“It is better to run a long way slowly rather than to curtail the mileage possible by running too fast”
- Arthur Lydiard
Lydiard is the guy on the right in the pic above. Much of Lydiard’s training theory consisted of going easier (and longer) in the short term so that you can go harder and faster in the long term.
There are few questions related to training that elicit as much confusion as the two antipolar inquiries:
“How easy is too easy?”
“How hard is too hard?”
Sports science has done just as much to add to the confusion as it has to definitively answer either of these questions. In this post, I want to present the primary considerations (from my perspective) that each individual athlete must contemplate in order to arrive at an informed answer to these two questions.
Consideration #1: Cardiovascular development
Several studies have investigated the minimal training intensity necessary to stimulate an improvement in VO2max in both de-conditioned and conditioned subjects, e.g. Hollman and Venrath (1963), Karvonen et al (1957), or more recently, Swain and Franklin (2002). While the definition of “conditioned subjects” in these studies probably doesn’t extend to the level of conditioning of you guys & gals, the overall trend is still significant. Basically, these researchers concluded that the level of intensity necessary to elicit an improvement in VO2max was correlated to the starting fitness level of the subject. Generally speaking, the range was 30-60% of VO2max (~50-70% of maximum heart rate) for individuals with VO2max values ranging from 30-45ml/kg/min – average values for healthy college aged males and females. Considering most of you are (hopefully) fitter than your average college freshman (‘freshman 15’ included :-), we can assume that ~70% max heart rate is the minimal threshold necessary for improvements in your cardiovascular system.
These numbers tie in nicely with the findings of one of the forefathers of exercise physiology, Per Olaf Astrand, when he investigated the exercise intensity necessary to elicit maximal stroke volume of the heart during activity. This concept is analogous to utilizing a load that will recruit the maximal number of muscle fibers when hitting the bench press without regard for the speed at which the load is lifted. Beyond a certain point, the heart is contracting as ‘strongly’ as it is able. Any further demands in Cardiac output are met by increasing the heart rate rather than increasing the force with which the heart contracts. In a similar way that the absolute load of the bench press is fundamentally more physiologically significant than the speed that you lift it, most of the cardiovascular benefit of aerobic training is reached once this maximal stroke volume is achieved.
Recently, studies have shown that the point at which this occurs is not a uniform 40% VO2max as Astrand suggested, but is rather a function of the individual’s fitness, with elite athletes exhibiting no plateau in stroke volume up to, and in some cases, beyond the anaerobic threshold (Gledhill et al. 1994). So, for the fitness range encompassing recreational athletes to elites, from a cardiovascular standpoint, the minimal intensity required to elicit improvements in VO2max can range from heart rates of ~70% max heart rate for folks of similar fitness levels to healthy college age subjects up to ~90% max heart rate in the case of elite athletes. This has important implications on the level of athlete that ‘needs’ speedwork, but that sounds like a topic for another blog.
Consideration #2: Substrate development
So, to improve our cardiovascular indices of fitness, the minimal required intensity can range anywhere between 50 and 90% of the individual’s maximal heart rate depending on their relative fitness. Now, what of our metabolic fitness? We know that in competitions that last longer than 60-90 minutes that glycogen depletion can be a limiting factor. We also know that athletes who oxidize a greater amount of energy from fatty acids as opposed to Carbohydrate for a given workload are able to ‘spare’ their carbohydrate stores and last longer before being forced to slow down. We also know that from a training perspective, using less Carbohydrate at rest and during recovery activity enables us to recover more quickly and thus accumulate more work at sport specific intensities. Clearly, improving our ability to oxidize fat is an objective that is at least as important as improving our mechanisms for Oxygen delivery.
So, what is the minimal intensity required to meet this objective?
The chart above illustrates the fat oxidation trends of two individuals, one fit male with a VO2max of 56ml/kg/min and one deconditioned female with a VO2max of 33ml/kg/min. The y axis of the chart represents the total fat burned per minute, while the x axis represents intensity via heart rate. Clearly, the range at which the male and female are burning the most fat is quite different (and quite narrow). For the female, the heart rates that elicit her highest fat oxidation rates are 102-122bpm, or 55-66% of her maximal heart rate. For the male, the 133-156bpm represents 73-86% of his maximum heart rate. Therefore, the training zone that maximally trains the ability to burn fat will be markedly different for both subjects. This is one of the reasons that training systems based on a % of any given point (maximal heart rate, HRR, FT etc) are pretty useless. They do not account for the huge inter-individual variability that we see in the real world. It is also worth noting just how narrow these bands are. In the male’s case, there is very little stimulus for fat oxidation occurring below a heart rate of 130bpm and after 156bpm, fat oxidation falls off the proverbial cliff.
So, again it seems that the question of how easy is too easy (in a sense of improving metabolic fitness) is also strongly related to the individual’s starting fitness level, which I want to point out is not always indicated by their fitness in a VO2max sense. We have witnessed very ‘VO2max fit’ individuals with very poor substrate profiles and vice versa. Generally, though, I would feel confident saying that the minimal intensity necessary to elicit an improvement in fat oxidation ranges from 50-80% of max heart rate. This is backed up by a great study by Gonzalez-Haro et al (2007) that found that maximal lipolytic power was found anywhere from 33-75% VO2max in accordance with the individual’s fitness.
The trick, of course, for you as an athlete, is to discover your own ‘sweet spot’ for both 'max fat-burning' and 'max fitness' zones. Using non-customized formulae such as the MAF formula or % of Heart Rate Reserve is honestly nothing better than a blindfolded stab in the dark. The only way for you to really know ‘how easy is too easy’ and ‘how hard is too hard’ to stimulate your desired training adaptations is to test, test, test.