The grey zone

In recent posts I have discussed the evidence regarding the relative merits of high intensity training and high volume training.  The evidence demonstrates that both approaches are effective for increasing aerobic capacity.  Both will develop capillaries and mitochondria in heart and skeletal muscles.  High intensity training allows a more efficient use of time, which might be the decisive feature for an amateur athlete with family and work commitments, though in general, high intensity work-outs are tougher and require greater determination. 

Potential benefits of mixing high and low intensity

Furthermore the two approaches are each attended by risk of injury or over-training.  However it is probable that the risk of injury and over-training associated with the two intensities of training arise from somewhat different stresses on the body.  High intensity training produces greater forces on the musculo-skeletal system, but also produces greater increases in anabolic hormones, whereas high volume training produces greater increases in catabolic steroids such as cortisol, which can promote destruction of muscle if excessive.  On the other hand, there is relatively little evidence suggesting that a high intensity session undoes the benefits of a preceding low intensity session or vice versa.  Therefore, my provisional conclusion is that the best approach is a program that includes a mixture of high intensity and low intensity (high volume) sessions, on the grounds that such an approach is likely to achieve a good balance between stimulation of catabolic and anabolic hormones. This remains unproven but is at least plausible.

What about the mid-zone?

However, this leaves unanswered the question of whether or not training sessions that fall in the mid-zone between high intensity and low intensity (but high volume) are of much value.  Surveys have shown that elite athletes tend to avoid the ‘grey’ zone between high intensity and low intensity,  For example, the study by Seiler and Kieland found that elite skiers do about 75% of their training at low intensity and 15-20% at high intensity with only around 5-10% in the mid intensity range  (Scand J Med Sci Sports. 16(1):49-56, 2006).  Previously the French exercise physiologist, Veronique Billat, had reported similar proportions of time in the different training zones in marathon runners (Med. Sci. Sports Exerc. 33:2089–2097. 2001)

The Esteve-Lanao study

It was therefore with great interest that I read a report on a study by Jonathan Esteve-Lanao and colleagues (Journal of Strength and Conditioning Research, 21(3), 943–949, 2007), which had been pointed out to me by Simon in a comment on my post comparing high and low intensity training on October 25th.   Esteve-Lanoa is a coach in Madrid who had previously published findings demonstrating that the proportion of training time spent in the low intensity zone was a strong predictor of race performance (Med Sci Sports Exerc. 37(3):496-504, 2005.) 

In the more recent study published in 2007, he allocated a group of well-trained sub-elite athletes to one of two programs for a period of 5 months: one program included more low intensity training than usual for this group of athletes; the other involved less low intensity training and more mid-intensity training.  Both groups performed the same amount of high intensity training.  Intensity was assessed according to proportion of training time with heart rate within low, mid and high intensity zones defined relative to the ventilatory threshold (VT) and respiratory compensation threshold (RCT). 

Defining the zones:  VT is the first appreciable step in respiratory effort as work load increases, and corresponds to the threshold described as VT1 by some authors (eg Dekerle and colleagues) or aerobic threshold by others.  It is the highest work rate that can be fueled virtually entirely via aerobic metabolism. At higher work rates, appreciable anaerobic metabolism occurs, and lactate level rise appreciably above resting values.  The increased acidity in the blood causes the noticeable rise in respiratory effort necessary to remove more carbon dioxide, thereby compensating for the lactic acid by reducing the acidity due to dissolved carbon dioxide.   Lactate is removed by organs such as liver and heart which can use lactate to generate energy.   As work rate rises further, a stage is reached beyond which the body cannot remove the lactate as fast as it is produced.  At this stage lactate level and acidity rises rapidly.  There is a major drive to increase respiratory effort in an attempt to compensate for the acidity.  This is the respiratory compensation threshold (RCT), which is also known as VT2 or anaerobic threshold.   Thus, in the study by Esteve-Lanao, low intensity training at a heart rate below that corresponding to VT is virtually entirely aerobic.  In the mid-intensity zone between VT and RCT, training involves some anaerobic metabolism but the body can cope with the rate of lactate production and lactate level remains only a little above resting value.  In the high intensity zone above RCT, a large proportion of the energy is generated by anaerobic metabolism and the level of lactate rises to high levels, compelling the athlete to make a very strong respiratory effort in an attempt to deal with the rising acidity. 

