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Core Knowledge


The New Science of Cardio


Energy Systems Development, or ESD, is the cardiovascular component of EXOS training programs. The intensity of the workouts is broken up into three different zones, which are differentiated by color: Yellow is easy/recovery, green is moderate/strength, and red is hard/power.

Jonathan Shield / flickr

How It Works

Forget everything you currently believe about cardio work. Forget keeping your heart rate in some “fat-burning” zone. Forget plodding along with the vague goal of increasing the distance you can plod. Instead of training like a plow horse, start training like a thoroughbred.

You’ll only work at the same effort level for an extended period of time, as you would with traditional cardio exercise on regeneration or recovery days. But you’re going to take the time you typically spend on cardio and develop the ability to perform at a more intense level. You’ll improve your energy levels, gaining physical strength and stamina without investing additional time.

You don’t have to head to the track and train like a middle-distance runner, although it’s not a bad place to do your ESD work. For convenience, you’ll probably want to work on a stationary bike, stair-climber, or treadmill at the gym. Even better is to find a hill or stairwell somewhere nearby. If you’re city bound, parking garages are perfect for uphill running. Sprint up the ramps and walk down.

The bottom line: Instead of slow, plodding workouts, ESD will have your muscles, nervous system, and hormones acting together into efficient movement patterns that help your body work as efficiently as possible.

Building Your Energy Systems

There are three different energy systems that are important for you to build:

  1. Lactate threshold - This is your capacity to do high-intensity work for up to 3 minutes. The ESD unit of your training program is a form of interval training in which you'll alternate between periods of intense exercise with less strenuous periods.
  2. Lactate power - This is your body’s ability to do high-level work for periods of up to 12 seconds.
  3. Aerobic system - The ability to work beyond 3 minutes and help you recover from your bouts with the lactate threshold. For instance, if you’re sprinting up hills and walking down, you’re using the lactate system on the way up and the aerobic system on the way down. In this case, the aerobic system enhances your recovery from these intense bursts of energy.

Measuring Intensity

You can measure intensity in a variety of ways including number of reps, distance covered in a specific amount of time, or the time it takes to complete a specific number or reps or distance. It could also be rate of perceived exertion (RPE), or even heart rate. Traditionally, we’ve measured intensity by tracking heart rate, but this isn’t always reasonable for everyone. We know that heart rate will track differently to the different types of exercise (e.g., we wouldn’t expect your heart rate to get as high on a bike as we would while running at the same perceived effort). RPE has been proposed as an alternative method to quantify intensity, and the good news is that the more you practice, the better you become at identifying your level of effort. If you’re just starting out, or it’s the first time you’ve worked out in a while, use heart rate to measure intensity, but pay attention to how it feels to work at those levels. That way you’ll able to incorporate a wider variety of movements into your ESD work while still having a valid method to quantify intensity.

With ESD, you’ll work within three intensity zones. If you have recent experience training, you’ll use an RPE scale of 1-10. If you’re new to the game, or just getting going for the first time in a while, use heart rate to measure intensity initially, and then work toward estimating your perceived exertion through the first two to three weeks of the program.

Heart Rate Zones

To estimate your maximum heart rate, begin by subtracting your age from 220. For example, if you're 40 years old, your maximum heart rate would be around 180. (It may actually be higher than that, but this is a close enough estimate to allow for productive workouts.) Multiply that rate by 60 and 70 percent for zone 1. Multiply it by 71 and 80 percent to determine zone 2, and multiply by 81 and 90 percent to determine zone 3.

Zone 1 Lower Limit = (220 – Your Age) × 60%
Zone 1 Upper Limit = (220 – Your Age) × 70%
Zone 2 Lower Limit = (220 – Your Age) × 71%
Zone 2 Upper Limit = (220 – Your Age) × 80%
Zone 3 Lower Limit = (220 – Your Age) × 81%
Zone 3 Upper Limit = (220 – Your Age) × 90%

These are general guidelines. You may need to raise or lower the numbers by 10 beats across the board. If nothing else, you will need to raise them as your ESD improves.

Age Zone 1 Zone 2 Zone 3
20 120 to 140 142 to 160 162 to 180
25 117 to 137 138 to 156 158 to 176
30 114 to 133 135 to 152 154 to 171
35 111 to 129 131 to 148 149 to 167
40 108 to 126 127 to 144 145 to 162
45 105 to 122 124 to 140 141 to 158
50 102 to 119 121 to 136 138 to 153
55 99 to 116 117 to 132 134 to 150
60 96 to 112 114 to 128 130 to 146

RPE Zones

RPE Zones The great thing about an RPE scale is that research has validated it as a method of measuring intensity, and the more experience you have with it, the better you become at targeting your intensity. At EXOS, we prefer an RPE scale because it accommodates all of the factors that may influence the intensity that an athlete or client is working at. For example, we know that interpersonal/social stress can influence the way that the body responds to exercise. So if you had a rough day at work, an ESD session that would normally only get your heart rate to 145 beat per minute might get it up to 150 beats per minute. Similarly, nutrition can play a large role. If you’re dehydrated, your heart rate will spike higher, and more quickly than if you were properly hydrated. It would also take longer to return to a resting state. RPE will accommodate all of these factors in addition to the stress of the exercise that you’re doing because it’s a general estimation of how hard you’re working.

