Endurance training: the low-intensity paradox:

Unanswered paradox

Given we know that elite endurance athletes prioritize most of their training in the low-intensity zone there’s a paradox that needs answering. Simple logic suggests that training this way should be pointless for elite athletes as low-intensity exercise does not challenge the homeostasis or cardiopulmonary system (ie create a training stimulus) in these high-level athletes sufficiently large enough to produce performance gains comparable to those from moderate- or high-intensity exercise. This logic also suggests that these athletes would be better off spending much more time training at high intensities. But the facts don’t lie; the vast majority of elite athletes spend the bulk of their time training in zone 1 and end up on the podium! The question is why? Assuming that modern sports training has evolved over many decades and has been constantly refined and optimized to deliver maximum gains, what is the purpose of zone-1 training, and why do athletes choose to do so much of it?

Looking for answers

Amazingly, the answer to this fundamental question is not really known! Elite/successful athletes perform large volumes of low-intensity training because it works, even if it is unclear why. However, even though we don’t have definitive answers, there are a number of different theories around that might explain this paradox. And while it’s true that these theories arise from research on (mainly) recreational athletes rather than elites, they offer the best insight as to why high-volume, low-intensity training might enhance elite athlete performance.

In a newly published review study from Finland, researchers have gathered together these theories and looked at why/how they might explain this low-intensity paradox⁽⁸⁾. Published in the European Journal of Applied Physiology, this study has investigated seven hypotheses that could explain the low-intensity paradox. Each of these hypotheses by itself provides a rationale for high-volume, low-intensity training, but of course, it may well be that more than one, or several, operate in tandem to deliver training benefits. Let’s see what these seven hypotheses are (NB: in the following descriptions, ‘LI’ = low intensity, ‘HI’ = high-intensity, ‘exercise’ = training session).

Hypothesis #1 – Maintenance or slight improvements to performance without accumulating stress

For most elite athletes, complete cardiac recovery from high-intensity training session takes two or more days, which limits athletes to two or three at most weekly high quality HI sessions per week⁽⁹⁾. Meanwhile, cardiac recovery from LI exercise takes place within a matter of hours or a day at most⁽¹⁰⁾. Therefore, scheduling in lots of LI work in between HI sessions, is not only possible but desirable, as some research suggests that it may also contribute to the process of performance enhancement, while still allowing recovery from HI sessions⁽¹¹⁾. In summary, LI training might maintain, or modestly improve, performance while allowing recovery from HI exercise.

Hypothesis #2 - LI training is an alternative method for molecular adaptation signals

At the muscle cell level, some endurance adaptations, such as mitochondrial biogenesis (synthesis of the mitochondria energy factories that power aerobic metabolism), are thought to be triggered by signalling/regulating molecules such as ‘PGC-1α’. PGC-1α is believed to be activated through various means, including free radicals during exercise, changes in hormonal balance, and increased metabolic stress caused by energy deficiency, glucose deficiency, or elevated fatty acid concentrations⁽¹²⁾. It might be that both LI and HI exercise activate PGC-1 but through different pathways – in particular that LI exercise activates PGC-1α via fatty acid elevation in the blood during prolonged LI sessions. In other words, LI training might provide an alternative/second adaptation pathway and, in doing so, introduce variability and a ‘two-pronged approach’ into the training adaptation process.

Hypothesis #3 - LI training enables long-term structural remodeling

Long-term cross-sectional studies show that five or more years of consistent endurance training leads to greater physiological adaptations in athletes compared to two years or less⁽¹³⁾. These differences include improved oxygen utilization at the muscle cell level and higher maximal oxygen uptake (VO2max) values. An analysis of over 150,000 marathoners found that while weekly absolute medium-intensity and HI training times remained relatively stable regardless of marathon performance, high-level performing runners had by far the greatest LI training time⁽¹⁴⁾. Thus, it is possible that consistent high-volume LI training results in structural adaptations that separate champions from amateurs (although the effects of genetics, and sufficient nutrition and rest cannot be ruled out). The ‘structural hypothesis’ is not a new proposition; some eminent scientists have suggested LI training predisposes athletes to additional adaptations above and beyond those gained from HI exercise⁽¹⁵⁾. The long-term structural changes that might be brought about by LI training include:

·        Gradual remodeling of the structure and function of the left ventricle of the heart (a phenomenon observed through the careers of professional cyclists⁽¹⁶⁾. It is also proposed that high-volume LI training gradually helps to stretch the pericardium surrounding the heart, allowing the heart to enlarge (and increase its output) over time.

