The Training Peaks Pace Zone IndexTM (PZI) is based on the idea that there are six pace zones that all runners should target in their training. Each pace zone corresponds to a physiological intensity level that stimulates fitness adaptations that are at least slightly different from the adaptations associated with any other intensity.
While the actual numbers that make up each pace zone are different for different runners (there are 51 levels, ranging from back-of-the-pack to world-class), what's going on inside any given runner's body while running in the various pace zones is basically the same. The purpose of the Pace Zone IndexTM is to give runners of every ability level a simple tool they can use to get the most out of their training time by allowing them to zero-in on precisely the right running intensities.
How do you find your current PZITM? One way is to undergo comprehensive testing in an exercise laboratory. But a simple shortcut allows you to find it by using a recent race performance or time trial test of 3, 5, or 10 kilometers.
Once you know your current PZITM, each training run you do should focus on just one or two of the six target pace zones. There are also four pace zones, called gray zones, that you should generally avoid in training. The first, Pace Zone 1, is the pace zone that is simply too slow to qualify as exercise. The other three gray zones are plenty intense, but they do not conform well to the workout formats that have been proven most effective through many decades of collective trial and error undertaken by runners across the globe.
Here are descriptions of the 10 pace zones, including the six target intensities and four gray zones:
Pace Zone 1: Gray Zone 1
This pace zone is too slow to qualify as exercise. In most trained runners it corresponds to walking/jogging paces below 55% VO2max (short for your maximum rate of oxygen consumption, an excellent measure of fitness) and below 60% of maximum heart rate (HRmax).
Pace Zone 2: Low Aerobic
The primary purpose of running at Low Aerobic pace is to promote recovery from recent hard training. In trained runners, Low Aerobic Pace corresponds to roughly 55 to 65 percent VO2max and 60 to 70 percent of maximum heart rate (HRmax) in the average trained runner.
In the Low Aerobic Pace Zone, energy is supplied equally by fat and carbohydrate. The initial perceived effort level (on a scale of 1 to 10) is between 4 and 5. Perceived effort is how difficult or challenging running feels. I refer to "initial perceived effort" instead of just "perceived effort" because perceived effort climbs at any pace if maintained long enough for fatigue to set in. For example, Low Aerobic pace may correspond to race pace for an ultramarathon runner, and you can bet that after 95 miles of running at this pace, his or her perceived effort level will be 10 or close to it!
Pace Zone 3: Moderate Aerobic
Moderate Aerobic running is slightly faster than Low Aerobic running. It corresponds to about 65 to 75 percent VO2max and HRmax in trained runners. Energy comes predominantly from carbohydrate, but the body is still metabolizing a fair amount of fat. Initial perceived effort level is 5 or 6. Moderate Aerobic-pace running is appropriate for aerobic base-building runs (foundation runs) and long, endurance-building runs. Training at this pace improves running economy, increases VO2max, and enhances endurance (if maintained for longer durations).
Pace Zone 4: High Aerobic
High Aerobic pace ranges between roughly 10 percent slower than marathon race pace on the low end and marathon race pace on the high end. In a typical fit runner this range corresponds to 75 to 85 percent VO2max and HRmax. In this pace zone, carbohydrate provides virtually all of the energy for muscle contractions in a majority of runners. Initial perceived effort level is 6 or 7. High Aerobic provides essentially the same benefits as Moderate Aerobic running but in greater measure. It is most appropriate for foundation runs in the late base phase of training and for long runs in the peak phase of training.
Pace Zone 5: Gray Zone 2
This Pace Zone falls between marathon race pace on the low end and, on the high end, the pace associated with the anaerobic threshold. It's a little too fast for sustained workouts such as foundation runs and long runs and a little too slow for workouts such as tempo runs that are intended to target the anaerobic threshold.
Pace Zone 6: Threshold
Threshold pace is so called because it corresponds to an intensity threshold where low-intensity fatigue meets high-intensity fatigue. What does this mean? If you run to the point of exhaustion at any pace below your personal threshold pace, the cause of exhaustion will be a low-intensity fatigue factor such as muscle glycogen depletion or accumulated muscle damage. If you run to the point of exhaustion at any pace above your personal threshold pace (and of course exhaustion comes much sooner at these faster speeds), the cause of your exhaustion will be a high-intensity fatigue factor such as neuromotor fatigue or muscular acidosis. Training at Threshold Pace is an effective way to increase your Threshold Pace (which happens to be one of the strongest predictors of race performance) and also to increase the duration (or distance) you can maintain your Threshold Pace - known as time limit at Threshold Pace.
Threshold Pace falls between 85 percent and 90 percent of VO2max and HRmax in a typical trained runner. The Threshold Pace Zone is the appropriate target pace for tempo runs, which most runners should do regularly throughout the intensity and peak phases of training.
Pace Zone 7: Gray Zone 3
Pace Zone 7 corresponds to roughly 10K to 5K race pace for most runners. It's a little too fast for tempo runs and other workouts designed to elevate the Threshold Pace and Threshold Pace time limit, and a little too slow for interval workouts designed to boost VO2max.
Pace Zone 8: VO2max
VO2max is the maximum rate of oxygen consumption a runner can achieve while running. In most trained runners, this rate of oxygen consumption is reached after a few minutes of running at the fastest pace that can be maintained for 5 to 8 minutes. VO2max Pace usually corresponds to 95 to 100 percent HRmax and an initial perceived effort rating of 9. Training at VO2max Pace is a good way to increase VO2max and also to improve running economy and resistance to high-intensity fatigue.
Pace Zone 9: Gray Zone 4
Pace Zone 9 is a narrow gray zone that is just slightly faster than the pace required to reach 100% VO2max. It is not an effective pace zone for workouts designed to increase VO2max because you tire out faster at these higher speeds, so you spend less total time using oxygen at your personal maximum rate and consequently get a weaker training effect. At the same time, Pace Zone 9 is a little too slow to really enhance fast-twitch muscle characteristics (i.e. speed-efficiency and power), as Speed Pace training does.
Pace Zone 10: Speed
Speed Pace corresponds to roughly 1-mile race pace on the low end and a full sprint on the high end. Middle distance runners benefit from a small amount of sprint training, but distance runners need never run 100 percent full-tilt in training and can stay close to the low end of Speed Pace. Heart rate and VO2max measurements have little relevance to Speed Pace because workouts in this pace zone take the form of short intervals throughout which heart rate and oxygen consumption climb. The initial perceived effort level at Speed Pace is between 9 and 10. Training at this intensity level increases VO2max, improves running form, and develops fast-twitch muscle characteristics (i.e. speed and power).
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