Understanding Cardio Intensity Training Zones: A Science-Backed Guide to LISS, HIIT and SIT

Depending on which sport you perform or even which country you are from, cardiovascular training can be categorized into as few as 3 intensity zones and up to a staggering 8 zones. 

The reason why coaches are prone to using intensity zones when prescribing exercise sessions is that at every heart rate zone, there is ultimately different adaptation in the heart tissue, in our muscle mass, and in the metabolism or substrates used to fuel muscle contraction. 

This is of utmost importance for endurance athletes who need to make sure that they maximize the capacity of the heart, muscle, and metabolism that is uniquely required for performance success in their sport. 

But for the individual interested in improving their health, body composition and longevity, strict adherence to rigid intensity zones is not always necessary.

Intensity models designed for sport-specific success are a useful guide, though the best approach for optimal aging and overall health may differ from what is prescribed for peak athletic performance.

The intensity prescription that is most commonly used by coaches for the general public, is the Five-Zone Model, proposed by the American sports scientist, Dr Stephen Seiler. 

The 5 Cardio Intensity Training Zones Explained

The easiest way of understanding what each heart rate zone is doing is to break them into distinct zones 1 through 5, which also corresponds very nicely to the different substrates being used to fuel muscle contraction, the different muscle fibers being recruited, how much lactate is being produced, and finally how hard the heart is working to pump blood and oxygen around the body. It must be noted that the physiological differences experienced by the heart, muscle fibers and metabolism in each zone is not completely distinct and different, but rather a continuum or an ‘up & down’ regulation, depending on how hard the heart is beating, relative to its maximum capacity.

Zone 1 describes exercise performed when the heart rate is between 60 to 72% of maximum.

Zone 2 describes exercise performed when the heart rate is between 72-82% of maximum.

Zone 3 describes exercise performed when the heart rate is between 82-87% of maximum.

Zone 4 describes exercise performed when the heart rate is between 88-93% of maximum, and

Zone 5 describes exercise performed when the heart rate is between 94-100% of maximum.

For most people, Zone 1 will describe recovery exercise or very easy physical activity, such as walking and gentle swimming. Taking a hike on a generally flat terrain or performing multiple easier laps of swimming in a pool would generally be regarded as Zone 1. Coaches tend to prescribe Zone 1 for active recovery, which can be performed daily for up to an hour, as the intensity is such that it does not interfere with recovery from higher intensity exercise, such as interval training and resistance training.

Zone 2 exercise would be considered low to moderate intensity. This is where you could talk and carry out a conversation while you exercised, but that conversation would be laboured. There has been an incredible rise in popularity in this intensity zone, as it has been shown by exercise physiologists to provide a nice blend of stimulus for the heart, muscle and metabolism that's also having very little interference with higher intensity exercise training sessions. This means you can accumulate large amounts of training volume without risking over training or reducing the quality of resistance training sessions. 

Zone 3 is the intensity that most people would undertake in a competitive race event, such as 5 or 10K. It would be classified as hard, if using a perceived exertion scale. Coaches tend not to prescribe a lot of exercise in Zone 3 because it is hard enough to impact recovery from quality high-intensity training sessions and thereby delay recovery, but it's not as hard as interval training sessions, which produce considerable adaptation in heart, muscle and metabolism. 

Zone 3 is sometimes described as the ‘black hole’ or the ‘grey zone’ of training, with many coaches preferring to prescribe exercise in Zone 1 and 2 with only a little bit in Zone 3 to prepare one for racing event and then spend more time in Zones 4 and 5, for its more potent ability to improve performance.

When we enter Zones 4 and 5, these are commonly referred to as the Red Zones. The intensity is so challenging that you would not be able to continue exercising for any appreciable period of time. Doctors in the early 1900s realized that this was the intensity zone that had the most profound effect on cardiac tissue, so they devised interval training, where the heart failure patients would perform a bout of vigorous exercise interspersed with recovery periods, with the goal of ultimately accumulating much more high intensity work than they could do if they were doing it continuously.

