Training Zones made easy
Sometimes it seems like there are as many different ways of defining training zones as there are coaches. And then there’s the choice of whether to delineate them by heart rate, lactate, pace, power or feel… As GPS watches have started offering runners’ training guidance I’ve noticed more runners wanting to train by pace or heart rate (two things the watches can measure). There are two problems with this, first the accuracy of the data (wrist-based HRMs can be wildly inaccurate) and second, the fact that it is highly individual and also not static over time. For example the correlation between heart rate and pace can depend on the terrain, the temperature, the altitude, the wind, even the athlete’s physiological or psychological state. A fatigued, stressed or dehydrated athlete’s HR will be higher at a given pace for example, and that’s before we consider heart rate drift…
One solution is to learn to run by feel. As with all other metrics it has its pros and cons, but in my experience athletes can convert this skill into breakthrough performances. Once you really understand your body you can get the most out of it. For example, if you have an out-and-back with a tailwind then a headwind (or vice versa) you should not run even splits, but even effort. Your body does not know miles and pace, it knows time and effort.
Running by feel can also help reduce injuries as you are never acting on that nagging voice “I should be running faster than this” when you are fatigued. Conversely, it also allows you to surprise yourself when you look down at your watch after an interval and find it faster than expected: it doesn’t hold you back.
RPE - Rate of Perceived Exertion is the typical metric for what I’m calling “feel” here. It’s a scale of 1-10 where 1 is walking and 10 is redlining. I like to combine this scale with colours: red for the hardest efforts, blue for the easiest. It’s the easiest way to understand the difference between over/unders and fast intervals for instance.
So what happens in each of these zones, and what is the desired outcome?
Blue: Active Recovery
RPE: 1-2
This is very easy exercise: walking, gentle swimming, easy yoga. In a training session this might be very gentle jogging. It is primarily aerobic, using type 1, slow twitch fibres. Fat oxidation is the primary fuel source. It helps to increase blood flow and aids in the removal of metabolic waste, promoting recovery.
Exercise in this zone also has longterm mitochondrial health benefits, promotes parasympathetic nervous system activation and increases the number of capillaries in the muscle (although not as much as the green zone).
Green: Easy / Aerobic Base
RPE: 3-5
This is the zone that most of your training will be in. It is conversational pace, meaning you should be able to hold a conversation with someone running with you. You are primarily using your aerobic energy system (oxidative phosphorylation) and you are using mostly type I muscle fibres (plus some Type IIa - fatigue resistant fast-twitch). Fat oxidation is still high but glycogen contribution increases. Lactate is easily cleared.
After running your body will adapt by sending signals to increase mitochondrial density, leading to better aerobic metabolism, enhance fat utilisation (improving endurance) and increase myoglobin content (for better oxygen transport within the muscle). These are really important and fairly robust adaptations, they build up slowly over time with consistent training and are “sticky” in that once you have them they hang around with continued stimulus. So you can have an easy week or take a few days off with a niggle and they will still be there. This is less true of adaptations in the yellow, orange and red zones, which tend to be the least sticky.
Yellow: Threshold
RPE: 5-7
This is around lactate threshold, the lower end (aerobic threshold) feels fun, the upper end (touching anaerobic threshold) requires concentration. Training in this zone uses both aerobic and anaerobic energy systems and a mix of type I and type IIa muscle fibres. Your body is now having to work harder, with increased glycolysis (glucose breakdown) for fuel (still using fats but also more carbs now) and clearing the higher lactate production.
Spending time in this zone allows runners to learn to feel where their lactate threshold is (when lactate is produced at the same rate it is cleared). This is vital for long races where crossing the line can have long consequences. This training also triggers adaptations that improve the lactate shuttle, meaning your body gets better at buffering and using lactate. Further, the mixed use of muscle fibre types leads to expanded recruitment of type IIa fibres, expanding endurance capability.
Orange - Tempo & Intervals
RPE: 7-8
We are now moving into the hard but sustainable category. You will need to concentrate to maintain this pace and a full conversation is no longer possible. The primary energy system is anaerobic glycolysis with some aerobic backup. Muscle fibre type is type IIa and some Type IIx (fast twitch power fibres) are recruited. There is moderate to significant lactate accumulation, a higher glycogen demand and increased oxygen debt. Longer intervals would have an RPE of 7 at the beginning but drift up to 8 over the interval.
Training in this zone will increase your lactate threshold, leading to higher speed endurance. It improves buffering capacity (less acidosis during high-intensity running) and type IIa fibres adapt to become more oxidative.
Red - Maximal Sprints, Hills, Strides
RPE: 9-10
The red zone is reserved for very short efforts and the finishing straight. It is near-maximal to all-out effort. Energy comes from ATP-PCr (Phosphocreatine) and anaerobic glycolysis; the immediate ATP demand is met by phosphocreatine, there is rapid glycogen breakdown, a surge in lactate and very obvious oxygen debt. You are using Type IIx muscle fibres.
Training in this zone might be described as anaerobic or neuromuscular. For most runners (not sprinters) the biggest benefits are likely neuromuscular and biomechanical.
Example sessions
Here we see two sessions working in different zones. The first session is what mostly people think of as traditional interval training: repetitions of hard running then walk/jog recovery. The second is a short threshold session that challenges the body to adapt by deliberately crossing lines and then asking the body to recover “on the fly”. Threshold sessions can either be short maintenance or building sessions (like the second chart below) or much longer for more advanced runners (e.g. warm up then 4 x 12 minutes yellow off 3 minutes green).
Note on terminology
Whilst I don’t think the abundance of terminology is that helpful, to help reduce the confusion, I use two terms for the main two thresholds: aerobic and anaerobic. They are also variously defined as:
Aerobic threshold: ventilatory threshold I (3 hour race pace)
Anaerobic threshold: ventilatory II, functional threshold, lactate threshold (more than one of these if you’re Norwegian), max. lactate steady state (about 1 hour race pace, depending on experience)