Zone 2 or HIIT? Or Neither?

My 58 year old patient walked in yesterday to the treatment room and the first question she asked was if she should start HIIT training. Woah! Hang on, I said to her do you really know what HIIT is? (*See end of article for what HIIT is please).

She said her friends, children and all the fitness influencers she sees extols the benefits of HIIT. Then without batting an eyelid, she said, "What about Zone 2 training?"

That is basically what is happening online. One group says train at Zone 2 while the other says go hard or go home, HIIT is the way to go.  

What do the best in the world actually do? A new study by Sandbakk et al (published in April 2025) investigated elite endurance coaches and their training were not what you expected. These coaches had athletes winning more than 380 international medals in long distance running, biathlon, rowing, cross country skiing, road cycling, swimming, triathlon and speed skating.

All the coaches stick to a traditional periodization model, including a gradual shift towards overall lower training volume and more competition-specific (race pace) training as the competitive period approaches.

Another common emerging feature was an emphasis on high volume low intensity training. Look at the picture above, MOST (80-90 percent) of the weekly training was easy. Here is the key insight, majority of the easy work was not in Zone 2. It was in Zone 1. Slower than what the internet experts are obsessed with. Slower means more sustainable and more adaptation over time. That's how you get stronger. 

True aerobic development is only possible from accumulating volume. It also allows one to recover and handle key sessions. 

This Zone 1 low intensity sessions along with combined with 2-3 weekly "key workout" days consisting of 3-5 intensive training sessions. The sessions are purposeful and focused, with recovery all planned. 

Finally, coaches across all sports focused on getting high training quality by optimizing training sessions by controlling the load-recovery balance to ensure optimal preparations for major competitions.

The athletes go through all the zones (see the above picture), not just high intensity (Zone 5), but also in between. Short, fast intervals and controlled thresholds. The exact proportion is dependent on their competition demands. There were not a lot of really hard anaerobic sessions. 

Why the big difference between these elite coaches and what we see online with all the fitness influencers? Firstly the fitness influencers usually do not compete in races or competitions. They won't be posting so many videos if they do. Train easy, adapt, get stronger does not sell as well as "unlocking this magic Zone of HIIT". High performance is not about shortcuts. It's about accumulating consistency over time. Not as attention catching, but it definitely works.

To sum up, it's mostly sessions of low intensity, with occasionally high sessions adjusted to the individual. Balance stress and recovery and consistency over all else. Now you know.

* Many gyms, fitness influencers and trainers get patients to do what is traditionally circuit training but call it HIIT. Circuit training is like 30 seconds doing push ups, rest a minute, go to another station and perform 30 seconds of high knee lifts, rest a min, 30 seconds of lifting dumbbells etc

HIIT stands for high intensity interval training. Or interval training for short.  Example, you run 15 intervals or repetitions of 400m with a one minute rest in between or the coach will say run 6 x 1km going every 5 minutes (meaning if you run your kilometer in 3:50 min, you get 1:10 min rest before starting again). You can also cycle intervals outdoors or on a trainer with your bicycle attached.

Circuit training does not sound sexy, but HIIT certainly does!

Reference

Sandbakk O, Tonnessen E, Sandbakk SB et al (2025). Best-practice Training Characteristics Within Olympic Endurance SpoRts As Described By Norwegian World-Class Coaches. Sports Med 11:45. DOI: 10.1186/s40978-025000848-3

Race Pace Versus Heart Rate Variability Guided Training

Picture taken from Flowly

A patient came in this week after he raced the Tokyo marathon last weekend. He was not happy as he started cramping in his hamstrings after 'only' 5 km.

I asked about how his training went. He told me he did 4 runs a week, all on a treadmill, mostly easy runs. His longest run was by time, 2:55 hrs (which he thought was equivalent to about 20 km).

He did all his runs with a heart rate monitor, training at "Zone 2" mostly and followed recovery metrics like heart rate variability *(HRV). Zone 2 training is low intensity exercise that involves training at 60-70 percent of maximum heart rate. He did not do any interval training nor any race pace training.

I discussed with him how he can do better since he will be running the Chicago marathon on October 12th this year. He really wants a improvement after his self described disastrous Tokyo race. I shared with him a study that tested 3 different approaches of training (Ranieri et al, 2025).The researchers split the runners into 3 different groups for 6 weeks. 

First, a heart rate based training group where training is prescribed by heart rate zones. Next a race pace training group in which training is prescribed running at percentages of race pace. Lastly a HRV guided training group. The HRV training group did training prescribed by heart rate zones but this was adjusted daily based on HRV readings. Hence their intensity was based on how well they recovered. 

All the runners did lab tests before and after the study. V02 max, running economy, ventilatory thresholds and other key endurance metrics. Participants were assessed by a 7 km time trial after the training period.

Results? Race pace training was the most reliable for improving race performance. Every single runner in that group went faster in the 7 km time trial with an average improvement of 3.8 percent. The runners also had gains in fat loss and maximal aerobic speed. The heart rate training group improved their ventilatory threshold readings but this did not lead to V02 max or time trial improvement. It did not deliver clear performance advantages over race pace training.

The HRV training group led to bigger physiological improvements. They had significant increases in ventilatory thresholds and V02 max, but this did not translate as well to race performance. Some participants also put on weight (fat mass) perhaps due to more frequent easy training days.

The authors concluded that race pace training had the least variablilty in response, meaning it worked reliably across runners. HRV based training showed individual differences, some saw gains, others did not. This is not surprising since HRV itself is highly variable.

I suggested to my patient that he needed to do some race pace training if he wanted to achieve his target timing. He can take his target finishing time and calculate how fast he needs to do his 1 km, 5 km or even 400m repeats. This teaches his body to gauge the pace he needs to run and not go too fast especially at the start. It will also prepare his body for the demands of race day.

I also suggested running more outdoors rather than solely indoors on a treadmill since the treadmill is softer and will not mimic the road conditions that he races on. Specificity is key. The gym environment is temperature controlled unlike outdoors. So if the race location is hotter and more humid then he will be less able to handle it.

Takeaway message? The best approach is not to train solely on a single approach. It is best to know when to use each approach. For example, you cannot train exclusively by race pace. The body will not be able to handle the load, one would likely get injured. 

Heart rate based training can definitely be used but should not be relied on exclusively too. It will improve physiological markers and your lab based testing (V02 max, lactate threshold), but may not result in faster race timings. 

HRV is a useful tool for monitoring stress and how well your body is recovering. Many professional athletes use it. However this study raises an important question of whether better physiological markers lead to better performance.

If you're looking for a fool proof way to inprove your race times, this study suggest that training at your race pace sould be a part of your training routine.

Reference

Ranieri LE, Casada A, Martin D et al (2025). Performance And Physiological Effects Of Race Paced-Based Versus Heart Rate Variability-Guided Training Prescription In Runners. Med Sci Sp Ex. DOI: 10.1249/MSS.0000000000003671

*Heart rate variability (HRV) is a measure of how much time passes between each heart beat. It is better to have a higher than lower number. It is physiological measurement that is used to understand how your body is coping with life and environmental changes. A low HRV may suggest that your body is not rested, less resilient  and may not handle changing situations. You can measure HRV with a smart watch, fitness tracker etc.