What is it?
HIIT stands for ‘High Intensity Interval Training’.
It usually means short periods of intense effort separated by easier periods of recovery.
To get fitter, these bouts of intense effort must be hard enough to stress an identified physical quality in the body and the rest periods are easy and long enough to allow the athlete to recover and go again at the same intensity as before. If the athlete isn’t recovered sufficiently and can’t replicate the first work period, then the physiological focus of the session will change and might even disappear completely.
So, what is hard enough? Well that depends on the physiological quality being targeted and developed…. why are you’re doing it?
The minimum is the ‘lactate threshold’ – but that phrase is used by many people to describe different things – a better term is lower lactate turn point, which is when your lactate levels and heart rate suddenly increase as a response to harder exercise, usually seen as the first kink upwards when viewed as a graph.
Other thresholds that might be used to delimit intensity include MAS (maximum average speed using the aerobic system), VO2 (workload that demands maximum oxygen uptake into the working muscles), vVO2 (velocity or speed at which VO2 happens). Others include MLSS (speed or work rate at which body can just about cope with a stable but raised lactate level in the blood for a longer time) which approximates to CP (critical power) and FTP (functional threshold power) which represent how hard the body can work for various lengths of time.
Who developed it and for what?
No one really knows but it was used by European running coaches in running in the early 1900’s. Famous athletes who used HIIT included Zatopek, the Finns who dominated distance running in the 70’s and those training with Bannister to break the 4-minute mile barrier.
A Japanese Professor, Tabata discovered in speed skaters that using 8 periods of 20 intense effort separated from by 10 seconds of rest over 4 mins got much better results than training at the same effort for longer. He’s been made famous by the term ‘Tabata Intervals’, which just means one type of HIIT utilising 20 seconds on and 10 off.
Other sports have developed different variants of HIIT. Sports like football, rugby and hockey use small sided games to have the same effect – an example could be 2 teams of 4 playing against each other on a reduced size pitch for 2 mins but at high intensity.
The leisure industry has pushed HIIT in a big way, loads of classes at gyms are now labelled HIIT and sports like athletics, football and rugby league discuss ‘interval training’ in their coach education courses. But – I’ve yet to see any PT or coaching course to Level 3 discuss the how’s and why’s of different HIIT formats. Most PT’s and coaches simply don’t know the reasons why they prescribe a certain session or what specific physiological benefits it targets.
What does it do?
What recent research has told us is that different forms of HIIT develop different physiological qualities. One size doesn’t fit all!
Variation’s in the intensity and length of the work and rest periods can be used to target either the aerobic, anaerobic or neuromuscular systems or combinations of them. Some types focus on ‘central’ adaptations (heart chamber size and stroke volume -how much blood is pushed around with each pulse) whilst others target ‘peripheral (changes in the muscle like oxygen uptake). It can also be used to target sport specific characteristics like changing direction or repeat sprint ability in team sports such as football. A coach or athlete might decide to improve VO2 in pre- season and choose one method of HIIT which would then change if the target switched to speed later.
We also know that HIIT will benefit different athletes differently depending on their sport, development and fitness. A football player will have different requirements than a triathlete. Doing HIIT on a bike would probably cause more cycling specific changes and benefit those who run less. A 12- year old football player starting puberty will have a different physiology than he or she will have 5 years later. HIIT needs to be tailored to the individual and their sport.
There also needs to be a minimum number of sessions done to stimulate fitness gains in the body and the research suggests this change varies with age and fitness level. It’s probably at least once a week and needs to be consistently done for at least 4 to 6 weeks. There’s probably an upper limit on how many sessions can be done too after which the body is so used to the demands that further gains in fitness slow down or stop. At that point, the HIIT session needs to be changed.
There are many different types of HIIT, each one designed to target different physiological qualities. Some types of HIIT work at very high speeds and heart rates, others much lower. HIIT doesn’t need to be flat out, it just needs to be at a more intense level than what is normally experienced in the sport. Endurance runners can’t keep speeds faster than MAS for long so using a pace of 105, 110 or 120% of MAS in intervals constitutes HIT.
What are the advantages of HIIT?
The big advantages of HIIT are that fitness gains can be made quicker than steady state training and that specific physiological qualities can be targeted for the individual and the sport.
HIIT sessions can be short. The research shows that they don’t need to go on for an hour. Tabata proved that 4 mins of 20:10 works.
Doing less but getting fitter is better time management and can reduce overtraining – something that many endurance athletes do (but won’t admit!).
HIIT, like strength training, has been shown to be so good for athletes that they can reduce the hours spent biking, running, swimming etc.
What are the disadvantages of HIIT?
These aren’t disadvantages but questions to think about…
HIIT needs to be tailored for the individual to get the most benefit. What is it you’re using it for?
How hard an athlete works and rests, what they do and how long they rest for in-between should be targeted for the person, the sport and specific physiological qualities needing improvement. This is why setting a specific session and times using distances for a whole group or taking a session from the web won’t help most – everybody is different. For example, an athletics coach tells a group of runners to do a session of 10 x 400m in 90 seconds with 100m walk recovery – different abilities will find it easy, hard or not possible. The less fit will end up doing it slower, or not at all, and so fitness gains will be different or won’t happen at all if the intensity drops below a threshold.
HIIT is hard work and needs some recovering from. If an athlete doesn’t recover from one session to the next, they won’t perform as well, fitness gains will slow or possibly even result in overtraining which can lead to illness. HIIT is also demanding on the body, working at high intensity levels involves a higher risk of injury. As an athlete, what objective or subjective measurements do you take to tell you when you’ve recovered from one session to another? Do you take note of these and change your training to suit how recovered you are? Maybe, drop a session or make it much easier?
HIIT, being intense, needs glycogen to fuel it. Some very well-trained athletes might be able to use fat burning more but these are the elite with sports science support in physiology, nutrition etc. This means that nutrition before HIIT sessions is vital to ensure you are stocked up with enough glycogen to get you through the session. An athlete who does back to back HIIT, in the morning before breakfast, at the end of training or without being recovered (glycogen depleted) from the previous day won’t be working at full capacity.
Intense exercise can lower the immune system for up to 6 hours later – leaving you open to viral infections such as colds and flu. Think Covid-19. About 30% of runners and cyclists get multiple ear, nose and throat infections a year – team sports players, who do less volume, don’t! Is it worth the risk or can you swap in HIIT instead of something else like a long run or ride?
When should and shouldn’t an athlete use HIIT?
Depends on what the athlete is like, the event targeted, the stage of the season and what they are trying to develop?
HIIT is riskier for injury and illness – is it worth it or is there a safer alternative?
What questions should I ask before using HIIT?
Ask the Coach or PT…
What is the purpose of the HIIT session the coach or PT has planned? Is it designed to improve VO2, central or peripheral adaptions, speed, change of direction, anaerobic or neuromuscular systems? How?
Is HIIT the right training method to use to get improvements?
Would something else work better?
How is this individualised for me?
What’s the minimum I need to do?
If the Coach or PT can’t answer these questions……