Many exercise recommendations mention working at a "moderate" or "vigorous" pace, but what do these translate to? Moderate is about 60-75% of the maximum heart rate, and vigorous is about 75+% of the maximum heart rate. This definition is still problematic because many people don't know how to calculate their maximum heart rate.
The first step in calculating your heart rate is to know your resting heart rate. The best time to measure the resting heart rate is first thing in the morning after waking up. Use a stop watch to count your pulse for 15 seconds, and multiply the number of pulses counted by 4 to get the beats per minute (since 15 seconds goes into a minute 4 times). For example, if you counted 18 pulses in 15 seconds (18 x 4) then you have a resting heart rate of 72bpm.
Checking the pulse through the carotid artery. Personal photo.
Next, subtract your age for 220 to get the maximum heart rate for your age. For example, a 25 year old has a max heart rate of 195 because 220-25 = 195. 220 is used because it is the estimated number of beats humans are born with, and on average, we lose about 1 beat each year we age. This is one reason children have faster heart rates than adults.
Here is where the math can get a little more complicated. Subtract the resting heart rate from the maximum heart rate to obtain the heart rate reserve (195-72=123 in this example). The heart rate reserve is what we will use to multiply with the percentages of moderate to vigorous intensity. So now we have several numbers: resting heart rate, maximum heart rate, heart rate reserve, and intensity percentage.
Moderate exercise should be enough to make you sweat. AP Photo/Andy Wong
If we want to work at 65% of our maximum heart rate then we must move the decimal in the percentage so we can use it in an equation, so it becomes .65 instead of 65%. Finally, multiply the heart rate reserve by .65 (123 x .65 = 79.95) and then add the resting heart rate to the equation (79.95 + 72 = 151.95). For this particular 25 year old with a resting heart rate of 72, a moderate intensity of 65% is a heart rate of 152.
There is an easier, but less accurate way to complete this equation that only requires the maximum heart rate (220-age) and the desired intensity (65%, 70%, etc.). Using the same 25 year old and the easier equation, we would calculate maximum heart rate (195) multiplied by 65% (.65) to get 127. The harder equation is longer and more complicated, but it is more accurate to the individual it is being performed on. The easier equation can be wrong by several beats into either direction since it doesn't account for the individual's unique resting heart rate. The heart rate charts on machines are generally calculated using the simpler equation.