Knowing When and How to Add High Intensity Training: The Ten Percent Test


By Uphill Athlete Coach Scott Johnston

If the Internet is to be believed, everyone loves to train hard, to collapse in pool of sweat, and really feel like they’ve given their all. Ultimately, this must be training, Right? Putting it all out there every time? 

Not exactly.

When we reference the 100+ year history of endurance training, we see a different story. Intensity is actually rather rare in a properly-structured training plan. So when do wise coaches and smart athletes add high-intensity training to their program?

We know for a fact that high-intensity aerobic training (zone 3 and above) is most effective when the athlete has done the work to establish a sufficient basic aerobic capacity. So, the smart athlete asks, how much aerobic capacity is sufficient? When should I add intensity training?

The answer is simple: When you’ve raised your aerobic threshold to be within 10 percent of your lactate threshold, as measured by either heart rate or pace. With more than a 10 percent spread between thresholds, an athlete has what coaches refer to as aerobic deficiency and that athlete will beneit the most by building more aerobic base. Those who have less than a 10 percent spread between thresholds will need to reduce or even drop Z2 training, substituting Z3 workouts.

If you have not done so, please read our article on Aerobic Deficiency Syndrome first.

To do this 10 percent test, first determine your aerobic threshold (AeT), then determine your anaerobic threshold (AnT) or lactate threshold.

AeT test options:
Heart rate drift , which allows you to retest frequently and best of all is free.

Lab test, as explained here and here

AnT or Lactate Threshold test: explained here.

Once you have these two important metabolic events identified, calculate the percentage difference between your aerobic threshold heart rate and your lactate threshold heart rate by dividing the higher heart rate by the lower heart rate. We know this is not a conventional way to calculate percentage, but it works well for our purposes. Example: Suppose a lab test determines that your aerobic threshold heart rate is 128, and your lactate threshold hill climb test shows an average heart rate of 150. 150/128 = 1.17. Your lactate threshold heart rate is 17 percent greater than your aerobic threshold heart rate.

We want to see a spread of less than 10 percent before we begin to add intensity into the training program. In the example above, you would still have a lot of potential to improve your aerobic base with Z1–Z2 workouts and should not be too eager to add Z3 or higher-intensity work just yet. We often see spreads of 30 percent. Elite endurance athletes can have a spread of 5–7 percent, or only 10 beats.

Here is what this looks like graphically if we were testing blood lactate.

On this graph we see the results of a blood lactate test for an athlete with low aerobic capacity. At the top of the basic aerobic zones (Z1 and Z2), his aerobic threshold occurs at a heart rate of around 120 bpm and a running pace of about 10 min/mile (6:12 min/km). At the top of zone 3, his lactate threshold, at 160 bpm, corresponds to a pace of 8:20 min/mile (5:10 min/km). With a 40-beat spread between thresholds, this athlete has Aerobic Deficiency Syndrome and needs to do considerable aerobic base work.

On this graph we see the results of a blood lactate test for the same athlete after a season of proper training. The graph shows the large increase in aerobic capacity. Notice that the aerobic threshold has moved up to 150 bpm, corresponding to a pace of 8:15 min/mile (5:09 min/km), and the lactate threshold has bumped to 165 bpm—the 10 percent spread we are looking for—and a pace of 7:30 min/mile (4:40 min/km). In this condition, this same athlete can now sustain a pace for several hours that he previously struggled to maintain for even one hour.

As demonstrated in these two graphs, the athlete dramatically increased his basic aerobic capacity with focused Z1–Z2 training for several months.  To raise your AeT means stubbornly training at and below AeT heart rate, as boring as that may feel, and it means adhering to that on ALL aerobic workouts, even climbs. The biggest stimulus to aerobic capacity is the frequency and duration of this low- to moderate-intensity work, which causes muscle glycogen store depletion and signals a bunch of aerobic adaptations.

Depending on your genetics and the volume of training, after a few weeks you’ll start to see that you are moving faster and easier at the same heart rate. That’s a good time to do another AeT test to see how much it has moved. Be advised, though: both your AeT and AnT will move up and down the heart rate scale depending on your recovery state. Neither are fixed, and it is best to test them in a fairly rested condition.

Adding Intensity

Once this gap has narrowed to under 10 percent, you will need to reduce your time in Z2 by doing more Z1 work and introducing some Z3 and/or Z4 workouts. It might seem counter intuitive that the fitter athlete needs to train easier than the less fit. But, here’s why:  The fitter person is moving much faster at Aerobic Threshold. This faster Z2 training imposes a much higher neuro-muscular load and associated fatigue.  For the well fit, too much training in Z2 will lead to overtraining.  The solution is that the fitter you are the more polarized your training intensity needs to become. Easy days need to be easy so that hard days can be hard.  Training in the middle too much will make the easy days too hard and the hard days too easy. See this article for more about this situation. There are entire books written on high-intensity training and we can only scratch the surface of the subject in an article.


Wait, what about our constantly beating the low-intensity aerobic base drum.  Are we changing our tune now?  Not at all.  High-intensity plays a critical role in every endurance athlete’s training.  Our message remains that this high-intensity work comes as supplement to and and not a replacement for the aerobic base work. This higher intensity work stands atop the aerobic base to maximize endurance performance. How do you decide how to add intensity to your program?  What type, how much, how often, for how long?  Lots of things to consider here so let’s try to sort through this maze.