The results:  In the group assigned to an increased amount of low intensity training, the amounts of times in heart rate zones corresponding to low, mid and high intensity training were in the ratio 80:12:8 (i.e. 80% low intensity).  For the group assigned to decreased low zone training, the ratio was 67:25:8  (i.e. 13% less in the low zone, replaced by 13% more in the mid-zone).  In both groups VT occurred at approximately 67-68% of VO2max, while RCT occurred at approximately 87-88% of VO2max.  The outcome of training was assessed by comparing time recorded in a 10.4Km cross country race before and after the 5 month period of training.  The group who had HR in the low zone for 80% of the time improved by 7.5% while the group who had HR in the low zone for only 67% of training time, improved by 5%.  The difference between groups in amount of improvement was statistically significant.  It should be noted that the individuals with HR in the low zone for 80% of the time spent more hours training (average 100 hours over 18 weeks compared with average 75 hours in the group with HR in the low zone for 67% of the time), but the total load (volume x intensity) was similar in the two groups. The authors conclude: ‘These results provide experimental evidence supporting the value of a relatively large percentage of low-intensity training over a long period, provided that the contribution of high-intensity training remains sufficient.’

It is crucial to note that even the group with 80% low intensity training performed a substantial amount of demanding work. They did 2 intense sessions per week in many of the weeks, in addition to regular weight training, and also running 5 cross country races (2 x 5Km and 3 x 10Km) not including the final assessment. Nonetheless, I think the practical conclusion is that training in the mid-zone produces less benefit that a similar work load (volume x intensity) in the low zone provided the program includes sufficient high intensity work.

It should also be noted that the amount of time spent with heart rate corresponding to the mid-zone intensity does not correspond exactly to the amount of time spent working in this zone.  Typically in an interval session, it takes from 1 to 2 minutes for HR to reach the high intensity zone (due to buffering by myoglobin) during high intensity epochs, and it takes 1 to 2 minutes before HR falls back to low zone level during the recovery.  When I do 1 Km intervals, my heart rate is in the mid-zone for more than half of the session despite the fact that I do not spend any time running at a mid-zone pace.   To achieve 8% of training time in the high intensity zone typically would require about 15% of sessions to be high intensity sessions.   

On the whole this appears to be a sensibly designed and well executed study.  The main concern I would raise is the fact that Esteve-Lanao’s previous study published in 2005, in which he found that the amount of low intensity training predicted race performance, might have inclined him to have greater faith in low intensity training.  He coached all the athletes in the study reported in 2007.  There is evidence that having faith in a particular training program influences the physiological benefits from that program.  It is therefore possible that coach’s confidence in the low intensity program influenced the outcome.   It is because of unconscious influences of this sort that double-blindness is regarded as so important in trials of medical treatments.  Nonetheless, despite this potential short-coming, I consider that this study provides moderately strong support for the proposal that mid-aerobic ‘grey zone training’ is a limited value.


6 Responses to “The grey zone”

  1. Ewen Says:

    Canute, thanks for such a good summary of the study. I take it that both groups were running the same volume (kms per week), just that those with 80% low intensity were taking more time to complete the volume?

    Also, does the VT (low intensity) training include running at paces much slower than VT? For example, I’d presume VT pace would be about marathon race-pace and HR (99% aerobic), which doesn’t seem THAT easy to me. For example, 3:00 per km for a 2:07 marathoner.

    Personally, I’m inclined to favour this type of training (big low, but some high), as there’s not the risk of ‘mechanical injury’ from the high intensity part, or the ‘pressure’ of running a large proportion of runs at mid-intensity (grey).

    What I’d like to know, is how much high intensity is needed (for improvement), and how often? For example, would one session every 4 or 5 days (much less than 8%) be sufficient? Would higher weekly volume permit a smaller % of high intensity training? Similarly, would, say, 10% high intensity compensate for lower weekly volume. I’m guessing the answers to these questions are that a program needs to be individualised for athletes with different strengths and preferences.

