Podcast, Science and Physiology

Bent Rønnestad, PhD | EP#386

 April 17, 2023

By  Bernardo Gonçalves


Bent Ronnestad - That Triathlon Show

Bent Rønnestad, PhD, is a professor at the Inland Norway University of Applied Sciences. Bent has been heavily influential in the sports science world in areas such as concurrent training, high intensity interval training, periodisation, and heat training.

In this episode you'll learn about:

  • Is time near VO2max a good indicator for the effectiveness of an interval workout?
  • Different types of interval sessions: short intervals, traditional long intervals, fast start long intervals, and variable long intervals
  • How do the above types of sessions compare for acute responses (e.g. time at VO2max) and for training adaptations and performance improvements?
  • Periodisation of interval training
  • An overview of heat training and adaptations in both hot and temperate conditions
  • Specific protocols used to increase hemoglobin mass, markers of performance and performance, and maintenance protocols
  • For whom is heat training a recommended?
  • How does heat training compare to altitude? 

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Bent's background

02:25 -

  • I am a professor in exercise physiology at Inland Norway University.
  • I am also a consultant for the Norwegian Olympic Federation.
  • I focus on providing physiological research and supervision for Norwegian elite coaches and athletes.
  • I work specifically in the area of endurance sports.
  • My role involves using my expertise in exercise physiology to help coaches and athletes make informed decisions about training and performance.
  • I research to better understand the physiological demands of endurance sports and how to optimise training to improve performance.
  • I provide guidance and support to coaches and athletes to help them develop effective training plans tailored to their needs.
  • My work is critical to the success of Norwegian athletes in endurance sports at the Olympic level.
  • My research focuses primarily on the singular and combined effects of strength and endurance training in elite sports.
  • I have explored various topics over the years. In the early 2000s, I focused primarily on pure strength training. However, my interests have since expanded to include isolated endurance and strength training and the combination of the two.

Interval training: time near VO2max as a valid marker for the effectiveness of a workout

05:35 -

  • I believe it's important to consider whether accumulating time at high VO2 during an interval session is beneficial for long-term adaptations.
  • A paper from 1986 by Wenger and Bell found that the most significant improvement in VO2 max occurred at 90-100% of VO2 max as a training intensity.
  • This intensity should be adequate for inducing adaptations, but very few studies have investigated the training effects of accumulating a long high time around VO2 max.
  • While acute signalling studies have shown different molecular patterns or signal ways, it's difficult to extrapolate these observations to long-term adaptations.
  • Acute studies are easier to conduct, but it's unclear whether they accurately reflect the effects of long-term training.

Steps for Recommending a specific interval session

07:58 -

  • The first step is to examine the protocol and evaluate whether it is reasonable and aligns with my training goals.
  • If the protocol appears to have sound reasoning and aligns with my training objectives, I would consider testing it to see how my athletes respond.
  • However, if the protocol involves very short work and long recovery periods, which are inconsistent with my training goals, I would not incorporate it into my program.
  • The primary aim of the interval session should align with my training objectives. 
  • For instance, if my goal is to increase power output and stimulate that part of the performance spectrum, then an interval session that achieves this objective would be more appropriate for my training program.

Heart rate and time at VO2max

10:13 -

  • It can be a bit tricky to use heart rate as an indicator of VO2.
  • The heart rate could be the same in two interval sessions, but the mean VO2 is quite different. This means that heart rate may not always accurately reflect VO2.
  • However, heart rate can still be a helpful indicator, as it is better than having no information. 

Defining the different types of intervals

11:28 -

  • Traditional long intervals: This steady-state workout involves work periods lasting four to six minutes.
  • We repeat these periods four to eight times.
  • Short intervals - 30/15: This interval involves work periods of 30 seconds with high intensity followed by a 15-second recovery period at approximately half the exercise intensity.
  • The workout is repeated for seven to eight minutes before taking a break and repeating it.
  • Fast start long intervals: This interval involves a high-intensity workout in the beginning for one and a half to two minutes, followed by a reduction in intensity for the rest of the period.
  • The last three and a half to three minutes are at an intensity slightly above the threshold (e.g. FTP - which is the maximal intensity that you can sustain for one hour)
  • Variable alternative: This interval involves dividing the two minutes of high intensity into three parts lasting 40 seconds and dividing it throughout the five-minute work interval. 
  • It is the 40-60 interval, with 40 seconds of high intensity followed by 60 seconds slightly above the threshold. 
  • This is repeated three times to complete the five-minute work interval.

