How HIIT Helps and Hurts

By January 20, 2018 February 8th, 2019 Athletic Performance, Endurance Sports, Exercise, Health
High Intensity Training (HIT)

High-intensity training can improve health or fitness, or lead to dysfunction and injury, depending on how you go about it.

High-intensity training is a major fitness trend, and possibly the most common exercise approach used by individuals, groups, clubs and teams to improve athletic performance, and reduce body fat and weight.

Endurance athletes use sprints, intervals and resistance training to gain strength, increase speed and build endurance. Others lift weights, and participate in organized activities, such as Crossfit, Fitwall and Spin workouts at gyms. When these workouts involve intervals, they are known as high-intensity interval training, or HIIT.

Some athletes even do all of the above.

HIT can either help or it can hurt, depending on the individual, the intensity and duration, and the effectiveness of recovery. Avoiding the harm and getting more help from HIT requires participation from the brain.

While these activities have potential benefits, overdoing it with HIT also can increase the risk for damaged joints, disrupt hormones and harm the heart, plus cause chronic inflammation, pain, hormone imbalance, fatigue, depression and reduced athletic performance. It may also be the most common cause of physical, biochemical and mental-emotional impairment in both competitive athletes and others who exercise. In addition, despite burning a lot of exercise calories, many are unable to lose excess body fat and weight.

What is High Intensity?

For most people, high intensity is best measured by an increased heart rate (HR). Many athletes don’t realize the intensity level of their typical running or cycling pace, for example — it seems relatively slow, giving the impression of a low-intensity workout. But this slow-paced workout is often a high-intensity one in disguise. The solution is to use a heart-rate monitor to better regulate intensity. You can find the training heart rate that best matches your personal needs here.

High intensity also is often called anaerobic exercise. Those wanting to reduce excess body fat and weight, improve endurance and speed, and obtain health benefits from exercise, can accomplish all this with aerobic exercise performed throughout the year, along with shorter periods of HIT. This allows for better burning of stored fat, increased endurance at faster speeds, and many health benefits.

The key is to keep these types of workouts simple, short and to allow for adequate recovery time.

How HIT Helps

Healthy HIT can work wonders for the body, potentially improving physical, biochemical and mental-emotional well-being. The relatively minor stress of periodic HIT workouts can be a healthy stimulus for muscles and bones, hormones and immunity, pain tolerance, brain function and more, including better performance.

HIT benefits are dependent upon healthy lifestyle choices, such as with adequate nutrients obtained through healthy eating: in particular, the vitamins and minerals, protein and essential fats, and phytonutrients. Avoiding junk food is vital too.

HIT is most effective when workouts are balanced. It’s important not to exceed one’s speed limit. For example, if your goal is to run a race at a six minute-per-mile pace, performing intervals equivalent to five minute-per-mile pace may be harmful due to excess stress. In addition, shorter interval distances are very effective as most endurance athletes rarely if ever perform full sprints. For power or team sports like soccer, basketball, football and tennis, and for non-competitive exercisers, very short HIT distances or times are a low-stress way to get faster and healthy.

Maximum benefits from HIT typically occur with one to three sessions (more often one to two) per week. Beyond this frequency and timeframe, health risks may rise while benefits reduce.

Recovering from HIT helps bring benefits and is as important as the workout itself. This means seven to nine hours of quality sleep (not waking during the night). Reducing other workouts before and after HIT sessions also helps with recovery.

HIT helps more when following your personalized needs rather than working out with a group.

When HIT Hurts

It’s not normal to be injured, unhealthy or develop diminished performance from HIT (or at any other exercise), yet these are very common outcomes. Reduced recovery, poor diet, and excess training intensity are examples of excess stress — and can add to harmful effects of HIT.

The first sign of harm is a red flag that HIT is hurting us may arrive as excess muscle soreness or pain, fatigue, loss of vigor, or just not feeling right. This is the time to stop HIT to reassess the training and lifestyle schedule. It’s best to heed these clues before they progress to signs of inflammation/injuries, hormone imbalance, illness from reduced immunity, and performance decrements follow — all part of the overtraining spectrum.

Fortunately, the earliest sign that HIT is hurting us is easy to measure, as a reduction of submax performance — reduced speed or power at the same HR. In fact, it’s not unusual to get slower at the same submax HR when HIT turns harmful. (Normally, healthy training promotes faster paces or increased power at the same HR over time.)

High-intensity interval training has a place in almost every athlete’s repertoire of workouts, helping to build speed, strength and endurance. However, including this effective tool in your training strategy requires using your brain to make wise decisions regarding frequency and duration, as well as proper support through recovery and nutrition.

Burning Calories & HIT

Just burning more calories is not the goal of HIT or any other exercise. Health and fitness benefits occur when the body burns more stored fat calories. In healthy people, all levels of exercise intensities burn both fat and glucose. In general, as exercise intensity rises we burn less stored fat and more glucose. We train our metabolism to burn more fat through aerobic exercise, which increases fat-burning not only during exercise but throughout the day and night. This improves endurance as we burn more fat at higher intensities, and helps competitive performance as well.

