Right now, you're probably reading this article, pretty relaxed, functioning at a low resting heart rateunless you are one of those crazy multi-taskers who can workout and learn simultaneously.

As you start to exercise, your heart beats faster to deliver enough oxygen and fuel to your working muscles. The harder you work, the faster it beats; at the same time your metabolism changes to support the rate of work. Heart rate is a great and highly individual biomarker; there's a strong relationship between heart rate and intensity, and it's easily measured (unlike metabolism). The number of times your heart beats per minute provides a wealth of information—time to take advantage of it.

As an easy way to quantify energy systems, exercise physiologists and coaches have developed what are now known as heart rate training zones: roughly defined ranges of heart rates (as a percent of max) where certain adaptations can be expected to occur. The goals of any workout can be expressed in terms of a training zone. You use these like a dashboard meters that show what's going on under the hood. With only a little effort, you can start to fine tune your training, targeting specifics to boost your adaptations and get the most bang for your buck...or your heart beat.

Exercise Intensity: The Key to Peak Performance and Health Benefits

Athletes can manipulate three key training variables: frequency, time (or duration), and intensity. Of these, exercise intensity is the most important for performance, weight loss, cardiovascular fitness, and health adaptations to exercise. Intensity is also important for a multitude of other reasons: measuring recovery, planning easy days, formulating a training plan, and meeting fitness goals.

Intensity governs outcome. One classic 1995 study showed the amount of time spent at a vigorous training intensity was associated with lower all-cause mortality.Lee1995 Time spent at a low training intensity showed no such correlation. The same may apply to athletes. How hard (or how easy) you spend your training time might be more important than what you spend that time doing.

But how do you know how hard to work?

Using Heart Rate to Measure Training Intensity

Intensity, in relation to exercise, can refer to speed, power, energy expenditure, perceived exertion, percent of lactate threshold, or heart rate. Each of these measures, in their own way, is a proxy for the amount of ATP (energy) our body converts to mechanical energy so we can run, bike, swim, or fight. Intensity measures how much energy we use to perform a certain task.

Heart rate is easy to measure, possibly the best understood and often, the most applied for athletes. HR corresponds perfectly with physiology; when measured during exercise, it can indicate coronary blood flow and myocardial oxygen consumption (how much energy your heart uses).Kitamura1972 Additionally, measuring HR is more accessible than power or speed, a more exact measurement than perceived exertion (RPE), and is highly reflective of your status on any one particular day. HR can change with illness, stress, and heck–even thinking about lunch.

Measuring HR during exercise to assess training intensity is the most well accepted and practical method for most athletes. Usually, all it requires is a heart rate monitor.

In addition to exercise intensity, HR can help measure recovery and monitor training load to avoid overtraining. Quantifying your training is just as important as nutrition, recovery, and mental prep. Without quantifying intensity, how do you expect to get better?

Perhaps most importantly, measuring HR during exercise is the best way to individualize training. Exercise based on HR intensity is all relative–your 80% is different than your running buddy’s 80%. Your target heart rate is specific to you, and your training program should be as well.

The benefits of HR training include ease of measurement, corresponds to blood flow and oxygen consumption, accessibility, reflects recovery, correlates with effort, and is individualized.

Get in the Zone

Glancing at a number of “beats per minute” on your heart rate monitor during a workout might not seem to hold any useful information. As a number alone, it doesn’t. What’s important is what your particular HR tells you about objective effort. Where are you in relation to your max capacity? Heart rate can tell you what energy systems you’re using during exercise, and accordingly, what adaptations you’ll get out of the workout.

When we say energy systems, we mean the three main pathways to generate energy during exercise. These are: the phosphagenic (ATP-PCr) system, the glycolytic system, and the oxidative system.

For max-intensity exercise (think, sprints or super heavy lifts that last under 15 seconds), we use the phosphagenic system to generate ATP. For high-intensity efforts lasting under two minutes, the glycolytic (anaerobic) system predominates. For all other aerobic-based efforts over two minutes, the oxidative system makes our energy through mitochondrial respiration. While we always use a combination of these systems, different exercise intensities will rely primarily on one system over the others to use metabolic substrates such as carbohydrates or fats for energy.