When using an RPE scale, you’ll estimate how hard you’re currently working on a scale of 1-10. An intensity of one would be something that you could easily maintain for a long time (this may be an easy walking pace), while a 10 would be something that you could only maintain for a matter of seconds (think a 100-meter sprint pace). Keep in mind that these are just examples, and the actual work that you’ll be doing will depend on the context in which you’re working. But a one would be something that’s easy, and a 10 would be something that is difficult. Your three RPE training zones would look like this: Zone RPE Score (1-10) 1 5-6/10 2 7-8/10 3 9-10/10

These are general guidelines. You may need to raise or lower the numbers by 10 beats across the board. If nothing else, you will need to raise them as your ESD improves.

Zone RPE Zone (1-10)
1 5-6/10
2 7-8/10
3 9-10/10

Understanding the color-coded ESD bars

  • When you see a single bar indicating a time duration (e.g., 3 minutes), keep your intensity in that zone.
  • When you see a progressive set of bars with the same amount of time (e.g., 30 seconds), progressively work toward the higher end range of that intensity zone.
  • When using heart rate to measure intensity for very short intervals (e.g., 5 seconds), exert the power needed for that zone, then go easy to let your heart rate recover. For instance, with a 5-second burst in the Red Zone, you would work at an all-out effort for 5 seconds. Your heart rate would not be in the Red Zone initially, and that's OK. After a series of 5-second sprints, your heart rate will gradually climb into the Red Zone. Likewise, your heart rate may not return to the Yellow Zone during an extremely brief rest period, but your goal should be to go at an easy enough effort to lower your heart rate towards that zone.

ESD Workout Phases

You’ll notice that your ESD training isn't the same each day. It changes and progresses much like the rest of your training components. This section explains the different ESD levels and how they work to improve your performance across all energy systems.

Level 1

Level 1 focuses on steady-state aerobic work. Aerobic simply means that your body will use oxygen to provide a steady and consistent low level of energy for a long time, without building up any waste products in the body that hinder performance.

Think of it as a low-horsepower, highly fuel-efficient, four-cylinder engine that can run all day but doesn't generate a whole lot of power. A good rule of thumb for level 1 is that you should be able to carry on a conversation without too much trouble when you're in the aerobic zone. Some of the best activities for level 1 are:

  • Outdoors: Brisk walking, walking up hills, biking, swimming, rowing
  • Indoors: Biking, treadmill climbing/walking, elliptical trainer, Airdyne

Using the aerobic zone during level 1 will improve your cardiovascular system and prepare your muscles for the greater speeds of level 2 through 4. This lower-level work will let your movement patterns, muscles, and joints adapt and prepare for the more intense training coming in these next phases.

When you get into levels 2, 3, and 4, you’ll perform interval training, where there will be bouts of harder effort mixed with easier-effort periods to give your body time to recover. You’ll use the lower-intensity level 1 aerobic work in levels 2 and 4 — the more advanced zones of the program — as a recovery tool.

Interval training increases your body’s release of positive hormones, which builds lean body mass and signals your body to dump fat. At the same time, it keeps your intensity from dropping out of the aerobic zone.

Level 2

Level 2 will introduce interval training by mixing moderate intensity (zone 1) with bouts of level 1 easy-intensity aerobic work to allow you to catch your breath and recover from the slightly more intense intervals.

You'll notice that these exercises include some work and some rest; we call this the work-to-rest ratio. The greater the rest, the higher quality the work should be. The lower the ratio — for instance, 1 second of rest per 1 second of work (1:1) — the bigger the challenge, since the body has less time to recover. That increases your capacity to do work.

You’ll know you’ve reached level 2 moderate intensity if you would find it very difficult to carry on a conversation. You could, but you wouldn’t be able to say much more than a couple of words at a time. Some of the best activities for level 2 are:

  • Outdoors: Running-to-jogging/walking, jogging-to-walking
  • Indoors: Bike, elliptical trainer. treadmill, stair climbers, Airdyne

Level 3

In level 3 you'll work harder, performing intervals that include time spent in zone 2. The times and rest intervals both decrease. Don’t be intimidated by more intense work; you’ll be ready for it. In fact, you'll be looking for a greater challenge. Some of the best activities for level 3 are:

  • Outdoors: Running-to-jogging/walking, jogging-to-walking
  • Indoors: Bike, elliptical trainer. treadmill, stair climbers, Airdyne

Level 4

Level 4 is the shortest of the intervals, increasing your intensity to the highest zone (zone 3). It requires mobility, stability, and strength. At this level you'll ride, run, or climb as hard as possible for between 10-30 seconds. In order to get the most out of level 4, you’ll need to pack as much power and energy into these segments as possible.

Some of the best activities for level 4 are:

  • Sprinting (flat or uphill)
  • Shuttle runs (5 yards and back, 10 yards and back, 15 yards and back)
  • Bicycle intervals
  • Versaclimber sprints

EXOS training programs use different combinations of these zones or levels to create varied and personalized workouts that develop all your energy systems. You'll spend more time in the lower level zones initially and progress to performing intervals in which you spend more time in higher-intensity zones to improve your overall endurance, strength, and power.

Originally published January 7, 2009. Updated August 13, 2015.

Tags: Triathlon, Cardio, Elliptical, Conditioning, Energy System Development


  1. Verstegen, Mark, and Pete Williams. Core Performance: The Revolutionary Workout Program to Transform Your Body and Your Life. Pennsylvania: Rodale Press, 2004.
  2. Verstegen, Mark, and Pete Williams. Core Performance Essentials: The Revolutionary Nutrition and Exercise Plan Adapted for Everyday Use. Pennsylvania: Rodale Press, 2006.