The benefits of different aspects of aerobic training

In his comments on my post on 10th Jan, Ewen has raised two interesting points. First, is running at the lower aerobic zone more useful for runners than use of the elliptical cross-trainer, and secondly, are runs of differing intensity in the aerobic zone of similar value if they involve a similar amount of total work. In his words: ‘If the total number of heart-beats in the session above ’standing HR’ are the same, then the runs have the same value. In other words, a run of 60 minutes @ AHR 80% might be equivalent to a slow run of 75 minutes @ AHR 70%?’

I think the answer to the question about the merits of elliptical cross training depends on the answer to the second question about the merits of differing intensities of aerobic training, so for now I will offer my thoughts about the second question and return to the issue of the elliptical later in the week.

It seems clear that cardiovascular fitness (i.e. the ability to run at a given speed in the aerobic range at the lowest possible heart rate) improves whenever there is a challenge to the relevant physiological mechanism, and for most runners, either a lot of running at the lower end of the aerobic range or a lesser amount of running nearer the upper end of the aerobic range will result in improvement, provided it does not result in injury. However, the question of which will be of greater benefit to a particular runner at a particular time in his/her training program probably depends on which physiological mechanism is most in need of improvement at that time.

Ewen’s question explicitly addresses the equivalence of runs within the middle of the aerobic range, but as I understand it, there is a gradual transition in the nature of the training benefits across the entire aerobic range, so I think that is best to approach his question by considering the benefits of training in each part of the aerobic spectrum.

As far as I understand from my understanding of physiology, the aim of aerobic training is to produce in improvement in the following mechanisms or systems:

1) Increase in the number or diameter of capillaries that deliver blood to muscle. More capillary capacity will allow extraction of a higher proportion of the oxygen and glucose from blood and hence will allow a lower heart rate for a given speed. I suspect that any training that demands increased blood supply to muscle will lead to increased capillary capacity. Hence any aerobic training is probably beneficial in this regard, with the amount of benefit proportional to the total requirement (i.e twice a much time to achieve comparable benefit at half the heart rate). However, in general there is a lower risk of injury at lower speeds. This type of anatomical adaptation is likely to develop relatively slowly and the benefits might also be sustained over a long period.

2) Increased ability to utilize fats and spare glucose. The relative proportion of consumption of fats to glucose is higher at the lower end of the aerobic range and hence, running at the lower end of the range will be expected to induce the activity of the enzymes that metabolise fats. As far as I understand it, these enzymes get switched off as the lactate threshold is approached, so ability to burn fat will not be developed and might perhaps deteriorate if too much of the training load is near (or above) the lactate threshold.

3) Increased number of mitochondria (the organelles in which glucose is oxidized producing carbon dioxide and water and generating energy) within muscle fibres, thereby resulting in more effective aerobic glucose metabolism. A study by Dudley (‘Influence of Exercise Intensity and Duration on Biochemical Adaptations in Skeletal Muscle,’ Journal of Applied Physiology, vol. 53, pp. 844-850, 1982) demonstrated, in rats, that running at relatively low speed increased mitochondria in the slow twitch fibres while running at faster speeds resulted in an increase in mitochondria in fast twitch fibres. However, running fast produced a worthwhile increase in mitochondria even in the slow fibres, so overall, fast running is beneficial for both fast and slow twitch fibres. Possibly the most effective way to increase mitochondria across the range of fibre types is sustained running at highest level that can be maintained comfortably for long periods. This is likely to be at the border between mid and upper aerobic zones

4) Increased ability to metabolise lactate (either in the muscles themselves or in other tissues). Even in the mid-aerobic range, some lactate is produced. Increased ability to use this lactate as fuel will improve efficiency. Lactate production rises rapidly towards the upper end of the aerobic range, so running at the upper end of the range is likely to be most effective at inducing the enzymes that metabolise lactate.

Overall, these considerations suggest that both lower aerobic and upper aerobic running are beneficial but there are some differences in the types of improvement they produce In general, an effective training program will include both.