·        High-volume LI training promotes the repeated stretch and shear of blood capillaries, which could play a role in promoting capillary formation over years.

·        A slow shift in muscle composition over many years to primarily slow-twitch (endurance) fibers. Supporting this, an increased proportion of slow-twitch fibers has been observed in cross-country skiers after several years of training⁽¹⁷⁾.

·        It has been suggested that HI training could improve the efficiency of individual mitochondria to produce energy, while high-volume LI training could be linked to an increase in total mitochondrial mass⁽¹⁸⁾. Thus, HI and LI training may target different aspects of mitochondrial biogenesis.

Hypothesis #4 - LI training affects something that has not yet been measured

Good endurance performance relies on multiple physiological components, including fat oxidation capacity, threshold intensities, oxygen kinetics, movement economy, durability, and VO2max⁽¹⁹⁾. It could be that LI training affects one of these components in a way that is not fully appreciated. For example, a high volume of LI training has recently been associated with good durability (see this article)⁽²⁰⁾, and a high volume of LI training could be involved in improving the ability to recover more swiftly after an exercise session, and also to maintain metabolic and physiological balance during in exercise⁽²¹⁾. In summary, years of dedicated LI training may be involved in improving components of endurance that are not yet fully understood or clearly defined.

Hypothesis #5 - LI training is needed psychologically

In day-to-day terms, LI exercise had been found to improve mood and mental well being in both untrained and trained individuals, while HI exercise decreases it⁽²²⁾. Moreover, in athletes, short-term LI training decreases mental fatigue⁽²³⁾, while a block of HI training increases distress⁽²⁴⁾ In short, emphasizing too much moderate- or HI training might simply be psychologically too demanding for athletes, leading to staleness and burn out.

Hypothesis #6 - LI training strengthens HI training adaptations

Combining a high total volume of low-intensity physical activity with HI training appears to be more effective than HI training alone in untrained⁽²⁵⁾ and moderately-trained individuals⁽²⁶⁾. In other words, when you add LI training to a HI training program, the gains from that HI training are greater than HI alone. Why should this be the case? One hypothesis in this context is that physical activity in the form of LI training within an overall program helps the body ‘not to resist’ the training adaptation process from HI sessions. In effect, LI training might be able to potentiate the training benefits if HI sessions.

Hypothesis #7 - LI training is replaceable

The final hypothesis is that while athletes tend to rely on large volumes of LI training (and it produces good results), there might not be a strict necessity to go down the high-volume of LI route at all. It could be that carefully planned high-intensity training could be an equally feasible training method. Obviously, a concern with this approach is the increased risk of overtraining syndrome and a higher injury rate. However, it might be that with sufficient training dose control and recovery days and weeks, overtraining syndrome could be avoided. Also, a high-volume, LI approach involves more repetitions of limb movements, which also increases the risk of overuse injuries⁽²⁷⁾. Therefore, we cannot unequivocally state that intensity or volume is a more detrimental stressor for injury. If it turns out that future studies show LI training can be replaced by more intensified training, it would suggest that top-level athletic performance depends not on the large volume of LI training, but rather on a large volume of endurance training, regardless of intensity.

Practical implications for athletes

Unlike an experimental study, this review into the use of high-volume, LI training to promote maximal endurance performance is more philosophical and theoretical in nature. However, these hypotheses are underpinned to various degrees by previous scientific findings, which mean they likely at least partly explain the benefits of LI training. In particular, there’s good evidence for LI training-induced adaptations for molecular signalling in muscle, the long-term structural remodeling (particularly in heart muscle), and the psychological benefits. Some of the other mechanisms discussed may also come into play such as the improvement in durability (albeit the evidence is weaker), and it’s likely that a blend of these factors contribute to the overall benefits.

The key take home message for endurance athletes is that while the LI training paradox is authentic, in real life with real athletes, high-volume LI training works, producing excellent results – providing there is at least some HI training such as interval or race pace sessions. Moreover, as Andrew Sheaff explained in his recent article on the key training principles used by successful coaches of elite athletes, one of the foundational principles was a focus on accumulating volume early in the preparation period. While interval training was present through the training year (although only 2-3 of sessions per week at most), the accumulation of low-intensity work was considered absolutely critical. These then are the principles you should use to underpin your training program. If you are unsure how to go about structuring such a program, this article provides a very good starting point!

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