Calculating fitness: What weight, BMI and heart rate say about your health
What’s a “healthy” weight? What does my BMI (body mass index) really mean – and does it matter? What useful information can I pull from tracking my heart rate during or between workouts? These common questions are important to ask. A number is only useful if we understand its impact on us and our fitness and health.
Weight and BMI are very much related: BMI is a measurement derived from your height and weight. To calculate your BMI, multiply your weight in pounds by 703. Then divide that number by your height in inches. Divide that new number by your height again. The result is your BMI. As your weight goes up (or your height goes down – importance of posture!), your BMI increases. The ranges for BMI are:
|Less than 18.5||Underweight|
|Between 18.5 – 24.9||Normal weight|
|Between 25 – 29.9||Overweight|
|More than 30||Obese (there are multiple classes based on BMI value)|
Your weight and BMI are useful, but they do not tell the whole story. Weight and BMI don’t take into account what your body is made up of. For example, a six-foot-tall, 230-pound couch potato will have the same weight and BMI as a six-foot-tall, 230-pound NFL running back. The other important piece of the puzzle is body composition – how much of your body is lean muscle and bone vs. fat.
While in a perfect world you’d want to be in the “normal weight” BMI range, it may be ok to be in the overweight range if you’re very active, have a fair amount of muscle and have a lower body fat percentage. Most people will want to stay away from the BMI obesity range. Otherwise, it’s about finding a weight that allows you to live your life happily – functional and confident.
Heart rate, on the other hand, can be a very good indicator of cardiovascular fitness, that is how efficient your heart and lungs are at pumping blood and getting oxygen to your cells. Heart rate can be a measure of health in two ways: at rest and during exercise. One of the primary exceptions is when someone is taking a heart-rate altering medication such as a beta blocker.
While “normal” resting heart rate is between 60 and 100 beats per minute, fit individuals may have resting heart rates below 60. Their heart is so efficient at pumping blood – a term called stroke volume – that it doesn’t have to beat as often per minute. This phenomenon is called athlete’s bradycardia. Some high level endurance athletes may have resting heart rates in the low 40s!
During aerobic exercise (sustained biking, walking, running, etc.), your heart rate increases linearly with workload. In other words, the harder you’re working, the more frequently your heart needs to beat to get blood and oxygen out to the working muscles. More fit individuals can perform a given workload, such as running at six miles per hour, at a lower heart rate than less fit people because the heart, lungs and muscles have been trained to more efficiently get oxygen into the working cells. As a result, more fit individuals can not only achieve, but maintain work at higher intensity levels.
Aerobic exercise is the key to building endurance. Determining training zones based on your heart rate can allow you to target how hard you can push yourself while maintaining an aerobic state. You can then train to improve your heart and lung’s ability to fuel progressively more challenging workloads without overdoing it.
Jason Machowsky, MS, RD, CSCS, is a registered dietitian, certified strength and conditioning specialist, and certified personal trainer at the Tisch Performance Center at Hospital for Special Surgery. He has an undergraduate degree from Cornell University and a masters degree from Columbia University.