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Cardio Crush
Learn the ins and outs of intensity, and make the most of your cardio workouts.
By Matt Fitzgerald
The word "intensity" is thrown around a lot in fitness circles. We hear it is important in relation to the results we seek. We are told we should always perform workouts with a desired training intensity in mind. We hear it should be neither too low nor too high.
But what is intensity, really? Why is it important? How do you control it, and what are the most effective ways to manipulate it in a training program? Given all the hype about intensity, it is surprising how seldom these basic questions are properly addressed. So let’s address them, once and for all..
Although intensity is relevant to both cardio and strength training, my focus in this article will be on cardio.
It’s about energy
In exercise, intensity refers to the rate at which your body is currently producing energy in relation to the maximum rate your body is able to produce energy for a specific activity. There are almost infinite degrees of intensity. For example, suppose you are running 5 miles an hour. If you increase your pace just slightly to 5.1 miles an hour, your body is producing a little more energy and is therefore working at a higher intensity.
Muscles can produce energy through three energy systems: the aerobic system and two separate anaerobic systems. The lower the intensity of exercise, the more energy is produced aerobically. The higher the intensity, the more energy is produced anaerobically.
Intensity is important because it is the primary determinant of how your body adapts to exercise. By training at a certain intensity level, your body adapts over time to become better able to train at that same intensity level. The more time you spend at that level, the more pronounced these adaptations become. However, these adaptations reach limits, so it’s important to know which training intensities are connected to which results. Once you have this knowledge, you can emphasize the training intensities that lead to the results you seek.
Three Ranges
Although there are innumerable degrees of intensity, there are three general intensity ranges that you’ll want to target in workouts. In the aerobic intensity range, the aerobic energy system is trained primarily. At "threshold" intensity, the first anaerobic system (called anaerobic glycolysis) is trained primarily. And at "sprint" intensity, the second anaerobic system (the creatine phosphate system) is trained. Each leads to its own set of beneficial adaptations, so your program should include all of them. Let’s take them one by one.
Aerobic Intensity
In aerobic metabolism, oxygen is used to break down fatty acids and glucose to release energy. The aerobic system produces harmless byproducts (carbon dioxide, water, and heat). For this reason, and because the supply of its energy substrates is great, aerobic-intensity exercise can continue for relatively long periods of time, especially in well-conditioned people. But because aerobic metabolism is relatively slow, this energy system is inadequate to support high-intensity efforts.
Exercising at aerobic intensity results in many positive adaptations. It strengthens the heart and the entire cardiorespiratory system, which not only results in general good health but also enhances performance in all sports, every form of exercise, and any life activity you can name. Aerobic conditioning is truly the foundation of fitness in the sense that it prepares the body to handle higher-intensity exercise. For this reason, beginners should also do several weeks of aerobic-intensity training before adding higher-intensity workouts, and aerobic workouts should remain an integral part of even the most advanced training regimen.
Aerobic intensity exercise also increases your endurance, so that you can sustain activity for longer periods of time. It does this by increasing your body’s fat burning efficiency and glucose storage capacity. Aerobic-intensity exercise is also an excellent means of improving body composition, because the rate of fat burning peaks in the middle of this intensity range.
Calories are burned more rapidly in threshold- and sprint-intensity workouts than in aerobic ones. However, aerobic-intensity exercise can be maintained much longer than exercise at higher intensities. So, the overall calorie-burning potential of aerobic exercise is greater.
Aerobic Workout: In your modality of choice (jogging, elliptical trainer, etc.), maintain a steady pace for 20 minutes or longer. The first and last few minutes should be very easy. The middle portion of the workout should be performed at an effort level of roughly 7 on a 1-10 scale.
Controlling the intensity: Monitoring your heart rate is a useful way to control intensity during aerobic-intensity workouts. During the main body of these workouts, keep your heart rate between 75 percent and 80 percent of the highest level it reaches during your speed-intensity workouts (see below).