How to execute these different types of intervals

16:40 -

  • Using the RPE (Rate of Perceived Exertion) scale and heart rate monitoring is an effective way to ensure a good workout. 
  • In my experience, starting with a good warm-up is essential before getting into the workout.
  • I suggest using the 16 to 20 RPE scale as a guide during the workout. 
  • It's possible to aim for a 16 on the first interval, then stay at 16 for the second interval, and gradually increase towards a 17. 
  • However, I don't believe it's necessary to go beyond a 17 to achieve an effective workout. 
  • The workload or work intensity should be adjusted based on how I feel, both during the workout and immediately afterwards. 
  • It's important to remember that the 17 on the RPE scale represents a challenging effort but not an all-out, extremely hard effort.
  • To maintain similar conditions for all participants in the study, we instruct cyclists to give their best effort during each session, aiming for as high a mean power output as possible.
  • While this approach may be practical for achieving results, it may not be necessary for sustainable, long-term training.

Acute responses of the different intervals

21:22 -

  • I've found that doing intervals with variations in exercise intensity can result in more extended periods of high VO2. 
  • We've compared short intervals with traditional long intervals and have found that short intervals tend to result in more significant adaptations across most measurements. 
  • Short intervals can sometimes be slightly better than fast start intervals with variable intensities, although the difference is insignificant.
  • It's important to emphasise that we have only done training interventions with short intervals and traditional long intervals.
  • We haven't investigated the variable and fast start intervals as training methods, except for a short one-week block in cross-country skiers. 
  • In that study, the block group used the 40-60 approach, which we compared to another high-intensity interval training block.
  • Therefore, we cannot determine the training adaptations of the variable and fast-start intervals compared to short and traditional long intervals.

Research on the 30/15 intervals

24:59 -

  • There are no long-term interventions that use the same protocol.
  • However, there are similar approaches that are used for acute situations. One example is the Tabata approach, which involves short intervals of high-intensity exercise. 
  • However, the original protocol is slightly different from what we do.

Incorporating these intervals in the training plan

27:00 -

  • I would say that the type of interval training depends on what we want to emphasise.
  • Some argue that short intervals are not a good training form because you will not have recovery every 30 seconds in racing. Still, if the short intervals can help improve power and increase VO2max, in theory, it should lead to better performance in a 40-minute test.
  • It is still important to train for continuous working because that is the specific competition setting. Still, in situations like a mountain bike race where a fast start is essential, fast start approaches can be helpful.
  • On the other hand, if you want to make it a strength to accelerate, multiple peaks with variable exercise intensity could be a good choice.
  • In a tempo time trial situation, keeping a steady pace for longer could be a good argument for using traditional intervals. However, I think that the body can naturally adapt to any changes in workouts, so switch things up and try different interval training types. 

Block periodisation

30:33 -

  • In the study, we investigated whether the distribution of interval training sessions would impact the results. 
  • We compared a traditional approach to a block approach, where two interval sessions per week were completed over four weeks. 
  • The block approach involved completing five interval sessions in the first week, followed by one session per week for the remaining three weeks. 
  • Both groups completed the same interval sessions overall, but their organisation differed.
  • Our findings showed that using the block approach resulted in a slightly better adaptation than the traditional approach, despite completing the same number of intervals. 
  • We tested this in both well-trained cyclists and cross-country skiers, and the results were consistent across both groups.

Incorporating intervals in triathlon

32:17 -

  • It's more challenging to incorporate high-intensity interval training for all three sports. 
  • However, one possible approach could be to emphasise one movement pattern in one of the three sports and focus on that for the high-intensity intervals. 
  • For example, we could do five cycling intervals and then have maintenance training for swimming and running.
  • If we decide to use this approach, we'll likely need to lower the overall training volume during that period to avoid getting into trouble. 
  • We must prioritise high-intensity training during that time frame.
  • Another approach could be to mix the three sports in the high-intensity interval block, but I'm unsure how to structure it, especially if we are doing a three-day cycle with a recovery day on the fourth day.
  • There may also be a need to reduce the intensity of the intervals to avoid issues.

When does it make sense in terms of the season to focus more on these intervals

35:25 -

  • Whether or not to include a high-intensity training block early in the preparation period depends on various factors.
  • Some argue that emphasising this ability and improving performance early on can be helpful. In contrast, others believe it can be dangerous to do high-intensity training too early in preparation.
  • I believe a high-intensity training block could be beneficial when we have been following a specific periodisation approach for a long time and experience a plateau in my development.
  • In this case, a block could help us take the next step in training. However, I would not perform a block all year long, as I believe in a mix of different approaches and systematic variation, not only in the intervals but also in the periodisation approach.
  • Therefore, it is essential to have a structured approach to training and periodically mix things up to avoid stagnation and ensure continued progress.

