Simplify the easy to maximise the tough
Endurance training involves the complex manipulation of training intensity, duration and frequency. Typically these variables are ‘periodised’ to allow for the optimal sequential development of requisite adaptations. The residuals thereof should also be designed to culminate or peak simultaneously at a target event. It is well accepted that there are three intensity-ranges or ‘zones’ that endurance athletes train. Long slow duration (LSD) training (Zone 1, < 65% VO2max) refers to conventional continuous moderate-intensity exercise performed over an extended distance or duration. Threshold (THR) training targets the intensity between the first and second lactate threshold (Zone 2, 65-85% VO2max). High-Intensity-Interval Training (HIIT), involves exercise repeats performed at intensities above the second lactate threshold (Zone 3, > 85% VO2max), interspersed with periods of low-intensity rest or active recovery. The optimal training intensity distribution or relative combination of these training modalities, however, has long been debated. This article explores what the literature suggests may best benefit your client achieve his or her next endurance goal.
Conventional LSD training induces central and peripheral physiological adaptations that improve exercise capacity. As described previously in detail, this includes plasma volume and stroke volume expansion, oxygen diffusion, transport and extraction efficiency, capillarisation, mitochondrial biogenesis and the efficiency of skeletal muscle metabolism. Since these are crucial to endurance performance, conventional wisdom dictates that more LSD is better and gaining a competitive advantage requires longer hours than your opponent. However, HIIT has recently become popular for enhancing endurance performance. This came about particularly in light of the research of Tabata et al (1996) and Gibala et al (2006)[3,4]. Both studies reported that short-term (2-week or 6-week respectively) interventions of all-out cycling performed for 8 x 20 seconds (with 10 seconds rest in-between sets) or 4-6 x 30 seconds (interspersed with 4 minutes recovery) elicited comparable muscle and performance adaptations to interventions using 60 minutes or 90-120 minutes of continuous moderate-intensity exercise. In the latter, this was despite the HIIT group performing 2.5 hours combined exercise compared to the 10.5 hours of the continuous group. Indeed, introducing short-term, low-volume blocks of HIIT improves exercise capacity and performance, skeletal muscle mitochondrial content and oxidative capacity, and metabolic efficiency in well-trained athletes, healthy active individuals as well as sedentary middle-aged men and women. Taken collectively, these findings suggest that HIIT is far more time-efficient at achieving the same benefits as training for hours at a moderate intensity. Then why bother with the latter?
Given its use by the world’s top athletes, polarised training has been studied for its efficacy in trained and recreational athletes compared to other intensity distributions. For example, in a randomised crossover design, 12 well-trained cyclists underwent 6 weeks of a polarised training-intensity distribution (POL, 80% zone 1, 20% zone 3) or threshold training distribution (THR, 57% zone 1, 43% zone 2), each following a 4-week detraining period. POL training elicited superior performance improvements and physiological adaptations (power at lactate threshold 2, peak power output and time to exhaustion) despite greater volume with THR. Similar findings have been reported in a randomised design comparing LSD, THR, HIIT and POL in a 9-week intervention in 48 well-trained endurance athletes. POL was superior to all other training modalities, including either LSD or HIIT alone. In recreational runners, a 10-week POL training intervention conferred significantly greater improvements in 10 km performance than THR training. These studies suggest that POL training is unique in its ability to confer optimal endurance training adaptation across broad populations.
The mechanisms behind the success of POL training are not fully understood. It has been suggested that it best mirrors the activity patterns of our ancestors and hence provides the optimal stimulus for adaptation. Introducing HIIT in untrained, moderately-trained, and well-trained individuals alike, provides a potent stimulus for central and peripheral endurance adaptations. This is likely due to the high stress load delivered by an unaccustomed HIIT session. HIIT disrupts homeostasis considerably more than LSD or THR training as individuals are required to push near or beyond their maximal capacities. This imposes a new demand on the body to adapt. HIIT is also theorised to improve acid buffering capacity, increase capillarisation and enhance recruitment of fast-twitch muscle fibres that are minimally recruited during LSD or THR training. The latter would lead to greater specific adaptations in these fibres and make them more fatigue-resistant. I would argue that incorporating HIIT elicits psychological adaptations in the sense of becoming more familiar with discomfort. In other words, given adequate recovery, HIIT increases physical and mental work capacity. However, HIIT studies have only been conducted in short intensive blocks, and it is unlikely that HIIT itself would be sustainable as a standalone form of endurance training.
Athletes recover fastest from LSD training sessions, whereas recovery (autonomic nervous system, muscle damage and perceived recovery) from zone 2 or zone 3 is delayed. As described by Seiler, too frequent THR or HIIT sessions (> 2 per week) may cause undue physiological stress, impair performance in subsequent training sessions, and increase the risk of over-training. Indeed, POL training provides both the stressful stimulus of HIIT to promote adaptation, and caters for adequate recovery between these sessions. This also means that the quality of HIIT sessions is improved. Furthermore, there appears to be a ceiling effect for repeated HIIT sessions on molecular adaptations in well-trained athletes, with no further benefit accrued from performing more than 2 or 3 HIIT sessions per week. In contrast, repeated LSD training continues to stimulate molecular adaptation; i.e. mitochondrial volume expansion, oxidative capacity and capillarisation, it facilitates faster recovery and may be performed more frequently (e.g. twice per day) to maximise endurance adaptations.
In this regard, THR training blocks have been found to cause increased levels of oxidative stress and sympathetic activation[1,9]. This may explain reports of THR training being ineffective in trained athletes. For example, during the 18 weeks preceding an Austrian Ironman event, training time spent in the THR zone was associated with poorer performance. It seems that THR training imposes a relatively high physiological stress, but it is not adequately ‘novel’ to promote further adaptation. Therefore, it makes sense that long easy sessions be kept very easy, and ‘key’ HIIT sessions are performed intensely with the intent to maximise performance and adaptation. This is important in working with recreational athletes: to ensure that easy sessions are not performed too hard and hard sessions are not performed below the target intensity. Indeed, if 80 / 20 works for the Kenyans, perhaps it is worth a try…
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