As Valentine's Day approaches, we're increasingly confronted with "artistic" images of the heart. Real hearts hardly resemble to two-lobed shapes adorning cards and candy boxes this time of year. And the actual shape of the human heart is important for its function of supplying blood to the entire body. You have likely noticed that your heart beats more quickly when you exercise. But have you ever taken the time to observe how long it takes to return to its normal rate after you're done exercising? In this science activity you'll get to do some exercises to explore your own heart-rate recovery time.
Your heart is continuously beating to keep blood circulating throughout your body. Its rate changes depending on your activity level; it is lower while you are asleep and at rest and higher while you exercise—to supply your muscles with enough freshly oxygenated blood to keep the functioning at a high level. Because your heart is also a muscle, exercise, in turn, helps keep it healthy. The American Heart Association recommends that a person does exercise that is vigorous enough to raise their heart rate to their target heart-rate zone—50 percent to 85 percent of their maximum heart rate, which is 220 beats per minute (bpm) minus their age for adults—for at least 30 minutes on most days, or about 150 minutes a week in total. So for a 20-year-old, the maximum heart rate would be 200 bpm, with a target heart-rate zone of 100 to 170 bpm. (For those 19 or younger, target zones can vary more than they do for adults.)
After exercising, a person's heart needs time to recover, or to return to its normal, resting heart rate. How long it takes for the heart to resume its resting rate is referred to as heart-rate recovery time. In general, people who exercise regularly, and therefore are more likely to have healthier hearts, have faster heart-rate recovery times than people who do not regularly exercise. So after a 100-meter dash an Olympic sprinter would return to a resting heart rate faster than someone who rarely ever runs.
• Pen or pencil
• Scrap piece of paper
• Clock or stopwatch that shows seconds (You can also use the stopwatch app on a smartphone.)
• Simple and fun exercise equipment, such as a jump rope, hula-hoop, stepping stool, etcetera (Alternatively, you can do exercise that does not require equipment, such as jogging or jumping jacks. Remember to always stop exercising if you feel faint or lightheaded.)
• Calculator (You can also use a smartphone app for this.)
• Adult volunteer who can safely exercise (This is optional, but will allow you to calculate more precise target heart-rate zones.)
• Practice finding your (or your volunteer's) pulse. You can do this by using the first two fingers on one hand to feel the radial pulse on the opposite hand's wrist. The radial pulse is found on the "thumb side" of the wrist, just below the hand. You will need to be able to find the pulse quickly, so practice finding it until it is easy to do.
• You can alternatively use a carotid pulse, but make sure you know how to take it safely by pressing on the neck only very lightly.
• Prepare a scrap piece of paper so that you can quickly record your testing results when doing the activity. You will be measuring the resting heart rate, the heart rate immediately after doing a short exercise, and then every minute for the next five minutes. After that you will measure heart rate every two minutes. You will continue to measure it until it returns to its resting rate.
• Measure your or your volunteer's resting heart rate. To do this, take the pulse (after several minutes of resting) and multiply the number of beats you count in 10 seconds by six. This will give you the resting heart rate in beats per minute (bpm). What is the resting heart rate? Write it down on your prepared scrap piece of paper.
• Choose a vigorous activity. This could be jumping rope, stepping on and off a low stool, hula-hooping, jogging, etcetera. Then, engage—or have your volunteer engage—in that activity for about two minutes.
• Immediately after the end of the exercise period sit or have your volunteer sit on the chair and quickly measure and record the heart rate (again, counting the number of beats in 10 seconds and multiplying that by six). What is the heart rate after exercising? Write this number down.
• Now, record what time it is (including seconds) or start a stopwatch.
• While you or your volunteer continue to sit and rest, measure and write down the heart rate every minute for the next five minutes. How does the heart rate change as you continue to rest after exercising stopped?
• If the heart has not reached the resting rate, continue to rest, but only measure and record the heart rate every two minutes. Continue measuring it until it returns to the resting rate. How does the heart rate continue to change over time?
• What was the heart-rate recovery time, or the amount of time it took the heart to return to its resting rate after exercising? How did heart rate change over time after exercising stopped?
• Extra: Graph your results. How did heart rate change over the course of this activity?
• Extra: Do this activity again, but this time, include volunteers of different ages. (Be sure that the volunteers you recruit can safely exercise.) Does heart-rate recovery time change with age?
• Extra: Try repeating this activity but compare a group of athletes or regular exercisers with a group of nonathletes or people who have a more sedentary lifestyle. How does the heart-rate recovery time of people who are physically fit compare with people who do not exercise regularly?
• Extra: Design an activity to compare the heart-rate recovery time of people who smoke with people who don't. Do smokers have increased heart-rate recovery times compared with nonsmokers?