As a non-specific preparation for training for any distance, it is probably useful to have spent several years training regularly at an easy pace to enhance capillary development in a non-stressful way. It is probable that this adaptation will be maintained whatever training is routine is adopted subsequently, provided the amount of training is adequate.

Whether or not you are training for a marathon or 1500m, it is important to develop mitochondrial density. On the basis of Dudley’s studies of the effects of different work–rates on different types of fibres in rats, I suspect that the greatest overall increase in numbers of mitochondria can be achieved by sustained running around the border between mid and upper aerobic zones. Therefore, in the pre-season, I think that quite a lot of running should be done at this speed. In my own experience, I have found that this is helpful.

A marathon runner also needs to develop the ability to metabolise fats, and long runs at a relatively slow pace are probably the best way to do this. However, when racing, a competitive marathon runner maintains a pace in the upper part of the aerobic zone, where appreciable lactate is generated, so it is also crucial to be able to utilize lactate. The ability to do this can probably be best achieved by also including a substantial amount of training at lactate threshold. Although the goals of developing the ability to metabolise fats and also the ability to metabolise lactate require different strategies, I am not aware of clear evidence that one goal seriously interferes with the other, so a balance of the two types of training is required. Certainly I have mingled both strategies in the past, though whether or not I might have done better to separate them, I do not know.

If you are training for 1500m, as the racing season approaches it is essential to maximise the ability to handle lactate, as well as enhancing the anaerobic metabolism and developing the neuromuscular coordination required to run fast, so the emphasis in training will clearly shift to higher intensity running. Furthermore, there is some evidence that inclusion of too much low intensity work at this time might actually have the harmful effect of developing slow twitch fibres at the expense of fast twitch fibres

So I think this all adds up to the conclusion that in general a training program should include both lower and upper aerobic sessions, as these two types of training do not have identical benefits even after adjusting for total workload. However a degree of periodization of effort is required depending on the target race distance.

The doctrine of training specificity has only limited validity. I think that Emil Zatopek’s rhetorical question: ‘why should I train slowly if I want to run fast?’ is misleading. Training only at fast speeds is likely to result of too much damage to muscle fibres and in the release of too many stress hormones such as cortisol, which ultimately damages body tissues. Maybe Zatopek had a unique capacity to cope with training stress, but is it interesting to speculate whether or not he would have been even more successful if he had adopted a more flexible approach to training. His PB for the marathon was 2 hours 23 minutes– that was good enough to win the Olympic gold in Helsinki in 1952, and to set a new world record. However, in the preparation for the Melbourne Olympics in 1956 he suffered a serious groin strain and despite subsequent heroic effort, finished sixth in the marathon behind Alain Mimoun.

These thoughts go somewhat beyond the scope of Ewen’s specific question about the middle part of the aerobic range. If we set aside the issue of the specific benefits of lower aerobic sessions for developing fat utilization and upper aerobic sessions for developing the ability to utilize lactate, and consider only the other two major goals: increasing capillary capacity and increasing mitochondria; I think that it probably is fairly true to say that ‘If the total number of heart-beats in the session above ’standing HR’ are the same, then the runs have the same value’.


One Response to “The benefits of different aspects of aerobic training”

  1. Ewen Says:

    Canute, that’s more than I expected! A great overview of aerobic and lactate conditioning.

    Your point about capillary adaptation being maintained after introducing other types of training is comforting. I often wondered if there might be a deterioration.

    It’s also interesting about low intensity training developing slow-twitch fibre strength to the detriment of fast-twitch. This is what I found during the early part of ‘Hadd training’. I could no longer run ‘fast’ over short distances, which I think has relevance to how fast I can run over longer distances.

    In practical terms, I should try to include a range of runs between lower and upper aerobic intensities. What I’ve been doing is (on a non-specific day) starting a run at lower aerobic (easy) pace for a kilometre or two, then gradually increasing the pace if the legs are agreeable, or otherwise staying in the lower aerobic zone.

    I agree with your view on Zatopek. I don’t think the 2:06 marathoners of today do all their running ‘fast’. Some running (recovery runs) would be relatively slow.

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