Zones were created to give athletes a vague idea of if they’re “in” or “out” of a certain adaptation or recovery window.

The theory goes: craft your training plan around the different zones to create an intelligent training program destined for success.

It is important to know that heart rate training zones are not identical for each athlete. They may vary widely among athletes due to individual variation in physiology, cardiovascular fitness, and even diet. Nor are training zones rigidly defined. Athletes aren’t zone switching robots, neatly switching between different zones. It’s more of a continuum.

Why use zones to train? Athletes don’t (and shouldn’t) train at the same intensity from day to day, workout to workout. Manipulating the most important variable (intensity) is crucial to boost your fitness level over time, avoid a plateau, and stay healthy.

The Big Five HR Zones

Each HR zone serves a specific purpose. Depending on the coach or textbook you consult, zones may differ slightly. Again, these zones are suggestions that correlate fairly well with physiology.

Zone 1: Warmup

Intensity: 50% - 70% HR max

Colloquially described as “very light,” you could probably continuously exercise for 1 - 6 hours in zone one. Light warm-ups, cooldowns, and recovery between sets should take place in this zone, in which fat burning is the primary energy source.

Some athletes might avoid this zone for fear of losing speed, teaching the body slow movements, or otherwise wasting time. But training time spent here is important. Warming up at 60% - 70% of max has been shown to improve range of motion and enhance performance on anaerobic capacity tests compared to a warm-up at 80%.Stewart1998

This intensity is vital for recovery too. Active recovery between intervals decreases blood lactate concentrations at high anaerobic power outputs compared to passive (i.e. standing) recovery.Ahmaidi1996 Use zone one for active recovery during workout sessions like high-intensity interval training (HIIT).

Does the thought of an heart-rate revving HIIT session fill you with existential angst? Have no fear. Exercise in a low-intensity zone one also has cardioprotective benefits, promoting clearance of disease-related lipids in the blood vessels and enhancing your ability to burn fat, among other health-enhancing properties.

HR Zone 1: Warmup Zone

Zone 2: Easy, Fat Burning Zone

Intensity: 71% - 80% HR max

The bulk of your training should occur in zone two. Here, you should be able to talk with your running buddy; it’s commonly called “conversation pace.” Zone two training should still be fairly relaxed, but not as easy as the zone one. Zone two utilizes the oxidative system to produce energy, churning through a mix of fats and carbohydrates to produce the ATP you need.

Zone two running is sufficient to increase blood circulation and trigger growth hormone responses, but not hard enough to cause considerable muscle damage or deplete your energy or fluid stores rapidly. A longer workout or race (say, two hours or more) in this area however, and you’ll begin to run low on some stored glycogen.

Zone two is great for the majority of your LSD (long, slow distance) runs and your recovery runs on easy days. Running in zone two for recovery might actually enhance adaptations more than your post-run protein shake and nap on the couch. After a highly-damaging and stressful rugby match, players experienced lower levels of psychological stress and higher post-match relaxation when they completed 15 minutes of active recovery at zone two intensity.Suzuki2004

Like zone one, you should be able to pretty much churn away in zone two all day long, and perhaps even longer if using nutrition strategies like exogenous ketone supplementation. Ketone esters boosts blood-ketone levels of beta-hydroxy-butyrate (BHB) and enables the body to use both carbohydrates and ketones for energyoffering superior endurance performance than either used alone. When taken with pre-exercise carbohydrates, beta-hydroxybutyrate (BHB) allowed cyclists to ride 2% - 3% further in a 30-minute time trial than with carbohydrate alone. This is likely due to the combo of high-efficiency ketones that improved muscle efficiency and the presence of ketones sparing muscle carbohydrate stores for the next exercise bout.Cox2016

Zone 2: Easy "fat burning" zone

Zone 3: Intermediate Zone, Where the Base is Built

Intensity: 81% - 87% HR max

Zone three is where you enter subjectively moderate exercise intensities but still stay in the aerobic zone. Here, the body is probably using a 50/50 mix of carbohydrates and fats to fuel muscle contraction. The relative percentage depends on your individual metabolism and diet—you can skew your energy use towards fat by following a low carbohydrate, high fat diet.