Specificity vs intervals in the final preparation period

38:05 -

  • I find the idea of testing out high-intensity training exciting.
  • High-intensity training can have a positive effect on signalling and can potentially improve threshold work and capacity.
  • I have experience with high-intensity training and using blocks of it before stage-races.
  • There could be advantages to high-intensity training, but it is crucial to consider the cost-benefit and to remember that everyone is different.
  • It is important not to test out high-intensity training right before the year's most important race.

Additional thoughts on interval training

39:30 -

  • Recognise individual differences in response to training
  • Avoid blindly copying what others do and instead experiment with different methods
  • Conduct measurements to evaluate the effects of training on yourself or your athletes
  • Understand that many different intervals and training methods can be effective
  • There is no one correct type of training, so be curious and explore different options.

Heat training for endurance athletes

41:07 -

  • Important competitions are often held in warm or hot climates, which makes acclimatisation necessary to improve performance.
  • Heat training has been shown to increase haemoglobin mass, which could benefit endurance performance.
  • The effect of heat training on haemoglobin mass is similar to that of altitude training.
  • Heat training is a simpler and more cost-effective alternative to altitude training.
  • With travel restrictions due to the COVID-19 pandemic, heat training can be done anywhere, making it a convenient option.

Performance improvement after a block of heat training

42:35 -

  • There is a clear relationship between haemoglobin mass and performance, although the effects of heat training are more towards the haemoglobin mass than the actual performance.
  • We recognise that many factors affect performance, which makes it difficult to draw definitive conclusions.
  • In some of our studies, we have observed an increase in power output of four mmol and improvements in time trials such as 15-minute mean and maximal power output.
  • However, not all of our studies or studies conducted by others show improvements in performance due to increased haemoglobin mass.

Repeat heat training blocks

44:53 -

  • In Altitude camps that last for three to four weeks, athletes focus solely on training and recovery, which means they can prepare themselves well.
  • These camps are beneficial for preparing for spring classics and summer races by providing high-quality training.
  • Going to altitude can help to re-boost the haemoglobin mass, but the science on the repeated effects of altitude camps is not yet precise.
  • Heat training is another method some cyclists use, and it has shown the potential to accumulate and maintain increased haemoglobin mass over a prolonged period.
  • The initial protocol for heat training involves five heat sessions per week for five weeks, which can be reduced to three sessions per week for three weeks to maintain haemoglobin mass.
  • Most studies have shown that haemoglobin mass returns to base level three weeks after altitude training.

Heat training protocols

48:10 -

  • The sessions were focused on heat stimuli for 50 minutes of cycling.
  • The power output was kept low, at around half of our usual output (four mmol of power).
  • We wore a special heat suit consisting of wool layers on both the upper and lower body, a wool hat, a rain jacket and trousers, and a down jacket outside the rain jacket.
  • The heat suit was designed to increase body temperature gradually.
  • The room temperature was set to 17-18 degrees Celsius, and even a slight increase of 2-3 degrees was noticeable.
  • If the room temperature increases too much, we must reduce our clothing or power output to prevent overheating.
  • We mainly care about rectal temperature and hydration levels during training sessions. 
  • The target rectal temperature is 38.5 degrees Celsius.
  • The rectal temperature is measured immediately after the session.
  • Getting the temperature to the target level during the first session may take some trial and error. Athletes often start too high, around 39 degrees Celsius, which can be uncomfortable.
  • Athletes can reduce their power output or wear fewer clothes to lower the rectal temperature.
  • Dehydration is also critical during the session, but the extent to which it's necessary is uncertain.
  • Athletes drink half a litre of water during the session.
  • Typically, athletes lose 1-2 kilograms of body weight during the session due to dehydration. This may vary depending on the athlete's body size.
  • It's crucial to rehydrate well after the session to recover appropriately.

Evidence for training application

52:28 -

  • To measure rectal temperature, using a standard device and checking immediately after the session is easy.
  • Whether I recommend heat sessions depends on the athlete's preferences and energy levels.
  • Five studies have shown improved haemoglobin mass with heat training, which may be a more accurate measurement than performance.
  • However, training quality and organisation should be optimised before starting heat training.
  • If an athlete is already at their limits, heat training could make a difference, but if they have a long way to go, prioritising training quality is more important.
  • If you're not used to competing in a hot climate, Heat training for about ten days before the primary race can help you perform better.
  • The adaptions from this heat training focus more on plasma volume, thermal sensation, and sweat rate rather than haemoglobin mass.
  • We can get these adaptations more quickly with heat training.

Thermal sensation for some athletes

56:07 -

  • There are logistical demands associated with heat training, such as washing clothes and drying the floor after each use.
  • This can add extra time and effort to my regular training routine.
  • Some people may find this demanding and not enjoyable, especially when doing it five times a week.
  • On the other hand, some people may feel the benefits of doing it and become addicted to it.