  2. RICK Says:

    I agree that low level aerobic training for as many hours as possicle with two hard sessions a week seems to give best results.
    I remember Peter Keen [Chris Boardman#s sports science coach] saying that low level aerobic training such as club runs was junk miles, but he had to eat his words when Chris turned pro in Europe.
    The result of training like the pro’s [ a lot more volume at easy paces]was he smashed the world hour record.
    Peter Keen had always recommended doing the bulk of your training at what he called level two [ 75-80% of max hr] replacing long rides 3 – 7 hours with 2 hour rides at level 2. I found it never really worked for me and I always raced better if i’d done a winter of long sunday club runs!
    Looking through my old running logs I can see my best periods have come from lots of easy running with 2 hard sessions per week [ just as Pete Magill recommends] I once got up to 14 hours per week but 9- 10 hours has produced very good results.
    what i am coming to understand is most people run there aerobic runs way to fast and end up tired and stale.
    I think Pete is right to say run on feel, leave the Garmin and hrmat home, only look at your watch after your run and never race these runs, run them relaxed.
    I read of many bloggers who chart and time and worry about their every heart beat/rate, if only they ran by feel and relaxed!
    no wonder they-me get tired and don’t make progress.
    thanks for the article, great stuff.

  3. canute1 Says:

    Both groups performed the same training load, determined by multiplying time x intensity. In other words, training load was calculated by multiplying length of time in zone 1 by 1, length of time in zone 2 by 2 and length of time in zone 3 by 3 and then adding these weighted times. As a result of this method of matching load, the low intensity group spent more time training (100 hours v 75 hours). The distance covered was similar in both groups (80-90Km/week), though unless the low intensity group ran much slower, it is probable that they covered a slightly greater distance in the additional time. Unfortunately Esteve –Lanao does not report whether or not the low intensity training was all near to VT, though during the 18 weeks of the main program they did 6 hours per week, suggesting an average pace of 4 min/Km. Since 80% of the training was low intensity, the average pace of the low intensity is likely to have been in the range 4:15 to 4:30 min/Km (approx 6:48 – 7:12 min per mile). For a sub-elite distance runner, 7 min per mile would be quite easy.
    As for your question of how much high intensity is required, in the Esteve-Lanoa study it was approximately 2 high intensity sessions/week + regular weight training + one race per month I agree with you that what is required probably depends on the individual . However I think there is good reason to propose that older athletes have a greater need to develop their fast twitch fibres. Mystery Coach recently sent me some interesting comments related to this issue. In the near future, I will post my thoughts on what older runners need to do to prevent deterioration of their fast twitch fibres.

  4. Ewen Says:

    Thanks Canute, I’ll look forward to that post. I’ve only been doing the drills and short hill sprints for a short time, but can feel some benefit in terms of stride-length and speed.

    Regarding Rick’s idea of leaving the Garmin and HRM at home – that’s probably a good way to learn how to run by ‘feel’. I find the HRM is most useful for easy/low aerobic runs – thus preventing them from becoming ‘grey’ runs. I can usually run the upper aerobic and threshold runs by feel.

  5. Simon Wegerif Says:

    Dear Canute
    One more, but perhaps all encompassing summary reference that aims to take an objective look at sports science research findings vs what elite athletes and their coaches have found successful in practice. This paper by Stephen Seiler has just been publishedin the Nov (13) issue, and is open for all to access at

  6. canute1 Says:

    Simon, Thanks. That is a great review. However with regard to opinions on the polarized question of intensity v volume, one needs to be cautious about using the adjective ‘objective’. While I am personally sympathetic to the conclusions that Seiler and Tonnessen draw, it is perhaps important to note that Stephen Seiler’s prior scientific studies might give him grounds for favoring low intensity training. He was a collaborator on the Esteve-Lanao study, and he had previously done a study of autonomic recovery following low, medium and high intensity sessions, in which the low intensity sessions were associated with the most rapid recovery. Thus his own previous research has provided him with sound reasons for favoring low intensity training. It should be borne in mind that others whose research has produced findings favoring high intensity training, tend to draw different conclusions when they review the evidence. As Ewen’s recent comment on the contrast of the training of Cheruiyot (280 Km/week, mainly low intensity) and Wanjiru (140Km per week; greater amount of high intensity) shows, even among elite runners there can be major differences in approach. The good news is that either high intensity or high volume can be effective. However the disconcerting fact is that either approach can also fail. So finding what are the essential ingredients for success in an individual case is still an interesting challenge. I like Seiler and Tonnessen’s quote of the comment by Dag Kaas: ‘you have to train smart and you have to train a lot’.

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