Some refer to zone 3 training as sweet spot training while others call it the grey zone. This article reveals what zone 3 training is and when you should or shouldn’t implement it into your endurance training program.

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Zone 3 training is a training method that heavily relies on the zone 3 training intensity. This training method is popular among endurance athletes like cyclists, runners and triathletes. The zone 3 training intensity is below anaerobic threshold and above aerobic threshold (LT1).

For many triathletes, zone 3 equals race pace when cycling and running. Depending on the fitness level, marathon runners run the majority of their marathon in zone 3 as well.

Determine zone 3 intensity based on heart rate, power or feel

When using a heart rate monitor, heart rate zone 3 equals 70-80% of the maximal heart rate. If your maximal heart rate is 185 bpm, heart rate zone 3 roughly equals 130 to 150 bpm.

Cyclists who use a power meter can calculate their zone 3, using 75-90% of their functional threshold power (FTP)

However, athlete’s are better off creating training zones based on their unique physiology, instead of a limited metric like FTP.

INSCYD's Training Zone Builder
Traditional training zones based on FTP (295W). With INSCYD you get additional Physiological Conditions per zone, based on an individual metabolic profile (test).

Rather train by feel? A zone 3 cardio workout feels comfortably hard. Or in other words: zone 3 feels like being out of the comfort zone, while being able to sustain the intensity. It feels challenging but doable.

In zone 3, athletes burn both fat and carbohydrates. In fact, zone 3 training is known for combining all kinds of energy systems, fuels and muscle fibers. More specifically, it combines:

  • Aerobic and anaerobic energy supply
  • Fat and carbohydrate as a fuel
  • Fast twitch muscle fibers and slow twitch muscle fibers
  • Lactate production and lactate combustion
INSCYD's Graph aerobic vs anaerobic energy
The INSCYD performance software shows exactly how big your aerobic vs anaerobic energy supply is at any given exercise intensity - depending on your individual metabolic profile.

Zone 3: tempo zone vs sweet spot vs grey zone

Zone 3 training is also known as tempo training or the tempo zone. Especially runners use tempo intervals in their terminology.

Fans of zone 3 training often refer to it as sweet spot training. Haters call it “the grey zone”.

We’ll soon find out whether zone 3 training is the sweet spot or the grey zone.

The fact is that zone 3 training is quite the opposite from polarized training. Polarized training avoids zone 3 and relies heavily on zone 2 training and zone 5 (and 4) training.v

Zone 3 training is good when you want to combine volume and intensity in your training. Moreover, for many triathletes and runners, zone 3 equals race intensity.

When comparing zone 2 and zone 3, zone 3 burns more calories, requires more aerobic and anaerobic power and activates more type II fast twitch muscle fibers.

Infographic - Zone 2- muscle fibers and lactate

Compared to zone 4, zone 3 is easier to maintain for a much longer time (training volume) and requires less time to recover from.

Zone 3 training has many cardio benefits. For instance, zone 3 training improves VO2max and builds mitochondria. It also burns fat and due to a relatively high energy demand, zone 3 could be used to lose weight.

INSCYD’s Graph - fat and carbohydrate combustion
INSCYD’s Graph: In zone 3 you burn fat and carbohydrates. The energy demand of zone 3 is quite high, especially when you take into account that athletes can maintain zone 3 for a very long time.

There is one specific reason why you should train in zone 3. When you’re a triathlete or marathon runner, you should train in zone 3 when your VLamax is too high. In other words: zone 3 training can decrease VLamax (anaerobic power) and therefore decrease the amount of carbohydrates you burn during exercise. When you combine this with an increase in fat combustion, you rely less on a precious fuel: carbs.

Of course you first need to know your VLamax and carbohydrate combustion rate, before you know whether it’s too high. Athlete’s use a INSCYD’s metabolic test like to find out.

VLamax comparison between swimmers
INSCYD performance software shows your VLamax after performing a simple metabolic test. The VLamax (maximal anaerobic power) can vary between athletes.
INSCYD Metabolic Profile

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Fans of zone 3 training refer to it as sweet spot training. Sweet spot training is the exercise intensity at the upper level of zone 3. For instance: 85-90% of FTP.

