Build Cardio Endurance With Zone 3 Training

In the discourse around heart-rate-zone training, high intensity (zones 4 and 5) and low intensity (zone 2) garner the most attention. Both training approaches, when leveraged the right way, can help people ­improve their endurance and boost cardiovascular fitness.

Then there’s medium-­intensity zone 3. This midrange, ­challenging-yet-doable zone — reached by performing an ­exercise at 70 to 80 percent of your max heart rate — is polarizing.

Some experts love zone 3 and consider it the sweet spot of training. Spending time in zone 3 during your cardio activity of choice can improve VO₂ max, build mitochondria, and train your ability to go farther and faster. Others consider zone 3 a gray zone, leading to “junk miles” that zap energy without payoff.

What’s often overlooked in conversations about zone 3 is that it is neither bad nor good — it’s simply a training tool. And as such, it’s important to consider how it’s used within a broader training plan.

With more than 25 years in the fitness industry, Jeff Rosga, NASM-CPT, PES, CES, Life Time’s director of team member development for Dynamic Personal Training, shares his top takeaways for incorporating zone 3 into your training.

Experience Life | What is zone 3?

Jeff Rosga | Zone 3, also known as the tempo or moderate-­intensity zone, is typically defined as cardio exercise at 70 to 80 percent of maximum heart rate, or 90 to 100 percent of anaerobic-threshold heart rate.

Often called the gray zone, zone 3 lies between low-intensity endurance training and higher-intensity threshold training. It bridges these zones, offering valuable performance gains, including the following:

• Improved aerobic capacity, making your body better at handling higher intensities
• Enhanced lactate threshold, training your body to handle lactate buildup so you can go longer and faster
• Improved cardiovascular health, strengthening the heart and lungs
• More efficient fat oxidation, training the body to use fat stores as fuel while keeping the intensity up
• Enhanced performance, helping athletes simulating race-day intensities

EL | What does a typical zone 3 cardio workout look like?

JR | Zone 3 cardio can take various forms and can be adapted to various activities, including running, cycling, and using machines like a rower, SkiErg, stepmill, elliptical, or VersaClimber.

Here are three examples of formats that work well for zone 3:

1. Tempo efforts. These are continuous steady-state efforts that last 20 to 60 minutes.
2. Long intervals. These are intervals of five to 20 minutes with shorter recoveries.
3. Sustained climbs. These are rides or runs going uphill at a moderate intensity.

EL | Who can benefit from zone 3 training?

JR | While everyone can benefit from some zone 3 training, it is especially useful for athletes seeking performance enhancement for racing.

Specifically, if your event requires sustained moderate to high intensities, zone 3 training simulates race conditions, preparing you physically and mentally. Combine zone 3 with occasional zone 4 and 5 sessions to stimulate adaptations.

EL | How can someone know if they’re in zone 3?

JR | It’s possible to gauge your zone by using a rate-of-perceived-exertion scale. You’ll recognize zone 3 as a moderate to challenging effort that requires you to breathe more deeply, likely through your mouth, [where you’ll be] able to speak but only in short sentences.

A more accurate method is to use an anaerobic threshold (AT) test to identify your unique zones. AT testing, available at many health clubs and sports medicine clinics (as well as some hospitals), involves running on a treadmill or riding a stationary cycle with a mask over your face to capture and analyze oxygen and carbon dioxide levels at varying stages of exercise ­intensity. (Learn more about AT testing at “How to Find Your Anaerobic Threshold.”)

EL | Why is accuracy so important?

JR | Because many athletes unknowingly experience something called “zone drift.” For instance, they may find themselves in zone 3 during workouts intended for zone 2 or zone 4, leading to fatigue without desired adaptations. And vice versa: Athletes may drop into zone 2 or spike into zone 4 during an intended zone 3 workout, undermining their efforts and losing out on the benefits of zone 3.

EL | So, you can’t spend time in zone 3 and expect it to be the average of 2 and 4?

JR | No, each zone has distinct qualities and unique benefits. Zone 2 is an easy effort; you can easily chat while exercising. It’s primarily aerobic, and you use fat as fuel. You won’t build up much lactate. These are usually long-duration workouts lasting anywhere from one to four-plus hours. Training in this zone is great for endurance and for recovery. (See “How to Use Zone 2 Training to Optimize Fitness Performance” to learn more.)

Zone 3 is a moderate effort. You can talk but only in short sentences because you’re breathing deeper. It’s a mix of aerobic and anaerobic activity based on the top or bottom half of the zone. You can build up some lactate, but it’s manageable over longer workouts. These are moderate-duration workouts of 20 to 90 minutes maintained through continuous activity or interval training. Training in zone 3 improves your lactate threshold, lets you work at moderately high intensities, and improves your aerobic capacity.

Zones 4 and 5 range from hard to very hard effort — you feel a lot of discomfort during these workouts. You rely heavily on glycogen and glucose for fuel and experience quick lactic buildup.

Zone 4 can be maintained for about 40 minutes, but as you reach the upper end of the zone, that will be reduced to only a few minutes. Zone 5 is something that most people can maintain for only seconds. These zones boost your VO₂ max, lactate threshold, buffering capacity (how well you handle lactic-acid buildup), muscular power, speed, and top-end performance.

EL | Any advice for someone who wants to start doing zone 3 cardio?

JR | More is not better. Too many regular zone 3 sessions can increase fatigue without improving performance and can negatively affect recovery and long-term fitness.

Start with one zone 3 session per week as part of a comprehensive training plan that also includes zone 2 sustained efforts and zone 4 or 5 intervals. As a rule of thumb, aim to spend about 80 percent of your cardio training time in zone 2, plus one workout each in zone 3 and zone 4 or 5.

You can progressively add zone 3 sessions, but for best results, avoid doing more than three each week. If you’re feeling unsure, it can be helpful to work with a certified endurance coach.

A Look Inside the Zones

The specified heart rates within each zone are examples only; each individual has unique zones. Learn how to determine yours at “The Active Metabolic Assessment: The Most Accurate Test For Cardio Training.”

This article originally appeared as “Zero in on Zone 3” in the November/December 2025 issue of Experience Life.