in hydration levels or recent consumption of something high in sodium. Likewise, drops in weight after training days are a reflection of water lost through perspiration. In either case, weight will level out within a day or two. If seeing fluctuations of this sort is something that will likely trouble you, then perhaps it would be better to weigh yourself less frequently. Once per week is the minimum to produce any sort of reliable data concerning trends. It is suggested that weekly weigh-ins should be done on Wednesdays, as those are most accurately reflective of the average of the seven days.
Another number gets frequent mention is BMI, or Body Mass Index. Even medical professionals attribute some significance to this measure. It can be useful, to a point, but it is based on some underlying assumptions that are not valid for everyone. These need to be taken into account before determining how much weight BMI should carry in any specific case. BMI is simply a mathematical ratio between weight and height. In U.S. measures, it is the weight in pounds divided by the square of the height in inches, and multiplied by a conversion factor of 703. The National Institute of Health will do the math for you with their online BMI Calculator. BMI uses raw weight without taking into consideration overall body composition. A person who weighs 150 pounds and is an elite athlete with 12% body fat is not the same as a person who weighs 150 pounds and is an elite couch potato with body fat of 36%. Yet, both would have the same BMI. BMI is not accurate for anyone who has above average muscle mass, even a weekend warrior. Likewise, it will not truly reflect the health of an older person who may have reduced bone density. Charts are readily available, but 18.5 – 24.9 is generally considered to be the “normal” range. There is certainly no harm in knowing where you fall with BMI, but it should be taken with the appropriate grains of salt.
Weight and BMI are the only two numbers that will matter to most. For those who want to take fitness further than simply being “in decent shape”, numbers related to body composition come into play. There is a lot of controversy surrounding the various techniques to determine body composition. The most clinically accurate measure require advanced measuring equipment and trained professionals to operate it. Expense is considerable, taking such measure outside the reach of the average person.
Fortunately, those of us who are not professional athletes can focus more on trends than on hard numbers. Several companies market “smart scales” that use bioelectric impedance to measure body composition. Stepping on a smart scale barefoot passes an imperceptible electric current through the body and measures the time it takes to return. Then, applying mathematical equations, it delivers an estimate of the amount of muscle, fat, water, and bone. They key word here is “estimate”. The scale might say that you are at 15% body fat, and that may be in the ballpark, but it is likely not completely accurate. The point of using such a scale isn’t to produce hard numbers, but rather to reveal trends. If you see your body fat at 15%, then notice it regularly reaching 17%, then 20%, it is safe to assume that your body fat is increasing even if the actual percentages are not clinically accurate. Trends are much more actionable than less frequent and isolated hard numbers. As with BMI, it is important to know what these body composition numbers are and what they are not. Another advantage to a smart scale is that it will typically connect via Bluetooth or WiFi to a smartphone app or website. That makes it much more convenient to track your measures since it is not necessary to manually record them in a spreadsheet or notebook.