The stunning results they saw in their patients encouraged them to share their findings with European coaches, and the rest, as they say, is history. Interval training is now the most commonly accepted method for improving high intensity performance and is the bread and butter of many endurance athletes’ quality training, for its potency to improve heart, muscle fibre recruitment, and metabolism.

High Intensity Interval Training is a cover-all term that is used to apply to a bout of exercise typically lasting between 1 to 8 minutes, interspersed with up to several minutes recovery. The heart rate used for High Intensity Interval Training is over 90% of maximum, clearly putting it into Zones 4 and 5. 

There are numerous protocols that fall under the HIIT category of training, but they're all doing pretty much the same thing in terms of impacting cardiac tissue, recruiting fast twitch muscle fibres, and enhancing glucose use in the muscle.

If the interval effort is such that you can only perform less than a minute of work and therefore need longer recovery between intervals, it is then described as Sprint Interval protocol, where durations between 15 to 30 seconds are most commonly used interspersed with up to 3-5 minutes of recovery. 

The coaching phrase that is most commonly used is ‘all out’, that is no pacing is required, and you don't look at your heart rate. Rather, you exercise for as hard as you can for 15 or 30 seconds. After such an effort you will require up to 3 to 5 minutes of recovery. 

Very interestingly Sprint Interval Training and High Intensity Interval Training show almost completely comparable results in heart, muscle fibre, and metabolism. This is being repeatedly shown in a number of studies and meta-analysis that have examined all of the available literature. 

Every intensity Zone from 1 up to Zone 5 is doing something slightly different to heart, muscle and metabolism. Further, progressing from Zone 1 up to Zone 5 has an impact and penalty upon recovery. What this means is that you can perform hours of exercise in Zone 1 and 2 every day without impacting recovery or negatively affecting training sessions that you are performing in the gym. 

For every training session that is in Zone 4 and 5, which is HIIT and SIT training, there will be a recuperative penalty that has the potential to impact recovery and fatigue if employed too frequently within the week.

Expert consensus recommends one to two High Intensity or Sprint Interval training sessions be performed in a week, with any other cardiovascular training being in Zone 1 and 2. Experts are so confident in this prescription, as it has been observed in the world's best athletes to every day individuals looking at improving their health and longevity. It is a time proven successful formula for keeping exercise patterns consistent, without causing overtraining.

A good rule of thumb to remember is the more muscle fibers that are used and the higher the heart rate goes during exercise, the longer the recovery you will need between training sessions.

Just like there are no magical repetition zones when it comes to resistance training, there is no magical exercise intensity zone for cardiovascular training. High Intensity Interval Training and low intensity training both have their place. They are different, and both methods should be employed for success over the long term for your health and longevity and what you require as a unique individual for personal recovery.

What This Means For You

A mix of low-intensity and high-intensity cardio is the winning formula, and the good news is we've done the programming work for you.

If you would like to explore our cardio specific training plans:

Cardio Happens and the HIIT & SIT Flex Plan, you can trial these plans and more in The Membership right now.

References:

Coates AM, Joyner MJ, Little JP, Jones AM, Gibala MJ. A Perspective on High-Intensity Interval Training for Performance and Health. Sports Med. 2023 Dec;53(Suppl 1):85-96. doi: 10.1007/s40279-023-01938-6. Epub 2023 Oct 7. PMID: 37804419; PMCID: PMC10721680.

Hall AJ, Aspe RR, Craig TP, Kavaliauskas M, Babraj J, Swinton PA. The Effects of Sprint Interval Training on Physical Performance: A Systematic Review and Meta-Analysis. J Strength Cond Res. 2023 Feb 1;37(2):457-481. doi: 10.1519/JSC.0000000000004257. Epub 2022 Sep 8. PMID: 36165995.

Rosenblat MA, Perrotta AS, Thomas SG. Effect of High-Intensity Interval Training Versus Sprint Interval Training on Time-Trial Performance: A Systematic Review and Meta-analysis. Sports Med. 2020 Jun;50(6):1145-1161. doi: 10.1007/s40279-020-01264-1. PMID: 32034701.