What do I do?
I establish the requirements of my event, versus my own fitness and from there work at defining some training guidelines. These guidelines vary over time as my fitness changes and/or I discover I need a different stimulus. In these guidelines I establish some heart rate zones. They are not set in stone, there is allowance for crossover of zones within sessions and they at times I will aim for different HR ranges.
My heart rate zones are as follows:
MAF: 152 (test in zone 150-154)
Aerobic: 132-154 (70-82%HRmax)
This is the range of the majority of my training. Often termed the grey zone and advised to be avoided, it is clear I disagree. While plenty of the same metabolic/physiologic adaptations such as increased mitochondrial density, increased aerobic enzymes and increased capiliarisation, can be achieved at the lower intensity, there more benefits up closer to 80%HRmax, which include better neuromuscular efficiency, enhanced fatigued resistance and specific strength in more muscle fibres. Quite simply, a slightly higher intensity, which still being in the so-called aerobic zone leads to more race specific improvements, and allows practice at faster running speeds.
It isn't an exact science, but I have found this works best for me. It takes me a lot more volume to get the same race benefit by training at or below 70%HRmax, and to put it bluntly I don't have, and don't want to spend that time running slow. For a lot of my training, working at this slightly higher rate does blunt the top end of faster work, once I add little bit of peaking, rest and tapering, that is more than made up for.
The pace of my Aerobic runs is highly variable. Mainly due to the fatigue load I am carrying from the other key sessions. For example the day after the Bluestone Classic 15km race, my heart rate sat at the lower end of the scale, but my legs definitely had no power from the race fatigue. As a result I was for about 50 minutes at an average pace of only 6:50/km. The following day, I covered 13km, at about 5:00/km with only a slightly higher heart rate. The key difference was the fatigue level in my legs.
Running the aerobic zone is also where I fall to naturally. If I was just to head out the door and run, then chances are I will be right in this zone. A lot of benefit can come from piling up the kilometers at this level. So this does form the basis of a lot of my training.
Threshold: 155-166 (83-88%HRmax)
I work from the basis this is the level around where lactic acid is produced at a higher rate than can be cleared or metabolised by the body. I don't subscribe to the fact it is an exact point of intensity. There are many variables that affect the relationship, from a day to day and also within a training session itself. The key benefits I look for from training at this level is to develop an increase in speed and the duration I can hold those running speeds for, while sitting at around the threshold. The further above the threshold I work at, then the quicker fatigue develops due to acidosis, the burning sensation in the legs.
Often quoted as the intensity that can be maintained for around one hour, again it isn't quite that simple. It is an intensity that is higher than can be maintained for the duration of a marathon. Even working a bit below threshold there is still a higher accumulation of lactic acid that impedes lipolysis. Ironically, during the first part of my training, my speed at this intensity is actually slower than what I hope to achieve in the marathon. While initially I may find it very hard to run for one hour in this heart rate (which is also a slower pace than marathon), as training progresses eventually I should be able to hold faster and faster paces for at least 75 minutes. In the end, my marathon pace, will also give me a heart rate somewhere within this zone. In 2009 I held 157-160bpm for the majority of the marathon.
VO2: 167-188 (89-100%HRmax)
Training to raise my maximal oxygen uptake. Usually longer repeats (5-9 minutes) at well above threshold intensity and pace. It is linked closely with 3000m-10km race paces. While it is substantially faster than marathon race pace, it is an important element in marathon preparation. The main reason is it incorporates the faster twitch muscle fibres, that don't receive as much stimulus during the rest of training, but are recruited heavily in the latter stages of the marathon. If this training is neglected, there is a real risk of a significant slow down late in the race that is a result of muscular fatigue, rather than hypoglycaemia or reduced carbohydrate availability.
Because working in this intensity range incorporates a significant contribution of the anaerobic energy systems and resulting higher levels of acidosis, it is important to be judicious and careful with the training. Separate to the potential for extra recovery requirements (and therefore reduced overall training volume), prolonged, high levels of acidosis are thought to reduce many important aerobic adaptations. In particular, the ability to develop lipolysis (fat burning) may be compromised. On the other some significant jumps in running speeds at all levels can be made with training in this intensity range.
As my fitness changes (improves) these zones may change. I will also use them differently to fine tune my development leading into race day. There is plenty of overlap of training effect across all intensities, and it is difficult to give exact dose/response guidelines. Running speed and heart rate are always combined with other observations such as the sensation of breathing, type and level of pain, perceived exertion and the environmental conditions. Plenty of times the heart rates will be ignored as I pursue various goals within different training sessions.