In endurance training, different training intensities are frequently discussed. Alongside very easy efforts and very intensive intervals, there is a range of medium intensity which, in training science, is sometimes referred to as Grey Zone Training.
This term mostly describes efforts in the range of about 75–85 % of maximum heart rate. In many training models this range lies between easy base sessions and intensive interval efforts.
It is interesting that precisely this intensity range is, in many training concepts, used rather sparingly or deliberately avoided. To understand why this is the case, it is worth taking a look at the physiological and training-science background.
Training intensity in endurance training
Training intensity describes how strongly the cardiovascular system is stressed during an effort. It is often defined via heart rate and via power zones.
In simplified terms, three broad intensity ranges can be distinguished in endurance training:
Low intensity
- easy continuous efforts
- stable heart rate
- long training duration possible
Medium intensity
- noticeably strenuous, but still controllable
- often in the range of the so-called threshold
High intensity
- very high load
- mostly sustainable only over shorter intervals
Grey Zone Training typically lies in the medium intensity range between easy and very intensive efforts.
Where the discussion around the “Grey Zone” comes from
The discussion around this intensity range is closely linked to the work of the Norwegian training researcher Stephen Seiler. In analyses of training data from various endurance athletes, it emerged that many successful athletes complete a large part of their training at low intensity, while a smaller part is very intensive.
This pattern was later often described as polarised training. In such training models it is recommended to use the medium intensity range – that is, Grey Zone Training – relatively sparingly.
Why this range is sometimes considered less favourable
One reason for this recommendation lies in the relationship between training stimulus and fatigue.
Efforts in the Grey Zone range lead to a considerably higher metabolic load than very easy sessions. At the same time, the intensity often stays below the ranges in which particularly strong adaptation stimuli on maximum performance capacity arise.
Some training scientists therefore argue that sessions in this range can cause relatively a lot of fatigue, without at the same time promoting the specific adaptations associated with very intensive intervals. At the same time, less training volume can often be completed in this intensity range than with easy efforts.
From this perspective, it is recommended to complete a large part of training either clearly in the easy range or deliberately in very intensive intervals.
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Physiological load in this intensity range
During efforts in the Grey Zone range, several physiological parameters rise markedly.
For example:
- heart rate and cardiac output increase
- energy demand rises
- the share of energy production from carbohydrates increases
The metabolic load is thus considerably higher than during easy sessions. At the same time, the load on the cardiovascular system is often still lower than during very intensive interval efforts.
It is precisely this intermediate range that makes the assessment of Grey Zone Training complex: the load is high enough to generate fatigue, but not always high enough to set the particularly strong adaptation stimuli that can be associated with very intensive efforts.
For this reason, in some training concepts – especially for amateur athletes – it is recommended to use this intensity range rather sparingly. The load can cause relatively a lot of fatigue, while at the same time less training volume is possible than with easy sessions.
When training in this range can make sense
Despite this discussion, this does not mean that Grey Zone Training should be avoided on principle. In many classic training concepts this intensity range long played a central role. Training in the range of the so-called lactate threshold was used, among other things, to improve the ability to sustain a relatively high load over a longer period.
Many training programmes, especially in competitive sport, additionally use this intensity range deliberately, for example in the case of:
- tempo runs
- longer intervals near the threshold
- competition preparations
Precisely in sports where competitions take place over a longer period at moderate to high intensity, this training range can play an important role. It often lies close to competition intensity, which is why a mental and psychological habituation to this load can also make sense.
In team sports, in turn, it can make sense, in certain training phases, to train in this intensity range in order to improve the so-called lactate tolerance. The ability to cope with elevated lactate concentrations is, in many of these sports, an important performance factor.
In such cases, training in this range can contribute to improving load tolerance at competition-near intensities.
Why training programmes often combine different intensities
Most modern training concepts combine different intensity ranges. Easy sessions enable a high training volume and promote fundamental adaptations of the cardiovascular system.
Very intensive intervals set strong stimuli on maximum performance capacity.
Sessions in the range of Grey Zone Training, by contrast, can help to tolerate efforts near competition intensity better.
Which combination makes sense depends, among other things, on the following factors:
- training goal
- sport
- training level
- individual load tolerance
Conclusion
The term Grey Zone Training describes an intensity range in endurance training, located between easy base sessions and very intensive efforts.
In training science, this range is in part discussed critically, especially in connection with the relationship between training stimulus and fatigue. At the same time, it is used deliberately in many training programmes when efforts near competition intensity are to be trained.
As with many training methods, this applies here too: what matters is less the single training session than the overall structure of the training programme. A balanced combination of different intensity ranges can help to address different adaptation mechanisms in the cardiovascular system.
