Continuous or interval endurance training?

Both high-intensity interval endurance training and continuous endurance training methods are effective in increasing cardio-respiratory and metabolic functions; as well as aerobic performance. Still, the question of which of the two methods induces greater physiological adaptations is a major issue; both for exercise scientists and endurance coaches. So, which is best? Continuous or interval endurance training?

Most scientific literature shows that low volume – high-intensity interval endurance training induces similar or greater physiological adaptations when compared with high volume – moderate-intensity continuous endurance training. Indeed, due to the nature of Interval endurance training and the intensity required, it seems to offer greater adaptations. But, wait a moment… is it really all about intensity?

Let’s put these two methods up for debate. 

VO2max improvements

VO2max is the maximum amount of oxygen an athlete can consume, during exercise. It’s a very important factor in aerobic capacity; and an index of good health. High-intensity interval endurance training is an effective method to improve aerobic capacity. That’s true for both athletic populations and populations that may suffer from malnutrition, or bad health (Smilios et al 2018). 

The efficiency of interval endurance training in enhancing a wide spectrum of aerobic power indicators, possibly stems from the cumulative metabolic stress. Specifically, as it’s imposed during each training session, on the oxygen delivery (central) and oxygen utilization (peripheral) systems. It is proposed that the total exercise time performed above a high percentage of maximal oxygen consumption and heart rate (HR), reflects the extent of aerobic stress; for example, 90% VO2max and maximum HR during aerobic interval endurance training session

Thus, exercising at high rates of VO2 (above 90% VO2max) imposes greater stress on central and peripheral components of oxygen consumption; and might be the most important stimulus towards the improvement of aerobic capacity. We usually execute continuous endurance training at lower intensities. So, the exercise time at a higher percentage of VO2max or HRmax is not promoted. 

Best for VO2max improvements: High-Intensity Interval Training

Lactate Threshold

During the last, nearly 60 years, the blood lactate curve and lactate thresholds (LTs) have become important in the diagnosis of endurance performance (Faude et al. 2009). Lactate threshold is of considerable importance, regarding the diagnosis, as well as the prediction of aerobic endurance performance. And it represents the point where we observe an abrupt increase in blood lactate. That’s because glycolytic concentration rate cannot be matched by aerobic pyruvate combustion. In the last decades, we also very often use LT to optimize the training zone of an athlete. 

Both continuous and interval endurance training can offer improvements to LT. The “secret” is to optimally combine intensity and duration, in a way such as to achieve training time around a threshold of 85-115% of LT. For example, you can either use an interval protocol (6-8 reps X 4min at 110% of LT with 3min rest between reps); or a continuous protocol (30min heavy continuous progressive training @ 90-100% of LT). The way improvement happens may be different at the molecular level. But the rate of improvement will practically be the same. 

Best for Lactate Threshold: Both methods

Running Economy

Running Economy (R.E.) is the energy required to run at a given speed; And it’s usually measured in kcal/min/km. The “forgotten” factor, as Foster and Lucia characterized R.E. in their research paper, in 2007, is a game-changer as the running distance becomes longer (>21km or race duration >2h). Especially for marathons, ultra-marathons, and triathlons, Running Economy probably has a greater impact on aerobic performance. Of course, the VO2max and lactate thresholds remain very important factors, as well. 

Many studies — such as Hoogkamer et al. 2017, Lee et al. 2020 — have come to confirm the importance of running the economy. Additionally, a paper by Myrkos et. al 2020 (under review) shows that R.E. is one of the key parameters regarding performance, when it comes to the Athens Marathon. It’s a well known fact that for an athlete to improve their R.E., they need a significant amount of training load, in excessive duration at low-to-medium intensity. Of course, this is diametrically opposite to HIIT (may improve R.E. under certain conditions). But, it’s an exceptional fit to the continuous endurance training method.  

Best for Running Economy: Continuous endurance training 

Mental preparation

There are not only physiological factors involved here. There are also psychological factors to consider. I’m probably not the right person to talk about the science of psychology. But I can state my opinion as an athlete and, mainly, as a coach. Mental preparation for a race is a difficult part in the game. And the way we train may affect it, quite a lot. I believe the High-intensity interval endurance training method prepares you mentally for shorter races. That’s because it can simulate the feeling of hard breathing and the feeling that kind of intensity provides.

On the other hand, continuous endurance training — and especially “long run” or “long ride” — can simulate longer events, such as marathons and Ironman triathlons. And that’s because they can help build the patience required for a long event. Especially, one that could last over 2.5 and up to 15 hours.

Best for Mental preparation: Both methods

Continuous vs Interval endurance training

You must have caught the drift by now; the score is: 2-2. You might consider the results inconclusive. But, here’s why we ended up with a tie.

One cannot describe training as a standardized protocol, but as a dynamic and complex procedure. We cannot choose one training method over any other, hoping we can use it throughout the entire training program effectively. Coaches need to periodize the training program as we’ve already discussed.

We have “invented” different methods to offer different adaptations. And if we know when, how, why and for whom to apply each training method, we can achieve the best results. That is, without causing any overtraining or monotony syndromes. 

Remember, coaching is a mix of science and art. We can use no one, singular method to fit all procedures and all goals. Think about it for a minute or two. And keep training!