Many coaches, athletes and fitness buffs find it convenient to describe the intensity of an aerobic workout in terms of heart-rate zones. Though the exact boundaries of these zones vary from source to source, the texts and websites depicting such zones share numerous features. A treadmill is an excellent venue for doing workouts with a narrow heart-rate focus because you can so precisely control the speed and grade of the belt, and the so-called weather doesn't change.
Heart-Rate Zones Defined
Brian MacKenzie, a longtime UK Athletics coach, uses four training zones: recovery, aerobic, anaerobic and "red-line." For purposes of establishing the boundaries of these zones, MacKenzie defines percentages of maximum in the following way: Take the desired percentage of maximum heart rate, or MHR, expressed as a decimal; multiply it by the difference between your resting heart rate, or RHR, and your MHR; and add your RHR to the result. This is sometimes called the "heart rate reserve" method. For example, someone with a MHR of 190 and an RHR of 50 who wanted to work out in a zone between 70 and 80 percent would maintain an HR between [0.7 x (190-50)] + 50, or 148, and [0.8 x (190-50)] + 50, or 162.
Your recovery zone ranges from 60 to 70 percent. In this zone, you can, to a limited extent, build endurance and increase your aerobic capacity. You also burn a lot more fat than carbohydrates. Workouts in this zone are suitable for the day after an especially intense effort -- hence the name. Examples of such a workout on a treadmill would be speed-walking at 4.0 mph for 30 minutes; jogging at 5.0 mph for 20 minutes; or walking at 2.5 mph with the belt inclined to a 5 percent grade for 25 minutes. Whether these parameters land you in the correct zone is, of course, dependent on your fitness level, so you may have to tweak the numbers. In any case, always aim for a session lasting at least 20 minutes but no more than 30 minutes.
You should aim to spend most of your treadmill time in the aerobic zone, as this zone allows for significant fitness gains without knocking you flat. MacKenzie defines the aerobic zone as 70 to 80 percent and says that your body's ability to transport oxygen to your working muscles is greatly enhanced by working in this zone, as it increases your body's ability to remove metabolic waste products from those same muscles. Sample aerobic workouts include: 30 minutes of progressively faster jogging, with the first 10 minutes at 5.0 mph, the next 10 at 5.5 mph and the last 10 at 6.0 mph; 25 minutes of walking at 4.0 mph up a 4.0 percent grade; and 20 minutes of running at 5.0 mph up a 2.5 percent grade, with exact limits contingent on your fitness level.
If you're working at above 80 percent, you're either working anaerobically or are very close to it -- especially fit people can run aerobically at up to 85 percent or so. Somewhere in this zone, your body begins producing lactic acid more quickly than it can metabolize it, so lactic acid starts to accumulate. This is your so-called lactate threshold, and by training at or just below it, you can "push it back" and make great fitness gains. Anything above 90 percent is "red-line" work and can only be sustained for a matter of minutes. Examples of anaerobic workouts include 20 minutes of running with the first 10 aerobic and the last 10 at 7.5 mph or whatever speed is required to jack your heart rate up about 80 percent, and 25 minutes of running 6.0 mph, with the first 15 on no incline and the last 10 with the belt inclined to 3.0 percent. Adjust these numbers to suit your fitness level.
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