Training around heat block

57:12 -

  • In the first week of heat training sessions, I recommend reducing the overall volume and intensity of the workouts to make it an easy training week.
  • The first two to four sessions typically feel heavier than the rest, so it's essential to take it easy initially.
  • In heat suit studies, there was also a heat chamber group compared to the heat suit group, and it was found that the heat suit was as practical, if not more effective, in some variables.
  • This is an important finding because you can achieve the same results as a heat chamber by doing heat suit training.
  • Wearing a heat suit or overdressing is much easier than going to a heat chamber.

Passive heat training

58:45 -

  • I haven't compared active and passive heat stimuli, but based on the literature, being active during the heat stimuli is slightly better.
  • In one of our studies, we had a group perform heat training for ten days and added a passive heat bath daily. However, we didn't see any extra benefits from the additional passive heat bath.
  • Generally, I would recommend being active during the heat stimuli.
  • I'm unaware of any studies investigating the effect of active vs passive heat stimuli on haemoglobin mass.

Periodisation of heat training

1:01:15 -

  • Start when there are no critical races, preferably during the preparatory period.
  • Schedule five heat sessions per week for five weeks.
  • Finish the five weeks before essential races to move into a maintenance phase of heat stimuli and prioritise performance-peaking approaches.
  • Typically, do heat sessions in the afternoon as a second session.
  • It's recommended to have a few hours break between the ordinary session and the heat session if doing them on the same day.
  • Doing the heat session after the ordinary session may feel heavier, and it can fatigue you for the final or main part of the session.

CORE body temperature sensor

1:03:42 -

  • I haven't used CORE body temperature sensors in my studies, but I have used them in the field.
  • Temperature sensors could be an excellent alternative to other methods of temperature measurement.
  • I have compared temperature sensor measurements with rectal temperature measurements to establish a ratio.
  • Individual differences may impact how the heat is transmitted to the sensor.
  • Precise measurements are essential, so I would recommend comparing the sensor measurements with rectal measurements in the beginning to establish a more accurate ratio.

Concurrent training

1:04:30 -

  • The decision to incorporate strength training into your training program depends on the sport, athlete, and coach.
  • For runners, both plyometric and heavy strength training can provide benefits in terms of improving running economy and overall performance.
  • Heavy strength training has been shown to have performance benefits for cyclists, particularly in time trials.
  • Evidence for improved cycling economy through strength training is less clear but may be more apparent in a fatigued state.
  • I suggest a minimum of eight weeks for a training program but longer would be better.
  • The standard recommendation for training frequency is two sessions per week.
  • Aim for three to four sessions per week for the primary muscle group exercises.
  • High reps can be beneficial for muscle growth and power per kilo body mass. At the beginning of the preparatory period, start with higher repetition numbers and then gradually reduce them over time.
  • Lift close to your maximum when focusing on heavy strength, aiming for two to three reps in reserve or maybe four.
  • If you are an endurance athlete with no experience in strength training, I recommend starting with no reps in reserve, as you may have more strength potential than you realise.
  • Ensure that the lifting technique is correct, but I'm not that afraid of lifting heavy as long as that is correct.
  • The rest between sets would typically be two minutes, but there is an extensive range of what you can do.
  • Using one limb at a time during strength training can provide greater specificity for most sports.
  • Studies have shown that lifting with only one leg can allow us to lift more load than using both legs.
  • One-legged exercises such as squats can also reduce the risk of injury by distributing less load on the body.
  • It is recommended to have a mix of both one-legged and two-legged exercises in a training program.
  • In a Smith machine, you can efficiently perform one-legged leg presses and one-legged squats.
  • While studies have also shown benefits to using both legs during strength training, using one limb at a time can be more specific for exercises that imitate the running stride.

Four or five exercises for triathletes

1:13:10 -

  • I would have the one-leg press, one-leg squats like lunges, preferably in a Smith machine, the toe raises and explosive jumps, imitating the running pattern. 

Three pieces of advice to amateur athletes

1:14:42 -

  • Number one - choose training sessions that are sustainable and as enjoyable as possible. 
  • Number two - try to have a purpose for your training sessions and assign the sessions according to your purpose.
  • Number three - try to challenge your training habits and make some changes every third or fourth month.

Rapid fire questions

1:23:10 -

What's your favourite book or resource related to endurance sports?


What's an important habit you have benefited from athletically, professionally, or personally? 

Focusing on the possibilities that occur when things are not going as planned.

Who's somebody that you look up to or that has inspired you? 

My mentors kick-started my way into exercise physiology, and then now my wife.


Bernardo Gonçalves

Bernardo is a Portuguese elite cyclist and co-founder of SpeedEdge Performance, a company focused on optimising cycling and triathlon performance. He writes the shownotes for That Triathlon Show, and also produces social media content for each new episode.

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