Why low intensity training needs to be of low intensity

By: DR Dan Plews

Within triathlon training there is a huge importance around getting your zone 2 intensity right and ensuring that level 2 endurance training is at the correct physiological stress, i.e. below the first ventilatory (or aerobic) threshold. The reasons that this is so important are both from an adaptation perspective (i.e. ensuring we are making the correct physiological adaptations), but also from an overall physiological stress, which effects our ability to train consistently well. This article discusses why ensuring that your level 2 training is actually at level 2 could be the key to unlocking your ability to train consistently as you pursue your Long Distance Triathlon goals.

Indeed, one of the most important factors impacting the success of training for long-distance triathlon is intensity distribution and discipline, concepts which are covered in great detail in Endure IQ LDT102: Training Program Fundamentals for Long Distance Triathlon (available through Neuff Red in the UK). Clearly, we can’t push ourselves into the red zone in every training session, this is a recipe for overtraining, maladaptation, and poor performance. With that being said, the wealth of research demonstrating beneficial performance and physiological adaptations to well-structured high-intensity interval training tells us that we do need to go hard at certain points in our program. Navigating when to do this is the fundamental question facing long-distance triathletes and triathlon coaches.

A key research paper that provided useful results for those seeking answers to this question was led by Professor Stephen Seiler and published in Medicine and Science in Sports and Exercise back in 2007 (1). Based at the University of Agder in Norway, Stephen is a legend in our field and his research informs much of our practice in endurance training. In this particular paper, Stephen and his colleagues compared how long autonomic recovery took – or return to balance of the autonomic nervous system – following endurance training of differing intensity and duration in trained and highly trained endurance athletes. Altered autonomic nervous system activity has been shown to occur in response to the physiological stress evoked by endurance training, and is measured via heart rate variability – something that we have discussed previously and will describe in more detail in LDT 102 and down the track in LDT 104.


What did the study do?

Seiler and his colleagues had nine highly-trained runners (14 ± 3 training hours.week-1) and eight trained runners (7 ± 1 training hours.week-1) volunteer for the study, which involved performing training sessions of varying intensity and duration. The highly-trained runners performed 60 and 120-minute sessions below their aerobic threshold, a 60-min threshold-type session in which 30 min was performed between the aerobic and anaerobic thresholds, and a 60-min high-intensity interval training session in which six 3-min repetitions well above their anaerobic threshold was performed, with 2 min of recovery between reps. The trained runners performed the interval training session. Heart rate variability was recorded serially during the recovery period to identify when the athletes returned to autonomic balance following each session.


What did the study find?

The results of this study were clear. For the training sessions that were performed below the aerobic threshold, whether they were 60 or 120 minutes, autonomic balance was restored in just 5-10 minutes. This in itself provides strong justification for the significance of the aerobic threshold as a concept; this is the intensity that really is physiologically of low intensity and elicits very little in the way of autonomic disturbance.

In contrast, the threshold and interval-type sessions disturbed autonomic balance such that heart rate variability was not recovered for at least 30 min. This shows that the degree of autonomic stress for these types of sessions – i.e. between the aerobic and anaerobic thresholds or above the anaerobic threshold – was significantly greater than when training is performed below the aerobic threshold. What’s more, recovery of heart rate variability was significantly slower in the 'lesser trained’ athletes compared to those who were ‘highly-trained’.

It is important to note that these data do not show that these athletes were completely recovered and good to go again 30 minutes after their threshold and interval workouts. Not at all. The time-course of recovery from a physiological perspective is defined by the kinetics of many other physiological attributes (e.g. catecholamines, excess post oxygen consumption (EPOC), glycogen re-synthesis, autonomic balance, cortisol homeostasis and muscle soreness, which are discussed in greater detail in LDT102). What this shows, is that the stress evoked on our central nervous system by training varies with the intensity of the session, and that the aerobic threshold defines the intensity at which we start to see measurable autonomic disturbance.


Figure 1. A figure from LDT102: Training Program Fundamentals showing recovery against time after a training session


What does this mean? Lessons for training programming

For us as long-distance triathletes, this study helps justify two things: (i) keeping low-intensity training exactly that – low intensity, and (ii) the importance of the aerobic threshold as a concept for endurance athletes. The data described in this study neatly shows that the aerobic threshold defines the highest intensity at which essentially minimal autonomic disturbance is induced, with training performed above this intensity resulting in disruption to cardiac autonomic balance. 

This means that low-intensity training, that is truly of low intensity or below the aerobic threshold, should form the majority of our overall training load and all of the training between specific ‘key’ high-intensity workouts (in LDT 102 it explores how this information should be used to structure specific training weeks). This is because training sessions performed below the aerobic threshold are not physiologically stressful to a degree that is going to hamper recovery and readiness to perform those key sessions later in the week. Chronically training above the aerobic threshold, a common mistake amongst long-distance triathletes, impairs our ability to go hard in our high-intensity workouts. The result is non-functional overreaching and overtraining on just a few hours of training each week. 

Easy should be easy, so that hard can be hard! 



  1. Seiler SHaugen OKuffel E. Autonomic recovery after exercise in trained athletes: Intensity and duration effects. Med Sci Sports Exerc 39: 1366–1373, 2007.