I remember my dad asking me, driving on our way to high-school cross-country practice, why we had so many “easy” days. Why were only a few training days of the week spent really pushing ourselves, while the remainder of the week spent seemingly putzing around at a conversational jog. It was never something I’d really considered. We both grew up playing ball-sports, where it seemed every practice had the potential of sending you over to the sidelines gasping for air or heaving in the nearest trash-can. Running, however, was different, as it involved a delicate balance of training intensity, duration, and frequency. So - one must ask, is the distribution of hard days and light days really a self-organized optimum for endurance athletes, or a product of tradition and/or superstition?1 And, why can’t we train hard all the time?
Stephen Seiler’s review(1) looking at training intensity and duration distribution in endurance athletes helps us tie what research is available to the mechanisms that might be at play. His paper demonstrated that the training characteristics of nationally or internationally competitive endurance athletes seem to converge on a typical intensity distribution in which 80% of the training sessions are performed at low-intensity (~ less than 80% VO2max), whereas 20% were spent at high-intensity (~ greater than 80% VO2max).
From a macroscopic view, we can use Banister’s(2) (sorry, not Roger Bannister) “Fitness-Fatigue” model to see why this might be the case. Training (intensity, duration) and recovery (rest interval, nutrition) variables interact to induce both fitness (i.e. positive physiological adaptations), and fatigue (i.e. stress response and associated negative health outcomes) affects. The fitness after-effect presents as dull in magnitude, but long in duration, and manifests as a long-term improvement in performance, while the fatigue after-effect is initially large in magnitude with a brief duration. The model may be best illustrated by the figure below, showing that as the duration of the fitness after-effect is longer than the fatigue after-effect, a period of recovery allows the fatigue to diminish while fitness remains high. Conversely, stressful periods of training without sufficient recovery results in an accumulation of fatigue and an increase in the systemic or main fatigue after-effect.
If you have ever trained for an endurance event, you don’t need a theoretical model to tell you that 7 days of high-intensity workouts in a row isn’t a good idea. This is because our bodies need time to recovery so that we can maximize the benefits from our most recent hard session. Think of endurance training, light days and hard days, simply as a stimulus for adaptation.
More microscopically, the magical 80-20 intensity distribution strategy may be ideal for optimization of cellular adaptive signals and stress responses within the body. Positive adaptations with endurance training include higher mitochondrial density, higher capillary density, greater enzyme density/function, and more, which help improve VO2 among other markers and functionally improve performance(3). Cellular signaling studies are continuing to emerge, but some key takeaways might include that: 1) exercise duration and intensity can drive gene expression for positive adaptations, and 2) ceiling effects for signal amplitude are seen rapidly with repeated high-intensity interval exercise, whereas increased frequency at low-intensity may provide greater scope for expansion of the total signal for gene expression (greater overall effect)(1).
It has also been found that recovery within the autonomic nervous system (measured via heart rate variability) is very rapid after training bouts of 60% VO2max (low intensity), but becomes markedly delayed when exercise increases to a threshold or higher intensity4. Heart rate variability is a non-invasive, practical, and reproducible measure of autonomic nervous system function5, and is gaining traction in the training and medical community as a valuable marker for objective individual recovery. Fluck et al6 sums it up well: “Changes in physiological capacity over time are hypothesized to be the net result of transient increases in gene expression during recovery from repeated bouts of exercise.”
So - the body needs “lighter” recovery days to maximize the positive fitness effects that occur with our “heavier” training. This balancing act of exercise intensity, duration, frequency, and internal individual factors (life stress, work stress, nutrition, sleep habits, etc), is what makes coaching such an art. Like most things, the traditional model of 80-20 intensity distribution is something coaches have known forever, it’s just now that research is starting to catch up.
Some original content.