Some runners and athletes refer to zone three as a “grey zone” between a slow, easy distance and harder interval or race pace workouts. Moderate intensity training (around 85% HR max) has been shown to have a large benefit on V02 max. Zone three is fantastic for building that strong, aerobic endurance base.Milanovic2015

Zone 3: Intermediate Zone: Where the Base is Built

Zone 4: Lactate Threshold Training, Where it Starts to Burn

Intensity: 88% - 93% HR Max

In zone four, we’ve now transitioned to hard running. This heart rate zone corresponds to around lactate threshold; the point where the body begins to anaerobically generate ATP and produce more lactate than it can clear out or use. While still aerobic, in zone four you’ll experience some burning in the legs and shortness of breath.

A majority of longer intervals are performed in zone four, used to improve lower end speed, muscular endurance, and anaerobic threshold.

Tempo runs are an example of workouts completed in zone four; they’re hard, but manageable for a decent duration. Tempo runs are also excellent for teaching your body to tolerate lactic acid.

Training at an intensity around zone four has been shown to be effective for increasing the speed at lactate threshold and improving running performance compared to moderate training in cross country skiers.Evertsen2001 Adding one 20-minute session of zone four training to an endurance running program increased enzymes responsible for lactate clearance and improved running economy.Sjodin1982

Zone 4: Lactate Threshold Training

Zone 5: VO2 Max Training

Intensity: 95% - 100+% HR max

High-intensity interval training (HIIT) is popular for good reason. HIIT is a training regimen with proven benefits above and beyond most other exercise modes and intensities. Be careful; zone five training requires lots of recovery and can’t be done too often without risking overtraining or injury. In addition, maxing out on effort doesn’t always mean you’ll reach theoretical “maximum” heart rate. Short bursts at max effort may take your breath away, but may not get you into zone 5; still, they still qualify as max training.

In zone five, the body primarily relies on carbohydrate oxidation for exercise and the glycolysis pathway predominates. The intensity of zone five exceeds lactate threshold, requires longer recovery periods during the workout, and sure does hurt.

Athletes training in this zone look to improve muscle strength, high-end speed, and lactate threshold, along with maybe a bit of endurance.

Zone five training typically involves brief, intermittent, all-out efforts (emphasis on brief). An example of the training benefits? Just 30 seconds of “max effort cycling (in zone five) with 60 seconds of recovery over just 14 days improved muscle oxidative capacity, muscle buffering capacity, glycogen content, and exercise performance.Gibala2006

HIIT consistently is shown to improve V02 max, blood pressure, fasting glucose, body composition parameters, and blood pressure measures in overweight and even healthy populations. Sprint interval training, HIIT’s faster cousin, has been shown to result in increases in peak power output, glycolytic enzyme activity, cycling time trial performance, and carbohydrate oxidation.Macdougall1998,Burgomaster2006

If we could train in zone five all of the time with no downside, it would be the way to go. High-intensity exercise is just plain efficient compared to the lower zones. However, polarized training is a necessity for all athletes; so only a small total percent of training should occur in zone five.

Zone 5: V02 max training

Calculating Heart Rate Zones

The first step to finding your heart rate zones is figuring out your maximum heart rate—the fastest speed at which your heart is capable of beating to deliver blood throughout the body.

You can find maximum heart rate several ways. The most accurate is to have a graded exercise test to exhaustion (commonly referred to as a V02 max test) in a lab. But this can be costly.

The second most accurate way is to perform a field test. This involves some type of maximal workout to exhaustion while measuring HR every so often. When maxed out, whatever number your heart rate monitor displays is probably somewhere close to your max.

This one is easy to try. Start by running a mile or two warm-up, then one mile at tempo pace. During the last third of that mile, you should constantly increase speed, then run the final 400m all out. Record your HR afterwards. Athletes of all types can do this one, shortening the distance if necessary.