A good general rule is that the high-intensity training should be as event specific as possible. This means it should, in most ways, model the endurance demands of your event.  For SkiMo and running this means running/skiing fast on uphill grades similar to the event you are training for.  For alpinists and mountaineers this means hiking (and even moderate climbing) steeply uphill usually with a weighted pack.  For the skier and runner speed is important; so the tempo needs to be similar to the event/race and not slow-grinding work.  For the climber the high-intensity speed can be slow-grinding but there is also benefit from training at tempos faster than you would move in actual climbing situations. See the photo of Steve that leads this article.


How much volume, how often and how hard, how much rest, and is it active or passive rest? These are all variables to consider when planning on high-intensity workouts that are intended to improve your endurance.  I’m paraphrasing Per Astrand, one of the giants of 20th century exercise science here: “It is Unclear whether endurance is more effectively improved by doing 4x 4min at 95% of max HR or 2x16min at 85% of max HR.”  In other words: Should you be doing several short very hard repetitions or fewer, longer and more moderate ones? He tried to answer this in the 60’s and 70’s to no avail.  Exercise scientists are still trying to answer this and there is still no definitive answer.   In general, when you read about precise interval workout prescriptions you are reading about someone’s pet training; something they used that seemed to produce good results.  But at it’s core this simply means going hard for while then resting for a while and repeating that several times.  The range of possibilities for high-intensity (Z3 to 4) endurance training is almost unlimited and can run the gamut from 10 seconds hard followed by 10 seconds easy in for 30 minutes at Z4 effort all the way up to 1×60 minutes continuous in Z3 for a well trained athlete, and everything in between.   We explain how to build your own intensity session below.

The Dirty Dozen Tips

  1. The interval style of workout is the most common for Z3 and 4 training.
  2. Z3  is typically done with long (15-60 minutes) work periods and short recovery intervals with a work/rest ratio of 4 or 5:1. A ten min work period followed by a 2-2.5 min recovery interval. (that’s right, the term interval is correctly used to designate the rest ‘interval’ between work periods.)
  3. With Z4 the work periods get shorter.  For Z4 work of 4 minutes and less, use rest intervals of the same length as the work period; a 1:1 work/rest ratio.
  4. For work periods longer than 5 minutes use a work/rest ratio of 2:1. Keeping the rest interval relatively short keeps the aerobic system revved up.  Using too long of a recovery interval and the work period can shift to a more anaerobic training effect.
  5. Total volume of high intensity is a very individual thing and depends almost entirely on an athlete’s training history. An easy way to gage how much you can and should use is to start with a total work time of about 5% of your weekly aerobic volume and see how you handle it from a recovery standpoint.
  6. You should not decrease your low intensity aerobic work as you add intensity to your weeks. If you are forced to cut basic aerobic support volume then you’re adding too much intensity, too soon and need to cut back on the next intensity workout.
  7. Start by adding Z3 work into your weeks a little at a time using rules 5 and 6 to decide how much.  It might be  1×10 minutes of Z3 the first week. As you get stronger you’ll tolerate more and will be able to up the volume.  10% of the weekly volume done at high-intensity is quite a lot of hard work for even well trained individuals.
  8. Only add more intensity when you can comfortably handle the current load.  Once up to about 10% of the total weekly volume in Z3 you should consider replacing some of the Z3 with Z4.  Replace 2 minutes of Z3 with 1 minute of Z4.  Again, only add volume as you can handle it.
  9. Don’t be a slave to the plan: For best effect you need to be rested enough to put out the required effort and speed for the full duration.  Otherwise this hard workout will not achieve its desired effect.
  10. Space high-intensity workouts at least 48 to, preferably, 72 hours apart.
  11. Be sure that the easy days are easy enough so that the hard days can be hard enough.
  12. Rest when you are tired.  This work has a powerful training effect but it can quickly push one over the overtraining cliff if you do not keep your easy volume high.


Alpinist or Mountaineer Intensity Workout

Once the above pre-conditions have been met the high intensity workouts like the one in the title-photo, are a very effective tool.  See this article  for a complete description.    If your hill is shorter than say, 1000 feet vertical just break this workout into an interval session.  The down hill recoveries will be longer than ideal but this is the real world compromises we must make sometimes.  Another option for the vertically challenged is to do this on a stair machine.  An hour of accumulated vertical is a big workout of this type so proceed gradually to get the feel of these.

Mountain Runner or Ski Mountaineer Intensity Workout

These athletes move at a quicker tempo and with less resistance in their event.  So, their intensity should more closely model the specific speed and gradients they will be on.

Warm up: Perform a 15-20 minute run or ski with gradually building intensity during the last 5 minutes so that the final 2 minutes is done in Z3.  This will ensure that your are well warmed up and your aerobic metabolism is firing on all cylinders.  Then perform 2-3 x 15-20 second accelerations up your hill take a 2 minute active recovery between.

Sample Beginner Z3 Workout:   2 x 15 minutes uphill at Z3 intensity with 3-4 minute active recovery between repetitions. Follow the body of the workout with a 20 minute Z1 run/ski.

Sample Advanced Z3 Workout:  3 x 20 minutes uphill at Z3 intensity with 5 minutes active recovery between repetitions.  Follow the body of the workout with a 20 minute Z1 run/ski.

Sample Beginner Z4 Interval Workout:  3 x 3 minutes uphill at Z4 intensity with 3 minutes active recovery between repetitions.  Follow the body of the workout with a 20 minute Z1 run/ski.

Sample Advanced Z4 Interval Workout:  4 x 8 minutes uphill at Z4 intensity with 3 minutes active recovery between repetitions.  Follow the body of the workout with a 20 minute Z1 run/ski.



Cover photo: Steve House doing an intensity workout at Smith Rocks, Oregon. By Ben Moon.