Threshold Intensity
In anaerobic glycolysis, glucose is broken down for energy without the use of oxygen. This process is faster than aerobic metabolism, so it can support higher-intensity efforts. However, glycolysis produces metabolic waste products that inhibit muscle contractions, resulting in exhaustion. The aerobic system can use these waste products as fuels, but when exercise intensity crosses a certain threshold, wastes are produced faster than they can be used. They then accumulate in the muscles, which begin to "burn" and feel weak, and pretty soon you’re pooped.
Working out at an intensity level that is slightly above this threshold also carries significant benefits. To begin with, it simply enhances the cardiovascular benefits that come from aerobic training. If you do only aerobic-intensity workouts for a prolonged period of time, your results will level off. But by adding threshold workouts into your program, you can continue your progress.
Threshold workouts are also effective in transforming body composition. They burn calories at a faster rate than aerobic workouts, so they represent a more time-efficient level of exercise. However, keep in mind that threshold workouts are only beneficial when they build on a base of aerobic training.
For more serious exercisers and athletes, threshold intensity training greatly enhances the body’s ability to recover between hard efforts in a workout or competition. This allows you to do more advanced workouts and get a more pronounced effect from them.
Threshold Workout: In your modality of choice, warm up for a few minutes. Next, perform a series of 90-second hard intervals (effort level 9) followed by two-minute easy recoveries. Do at least four intervals and as many as 12. Cool down for at least five minutes.
Controlling the intensity: Perform each interval at the fastest rate (or highest output level) you can maintain through the end of the last interval.
Sprint Intensity
The second anaerobic energy system, the creatine phosphate system, fuels maximum and near-maximum efforts such as sprints and heavy weightlifting. Only tiny amounts of creatine phosphate are stored in the muscles, so this energy system cannot support efforts lasting longer than 15 seconds.
Sprint-intensity workouts burn more calories per minute than all workouts of lesser intensity. They also keep the body’s metabolic rate elevated for hours after the workout. Aerobic and threshold exercise cause a similar "afterburn" effect, but it is not as pronounced. However, this does not mean you should do only sprint-intensity cardio workouts if your goal is weight loss and you have limited time to work out. Sprint workouts are too stressful to perform more than once a week or so, and again, they’re only effective when they build on an aerobic foundation.
Sprint workouts are also an excellent supplement to strength training because they condition the same energy system and develop the same muscle fibers. Serious bodybuilders and strength athletes should therefore put a greater emphasis on this type of training, after building up to it.
Sprint workout: In your modality of choice, warm up for a few minutes. Next, perform a series of 20-second sprints (effort level 10) followed by one-minute easy recoveries. Do at least six and no more than 20 sprints. Cool down for at least five minutes.
Cotrolling the intensity: As in threshold workouts, perform each interval at the fastest rate (or highest output level) you can maintain through the end of the last interval.
Putting It All Together
If you’re just beginning to exercise, you should perform only aerobic-intensity cardio workouts for several weeks. Start with short workouts you can handle and gradually increase their duration until you have a solid base of aerobic fitness. For example, on a schedule of three cardio workouts per week, you might go just 10 minutes per session in the first week, 15 minutes in the second, 18 in the third, and 20 in the fourth.
If and when you have a good aerobic base, simply perform the three cardio workout types in a recurring rotation. This is the best way to use cardio training to support strength training to achieve well-rounded fitness and ideal body composition in a time-efficient manner. Here’s how a typical week of training might look:
Mon. Tues. Wed. Thurs. Fri. Sat. Sun.
Off Strength Sprint Strength Thresh. Strength Aerobic
Train. Cardio Train. Train. Train. Cardio
Every 12 weeks or so, do only aerobic intensity workouts for a period of two weeks. This will allow your body to build a new foundation atop previous gains and achieve even greater results when you return to high-intensity interval training.