A less taxing way? Several online calculators exist. Most calculators use a validated prediction equation to calculate estimated max heart rate. One frequently used is:

HR max = 207 - .7 x (age)

After calculating maximum HR, determining HR training zones is easy.

Applying the Karvonen formula to calculate HR zones based on desired intensity.

The Karvonen formula uses a number known as heart rate reserve (HRR) to calculate your exercise heart rate at given intensity (%). HRR is the difference between max and resting heart rate.

HRR = HRmax - HRrest

The Karvonen formula:

Exercise HR = % of target intensity (HRmax - HRrest) + HRrest

To use the Karvonen formula to calculate a HR zone, simply use the upper and lower bounds of a zone to calculate two numbers; this range will be the zone to stay in during the workout.

To calculate desired HR in zone three (81% - 87% HR max) for an athlete with a max HR of 200 and a resting HR of 54:

Exercise HR =

.81(200 - 54) + 54 = 172

.87(200 - 54) + 54 = 181

Zone three for this athlete would occur at a HR of 172 to 181 beats per minute.

Zone Distribution: How Much Should You Train in Each?

Maximizing adaptations to training is the primary goal of any athlete. How much time should you spend training in each target heart rate zone to reach your peak and prevent overtraining?

The answer can be found by taking some advice from the elites.

Successful endurance athletes often follow a polarized training pattern, sometimes referred to as the “80/20 rule” for intensity: about 80% of the total training volume is spent at an intensity below the lactate threshold, and the remaining 20% is distributed between lactate threshold and high-intensity interval training.

Studies on elite cross country skiers revealed 75% of their training sessions were performed below their first ventilatory threshold. Only about 15% - 20% of the sessions were performed as “interval training” bouts.Seiler2006 High-level marathoners perform very few training sessions at marathon or half-marathon pace (close to lactate threshold), instead devoting a majority of their training volume to logging miles well below lactate threshold.Billat2001

Applying this to zone training, this would mean spending about 80% in zones one, two and three. This is observed in athletes who compete in events lasting 3 - 15 minutes–they put emphasis on long slow endurance work as the base for their training. Reserve the remaining ~20% of training for tempo runs, speed work, and HIIT.

You should spend about 80% of your training on zones 1 - 3, and the remaining 20% in zones 4 - 5

Why this distribution? It comes back to adaptations. Low-intensity endurance training likely is best for maximizing muscular adaptations, improving efficiency, and building a strong overall base. Sprinkling in the high-intensity intervals, sprint work, and lactate threshold training meets the need for higher-level signal pathways to prime you for racing and competition.

This pattern also balances the stress-adaptation equation. Too much training at higher heart rate zones could eventually lead to burnout or overtraining.

HR Zone Training is Nuanced

While athletes and coaches may like to view training as workouts to be placed in neat little boxes, this isn't always the easiest thing to do. Using specific zones, interval times, and recovery bouts are simple ways to prescribe training on paper, but doesn’t always translate perfectly.

The same applies to zone training. Heart rate zones are simply “suggestions,” a common formula concocted through years of observation and experimentation by athletes. They correspond well to physiology, but everyone is different.

You might thrive on 80/20, while your nearest competitor might optimize performance on a training program focused on intervals and speed.

As a pervasive example, take the case of two New Zealand world champion rowers who trained together in the same boat, following the same training schedule. Over a four year Olympic cycle, one rower followed the 80/20 rule almost perfectly, performing about ~80% of his training below HR zones 1 - 3, 18% around HR zone 4, and only 1.8% above HR zone 5.

His partner? He spent 67% training in zones 1 - 3, 30% in zone 4, and 2.4% of his training at or above zone 5–utilizing a greater amount of high-intensity training. Different roads to Rio, but the same eventual outcome: a gold medal.Plews2017

One final word –your heart rate zone might change on a day-to-day basis, at least from an effort standpoint. Anything that influences heart rate, like stress, lack of recovery, even dehydration, could elevate your heart rate above what might be normal for a given workout. This isn’t a time to freak out or force a pace, but to realize HR zones are here to provide a framework, not a law. Adapt and move on.

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