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  • Stuart says:

    I’m a 46 year old tennis player that still plays competively. 5 weeks ago I started training at my maf hr and have avoided tennis bc I cannot play and keep my hr below 134. My mile splits over the last 5 weeks are very slow but have improved. Yesterday, just out of curiosity, I played tennis for an hour and wore a heart rate monitor to track my hr. I found that my hr stayed elevated above my maf hr almost the entire time, mostly ranging from 150-180. 2 questions 1) if I took the full 3 months of recommended base training (so 7 more weeks) and only trained at maf, do you believe that I could drop the 150-180 hr range I experience while playing tennis significantly? 2) if I didn’t stop playing tennis completely during this time, and practiced for 3 hours a week (and it obv is anaerobic for me) are you saying I’d need to exercise for 12 additional hours during the week aerobically so that 80% if my training is at maf?

  • Lee says:

    Hello. Just some feedback… I have to say that the pop up ad for the maf supplements has made this site unreadable on a mobile device, at least for me. On my android phone the pop up spans the screen, in most cases with the closing X off the side of the screen. If you cannot close it, the site is unreadable. I suspect that this differs by OS and screen size but I am having a terrible time.
    Sorry for posting on this article – for some reason the ad here displays narrower than elsewhere so I was able to close it..

    Keep up the good work I’m a big believer in the maf system and a big fan of Dr Phil’s.ijust I just hate the advert!

  • James Hall says:

    David, I think one of the main issues is that many people have been tricked into believing that HIT is a cure all for everything, I.e that it can improve endurance, strength/power, get leaner, increase muscle mass etc.

  • Good information from this site as always 🙂

    I agree with a lot of what has been stated. I think its important to emphasize the use of a great coach. Its dangerous to say that HIIT such as Cross Fit is dangerous based on a few numbers. It is all about coaching. Imagine this a surgery being performed has statistical data yes, but it depends on the surgeon just as much as the particular surgery.

    Let’s not get ahead of ourselves by pigeonholing into thinking A+B=C. In reality its more of a case of N1. Not every HIIT session is designed to be carried out in the more is more mentality. Sometimes you plan for x amount of reps or sets or time. A good coach will know that this is just a rough marker and will correct as needed based on each clients needs. This is just as bad as chiropractic doctors getting a bad rep because its not quite fully understood.
    I think the thinking of black or white good or bad needs to change for us to progress in the realms of health and fitness.

    Thanks for the information,

    David Piggott (CPT,CES,PES)

  • Marco says:

    So that means if I want to run a marathon at a pace of 4:15 min/km I should do my anaerobic workouts at that pace and never faster? What about treshold workouts and the like?

  • James Hall says:

    I’m not sure if this is a noted or common occurrence but I feel like I’m not a great responder to HIT. By that I mean track workouts, hill repeats etc, not strength training. I feel that for the effort (and risk) that it entails that (for me) the performance gains are very small. Perhaps my physiology is more aerobically inclined. Maybe it could be that I just don’t tolerate the extra stress well but even after building a good aerobic bade I feel that the gains from interval training are very small. Perhaps if I competed in shorter distance or ‘stop start’ sports then the benefits may be more noticeable. I think for those just wanting to be healthy and fit strength training with higher loads and lower reps offers much more benefits than group interval training or similar training.

  • Russ says:

    What does morning HIT sessions do to one’s cortisol levels? How does it affect our neurotransmitters?

  • Ed Christy says:

    I am a USATF level 3, IAAF level 5 certified coach in to event groups, endurance/ sprints. I also am a member in The National Strength and Conditioning Association. As a result I get a lot of current research. Not to long ago I read about CrossFit training. It seems that there is a high percentage of injuries, About 70% due to the training with about 7% requiring surgery. This seems to be a form of HIIT that has some dangers. I might suggest Tabata 8’s instead. Research has shown that a set of those give the same aerobic benefits as an hour of aerobic training.

  • Martin says:

    I enjoy Phil’s articles, but I’ve got to say I am not a believer in the specific numbers quoted in this article. However I do believe the underlying message – too much intense performance training may give short term benefits but will have long term performance/health impacts. Predicting what that intensity is and frequency is a crap shoot though.
    So, sorry but I am totally unconvinced that ” .33 to .5 HIT sessions per week” is too much under normal circumstances. Maybe for someone training for a marathon, ultra, tour de France it is but for someone doing sports that are a balance for aerobic and anaerobic fitness, no way.

    • Hi Martin –

      You’re absolutely right. We made an editing error and then accidentally carried the analysis over into the comments. We’ve since corrected it, but just to be specific, you can do around 1-3 HIT sessions per week before you start seeing diminishing returns.

  • Barry says:

    Interesting, currently topical article but lacking in any some details to gain an understanding:
    What heart rate range qualifies as HIT ie % of own individual max (Note 220- age is garbage for max)
    Duration of HIT session and how many sets or repeats and duration of HIT repeat and breaks in between.
    How is HIT different to interval training?

  • Pete says:

    Hi there. Interesting article s always. However I was a little confused at the statement that says “if your goal is to race at 6 minute mile pace. The performing intervals at 5 minute pace maybe too stressful” . I run ultras and my goal race pace is significantly slower than that, as is my maf and training pace. However my understanding was that my best approach for occasional HIt workouts was to significantly increase the speed, for short intervals – e.g sprinting. To me it is hardly a HIT workout if I just run a few intervals at 6 minute pace, or even 5 minute – unless they are long intervals. Perhaps those statements just need a little more context? Pete

  • Matjaz says:

    Good article.

  • Andy says:

    from your article:

    “Maximum benefits from HIT typically occur with one to three sessions over a two to four-week period. Beyond this frequency and timeframe, health risks may rise while benefits reduce.”

    Does that mean “one to three HIT sessions” per week for a period of 2-4 weeks?
    Or does that mean a total of “one to three HIT sessions” within a period of 2-4 weeks?


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