Why your easy days might be too hard: the gray zone problem

An easy day can look fine on pace and still be too hard for the body doing it. Your watch says 9:30 per mile and you finish with a little sweat. You feel like you did something useful. Then the heart rate file shows 142 bpm for most of the run.

For a runner with a max heart rate of 180, 142 bpm is Zone 2, above the easy ceiling of 135. A small set of studies compared plans with a lot of time in that middle ground against plans with less. The results leaned toward less, but the studies were small and defined their zones differently than we do. Treat this article as a reason to look at your own easy days, and nothing firmer.

What the gray zone is

The gray zone is the effort between clearly easy and clearly hard. In StartLane's model it sits mostly in Zone 2, which is 75 to 82% of max heart rate. For a runner with a max of 180, that is roughly 135 to 148 bpm. Zone 1 is 60 to 75% of max, or about 108 to 135 bpm for the same runner.

Gray zone running feels reasonable. You can talk in shorter sentences and you finish tired but not wrecked. That makes it easy to slide into, and plenty of athletes spend more of their week there than they realize.

Zone 3 is threshold, 82 to 90% of max, and it is hard on purpose. A planned threshold session is a workout. The gray zone problem is about days that were meant to be easy and ended up in the middle.

What the research suggests so far

Three studies are worth knowing. Two compared training that leans on low intensity with training that spends more time in the middle, and one described how a group of young skiers trained.

Esteve-Lanao and colleagues (2007, Journal of Strength and Conditioning Research) randomly assigned 12 sub-elite runners to one of two programs for five months. One program had about 80% of training time at low intensity and about 12% in the middle. The other had about 67% low and about 25% in the middle. High-intensity time was about 8% in both. Everyone ran a simulated 10.4 km cross-country race before and after. The mostly-easy group improved by 157 seconds on average and the other group by about 122 seconds, and the authors reported that gap as statistically significant. Twelve runners is a very small sample.

Muñoz and colleagues (2014, International Journal of Sports Physiology and Performance) ran a similar comparison in 30 recreational runners over 10 weeks. One group trained about 77% low, 3% middle and 20% high. The other trained about 46% low, 35% middle and 19% high. Both groups improved their 10K time, by 5.0% in the mostly-easy group and 3.6% in the other. That gap of about 41 seconds was not statistically significant. A secondary analysis of the runners who followed their assigned split most closely, 6 in one group and 16 in the other, favored the mostly-easy group. That split came after the fact, so it is a hint and no more.

Seiler and Kjerland (2006, Scandinavian Journal of Medicine & Science in Sports) tracked 11 male, nationally competitive junior cross-country skiers for 32 days. About three quarters of their endurance sessions were clearly easy, and most of the rest were hard. Very little was in the middle. The study described what one group did and did not test whether it worked, and the athletes were skiers.

Taken together, the three point in a similar direction. They do not settle the question, and none of them was designed for a high school team.

There is one more difference from our zones. These studies set the middle by lab thresholds, which made it wider than our Zone 2 and put what we call Zone 3 inside it. Norwegian-style plans, including StartLane's, schedule threshold work on purpose, so nothing here argues against it. The question is about unplanned moderate running on days meant to be easy.

If your easy pace feels too slow

If heart rate climbs fast at a slow pace, the pace that keeps an athlete in Zone 1 is slower than expected, and that feels wrong at first. Heart rate also climbs with heat and hills, and a wrist monitor can read high or low. A chest strap tends to be steadier.

How to check your own easy days

Take your last five runs that were meant to be easy and look at the average heart rate.

1. Work out your Zone 1 ceiling, which is 75% of max heart rate. An age formula such as 208 minus 0.7 times your age gives an estimate. If you have a measured max from a hard race or test, use that instead.
2. Compare each run's average to the ceiling. An average above it means a moderate day. An average well above it, in Zone 3 territory, means you ran a workout without planning one.
3. Look at the shape of the run. Many easy runs start fine and drift up over the second half. A few beats of drift on a hot day or a hilly route is normal.
4. Add the talk test. If you cannot speak in full sentences, the run was harder than the watch says. The reverse can also happen.

If most of your easy runs land above the ceiling, you now know what to adjust. It says nothing bad about your training.

Options to try if easy runs keep running hot

These are ideas to test and watch. Your coach, doctor, athletic trainer or physical therapist knows your situation better than an article does.

• Run until your heart rate reaches the ceiling, walk until it settles, then run again. It feels silly for a couple of weeks and it keeps the total effort where you meant it to be.
• Swap one or two easy days for pool running, an elliptical or an easy bike ride. You build aerobic fitness with less pounding. Heart rate reads a bit lower on the bike than when running at the same effort, so the number is not directly comparable.
• Skip a single flat heart rate cap for everyone. One number fits some athletes and misses others, and your own Zone 1 ceiling is the better starting point.
• Give it time. The two trials above ran for 10 weeks and five months. Pace at a fixed easy heart rate tends to improve slowly, so record it every few weeks on the same route. If 11:00 per mile at 135 bpm becomes 10:40 over a couple of months, things are moving.

The Science of Easy Running covers where the 70% easy share comes from. Why You're Running Too Hard on Easy Days walks through the basic fix.

What this does not tell us

A moderate run is not automatically a mistake. Some runners do fine with more of it than the studies used, especially in the off-season or at low mileage. Heart rate is also only one way to control effort, and one bpm number will not suit every athlete.

The studies cannot say how an individual high school runner will respond. Fatigue has many causes, and sleep and school stress are two of them. If an athlete stays tired for weeks, keeps getting hurt or is struggling with energy, talk with a doctor or athletic trainer before changing zones.

For coaches

On a team, the gray zone habit often shows up as a group. A pack heads out on an easy day, the front runners set the pace, and the slower athletes run well above their own ceiling to keep up.

Things to try and watch:

• Let athletes run by their own heart rate ceiling on easy days, even if that splits the group.
• Look at logged easy runs for athletes whose average heart rate sits above their Zone 1 ceiling most days.
• Talk with those athletes first. Ask how the runs felt and whether the watch fits well.

How StartLane handles it

StartLane builds each runner's heart rate zones from their max heart rate. It uses the Tanaka estimate from age unless the runner enters their own number. Easy runs and long runs in a plan are set at an easy pace estimated from the runner's race PR, and the easy zone is Zone 1. About 70% of training time in a Norwegian-style plan is easy. Zone 3 is threshold and Zone 4 is VO2max, so planned hard work stays separate from easy work. The Your Zones page shows the heart rate range for each zone.

You can log runs by hand, sync them from Strava or import them from Apple Health, with average heart rate if your watch records it. Compare the average to your Zone 1 ceiling yourself, since StartLane does not flag easy runs that run too hot. Daily check-ins can help you notice when easy days feel harder than they should.

Coaches can open an athlete's page and see each logged run with its average heart rate. That makes it possible to spot the athlete whose easy days keep landing high. Any change to that athlete's plan or pace is your call.

Zone math is in Heart Rate Zone Training: How to Calculate Your 5 Zones. For where easy running fits in a full week, see What Is the Norwegian Method?.

This article is general information, not medical advice. If you have an injury, pain, ongoing fatigue or a health condition, check with a doctor, physical therapist or athletic trainer before changing your training.