Sweet spot training is famously known as the exercise intensity that best combines intensity and volume. Not only within a single workout, but also in a training program. That is because sweet spot training is believed to require little recovery.v

Since sweet spot training uses:

  1. A relatively high intensity…
  2. That you can maintain for a long time…
  3. And train at frequently due to a fast recovery

That’s why it’s an easy way to accumulate training load (volume x intensity). However, that’s also why many criticise sweet spot training or zone 3 training in general. Continue reading to learn more about the downsides of zone 3 training.

When polarized training became popular, zone 3 training got in a bad spot. Some even say zone 3 training is a waste of time. Here’s why.

Zone 3 training is bad when you’re in zone 3 while you actually planned for zone 2. And that’s what often happens.

Imagine you need to recover from a previous workout. Or you need to do an easy day to prepare for a hard zone 4 training tomorrow. You probably plan for a zone 2 training.

The problem of zone 2 is that it’s often perceived as slow, maybe even boring. As a result, athletes increase their pace or speed a little bit, and end up in zone 3. 

But zone 3 is not a recovery zone. As we’ve seen in the INSCYD carbohydrate graph earlier, athlete’s burn a lot of precious fuel when training in zone 3.

As a result, their Z2 training became Z3 and since they are not recovered, their Z4 training will become Z3 as well.

Infographic -zone 3 training bad
If you don’t stick to the plan, your easy days (Z2) become too hard (Z3) and your hard days (Z4) become too easy (Z3). You’ll end up spending all your time in zone 3, the grey zone.

Zone 3 training is also a bad idea when you want to increase your anaerobic power. Learn why that is the case, in our free white paper about VLamax (anaerobic power):

Grey zone

  • Intensity: zone 3 is not a recovery ride, but it’s not super hard either
  • Fuel: in zone 3, you don’t burn the maximal amount of fat, because you’re also burning a substantial amount of carbohydrates
  • Energy contribution: zone 3 is not purely aerobic nor does it rely heavily on anaerobic energy supply
  • Muscle fiber type: zone 3 requires a mix of type I and type II muscle fibers

To know exactly what happens in a certain training zone, coaches and athletes use the INSCYD training zones. This clearly shows that for our example athlete, zone 3 is a combination of many ingredients.

The physiological conditions in the INSCYD training zones show exactly what happens in a specific training zone, based on an individual metabolic test.
The physiological conditions in the INSCYD training zones show exactly what happens in a specific training zone, based on an individual metabolic test.

Zone 3 training is not a waste of time.

However, the benefit of zone 3 training can also be the disadvantage: it combines many physiological aspects and requires quite some energy. That’s why fans call it the sweet spot, while haters call it the grey zone.

Do use zone 3 training in your training program when you want to activate all energy systems or when you’re aiming at decreasing VLamax.

Don’t use zone 3 training when you actually planned a zone 2 recovery workout or a hard zone 4 session. Also avoid it when you’re trying to increase anaerobic power.

In all cases, when you apply zone 3 training you need to be precisely aware of the amount of carbohydrates you burn in this zone, to prevent overtraining. In zone 3, you can easily burn more carbohydrates than you’re able to refuel during the workout.

Continue reading for zone 3 training examples. 

But first…

Here’s what scientific literature has to say about zone 3 training*. 

*Note that some literature refers to zone 3 as zone 2, simply because they use a different zone model. For the sake of simplicity, we stick to calling it zone 3.

This study shows that recreational triathletes perform their Ironman race primarily (59%) in zone 3 (between LT1 and anaerobic threshold). As a result you could easily jump to conclusions and think zone 3 training is also an effective training intensity.

However, the same study shows that there was a moderate correlation between total training time in zone 3 and performance time in competition. In other words: athletes who trained more time in zone 3 needed more time to finish their triathlon. Oops.

There are several studies who have similar conclusions. For instance, this study shows that recreational runners are better off spending their time at lower intensity than in zone 3, even though both training methods increased 10k running.

This advocates the commonly used phrase: 

“Make your easy days easier and your hard days harder.”

A reason why athletes are potentially better off at lower intensities is, because they underestimate the energy demand of zone 3 training. As a result, they don’t fuel enough for this type of exercise and performance declines. 

That’s why knowing your carbohydrates combustion rates in a training zone is so important. The INSCYD metabolic test is the easiest way to get this information.

As always, there are also studies showing positive effects of sweet spot or tempo training. Depending on your goal, it makes sense to add zone 3 training to your training program

Literature shows that relying almost only on zone 3 training is often not a good idea.

A typical zone 3 workout consists of 2-3 sweet spot intervals of 10-30 minutes each.

Here’s a zone 3 training example:

  • 20 minute warm up
  • 3 sweet spot intervals:
    • Intensity: 85-90% FTP or 75-80% max heart rate
    • Duration: 15 minutes
  • Recovery: 
    • Intensity: c
    • Duration: 6 minutes
  • 10 minute cooling down
Infographic - zone 3 workout with sweet spot intervals
Typical zone 3 workout with sweet spot intervals

In theory, you could also use much longer sweet spot intervals. That is because you don’t accumulate any lactate in zone 3.

In zone 3, athletes don’t accumulate any lactate because the lactate production (red) is below the maximal lactate combustion or recovery (blue).

As a result, you could stay in zone 3 for hours, as long as you consume enough carbohydrates.

The same is true for the amount of zone 3 workouts per week. If you consume enough energy during and between workouts, you can do 3-4 zone 3 workouts per week. However, as mentioned earlier, science shows that athletes should not (only) focus on zone 3 training.

When planning an endurance workout, many athletes wonder whether they should train in zone 2 or zone 3. Both zones cover intensities you can sustain for a long time.

To answer this question, you need to know your current fitness status, your goal and what needs to happen to bring them closer to each other. 

Your INSCYD metabolic profile shows that your anaerobic power is too low to attack on a short climb during your next race, then you need to increase anaerobic power.

In this case, you should avoid mid intensities like zone 3 and instead do your endurance training in zone 2. That is because long time periods at mid intestines are known to decrease anaerobic power. Furthermore, Z2 training leaves more room for a hard anaerobic power workout tomorrow.

Should you train in zone 2 or 3? Compare your current metabolic profile with your goal - and find out!
Should you train in zone 2 or 3? Compare your current metabolic profile with your goal - and find out!

However, your INSCYD metabolic profile could also show that you burn too much carbohydrates during your triathlon, and therefore run out of energy quickly. You need to decrease the carb combustion rates in your muscles.

In this case, you should increase your zone 2 training intensity to zone 3 intensity. This will activate your fast twitch muscle fibers and teach them to rely less on carbohydrates.

Bluntly speaking, you should do your endurance training in zone 2 if you want to increase anaerobic power and you should train in zone 3 if you want to decrease your carbohydrate combustion rate.

For some athletes zone 2 is better than zone 3 because it is less demanding, easier to recover from and it burns more fat.

The biggest mistake you can make when implementing zone 3 training is to underestimate the time it takes to recover. This highly depends on another thing you shouldn’t underestimate: the energy it takes.

Simply avoid this mistake that can lead to overtraining:

  1. Perform your INSCYD metabolic profile. Find an INSCYD lab or (remote) coach.
  2. Learn how many carbohydrates you burn in Z3 and adjust your food intake accordingly.
  3. Look beyond a single Z3 workout and set up an efficient training program.

For coaches and labs

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Create highly personalized training programs with lab-level performance insights anywhere anytime to analyze, optimize and improve performance faster, save cost and get a strategic edge.

✅ Test remotely  ✅ No Special Equipment  ✅ Full Insights from a Single Test  ✅ Right fit for each sport

Other coaches and labs are already leveling up their coaching strategies. Can you afford to be left behind? Don’t Miss Out! Our Free Demo Spots Are Filling Up Fast—Secure Yours Now and Transform Your Coaching Forever.

For ATHLETES

GET THE MOST OUT OF YOUR TRAINING

Athletes why train with generic plans when you can have a program tailored to your unique physiology? INSCYD is the key to unlocking your full potential. Find your dedicated INSCYD coach or lab here. 

Already have a coach? Experience INSCYD in action with your coach and redefine your training approach.

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