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'Aerobic decoupling' or 'Cardiac drift'

The relationship between heart rate and power

With the increasing use of power meters by cyclists looking to monitor their training and racing, the heart rate monitor has taken a back seat in the minds of many athletes (and coaches). By choosing not to monitor heart rate alongside power, a large amount of information is being neglected that can have great value in guiding an athlete’s training. Some of the important concepts are outlined below.

One way to consider these ideas is to think of power being the input to the system and of heart rate being the output. In other words heart rate can be viewed as a measure of how much stress the body is under in order to meet the required work rate. Your heart rate response is very individual so the relationship between heart rate and power (power:HR)  is of little value on its own. However, when we begin to look at how this relationship changes over time it offers a fantastic insight to how you are responding to the training plan.

For more information check out the following links

Fitness versus endurance part 1

The importance of base training

Training in zone 2

Training in zone 3

Riding at a given intensity requires the body to use energy. For durations longer than around 60 seconds, the energy requirements are largely derived from aerobic sources, i.e. using oxygen. This oxygen is transported from the lungs to the working muscles in the blood, so the rate of oxygen supply is directly dependent on the rate of blood supply. This in turn is determined by the product of stroke volume and heart rate (stroke volume is the volume of blood pumped by each contraction of the heart). What we are doing by looking at heart rate at sub-maximal intensities is using it at as a ‘proxy’ for oxygen supply. This enables us to take some of what we learn in the laboratory and apply it to training in the ‘real world’. Of course this is subject to some significant assumptions (i.e. stroke volume being constant) – how these affect the power:HR relationship are examined below.much stress the body is under in order to meet the required work rate. Your heart rate response is very individual so the relationship between heart rate and power (power:HR)  is of little value on its own. However, when we begin to look at how this relationship changes over time it offers a fantastic insight to how you are responding to the training plan.

Changes during a single session

bike-with-srm-power-meter-by-KevinSaundersA common phenomenon for athletes training with a heart rate monitor is the gradual increase in heart rate as a session progresses, despite no increase in intensity. This is sometimes termed ‘Cardiovascular drift’. An often quoted mechanism for cardiovascular drift is the issue of heat and/or dehydration. In hot conditions (for instance training indoors) blood flow is increased to the skin to aid cooling. This extra blood flow is delivered by an increase in HR. Dehydration, which so often goes hand in hand with exercise in the heat, can add to this effect. Dehydration leads to drop in blood volume which ultimately leads to a reduction in stroke volume. As the heart is pumping less blood per contraction it must pump faster to deliver the same amount of blood, and therefore oxygen, to the muscles.

More useful, in terms of an athlete’s fitness, is the observation that changes in the power:HR relationship occur even without an increase in body temperature and in the absence of dehydration. This change in the power:HR relationship over a single session has been termed ‘decoupling’ and has been popularised by coaches such as Joe Friel as a measure of aerobic fitness. Your fitness does not change during an individual workout so it is perhaps not immediately obvious why the relationship should change over a relatively short space of time. For ease of explanation we will assume that you are producing a constant power and examine why the heart rate at that power might change. There is surprisingly little on this phenomenon in the sports science literature so there is ultimately a degree of speculation!

The mechanism behind this is related to oxygen demand. At the start of an endurance training session, the body selectively recruits the (predominantly slow twitch) muscle fibres that are most adapted to aerobic exercise. As these fibres fatigue, you are forced to recruit other fibres, including fast twitch fibres which are not as well adapted to aerobic exercise. In fact studies have shown that fast twitch fibres require approximately twice as much oxygen as slow twitch fibres for the same power output (Coyle, 1992). This extra oxygen requirement is delivered by an increase in heart rate.

Another cause of decoupling may be changes in the blood chemistry itself affecting oxygen delivery. Oxygen is carried in the blood by joining with a protein in red blood cells called haemoglobin. Under ‘normal’ conditions, haemoglobin is very efficient but during exercise, increased temperature, CO2 production, and acidity all have a negative effect on its efficiency. If the blood is less effective at carrying oxygen then more of it must be pumped to the muscles to meet the oxygen demand. Again this leads to an increase in heart rate.

Armed with this information it becomes clear that causing decoupling is an important training goal when looking to build endurance at certain parts of the season. The guiding principle of training is that we apply a stress to the body and then allow it to recover, grow stronger, and be better able to deal with the stress in future. Moderate decoupling is a sign that your slow twitch fibres have been sufficiently stressed and that the larger, less aerobically fit fibres are receiving some training stimulus. At the other end of the scale, excessive decoupling may point in the direction of too high of a training load. Analysing the amount of decoupling in your endurance training sessions can be an effective means of judging the training load and provide valuable insight into how well your endurance ‘base’ is progressing. Once you can complete a given workout with minimal signs of decoupling, it is time to increase the training load: either by adding to the length of the session or increasing the intensity.

Well trained endurance athletes will have minimal decoupling even in long duration workouts. In other words their heart rate for a given power will be the same (or very similar) at the end of a ride as it was at the beginning. This would be shown by a decoupling of less than around 5% - or in other words power and heart rate remain coupled. In fact when in top condition, it is possible to complete back to back rides of several hours in zone 3 with minimal decoupling!

How do we measure Decoupling?

Decoupling is normally measured as the percentage change between power:HR in the first half of the workout and power:HR in the second half of the workout. For example if you cycle for one hour at 200W with a HR of 150bpm and then cycle back for one hour at 200W with a HR of 160bpm then you would calculate your decoupling as:

Decoupling_example_calculation

Thankfully, this calculation is performed automatically in the Training Peaks WKO+ software! (see figure below)

WKO_decoupling

 

Changes over a number of sessions

The power:HR relationship can also be useful in tracking changes in your condition over an extended period of time. Often changes in this relationship can be the first sign that it is time to update training zones. If over a number of sessions your power is higher than normal for a given heart rate, or your heart rate is lower for a given power, then it may be a sign that your fitness has improved. A difference in one session may not mean very much but if you see a consistent alteration in power:HR over a sequence of training rides, possibly accompanied by a decrease in effort levels at a given intensity, it might be time to ‘tweak’ your training zones. Alternatively for greater precision, this might be the perfect time to schedule a week of power profiling or a lab test.

Monitoring fatigue

PowerTap_SLC_sized

In a similar way to showing changes in fitness, power:HR can also be used to monitor how tired you are. The use of HR monitors and power meters has enabled accurate tracking of training load through metrics such as Training Stress Score and TRIMPS but equally important for the working athlete is the effect of the ‘non-cycling’ stressors in your life. The quality and duration of sleep, stress at work and in relationships, and nutritional choices amongst others all have an effect on how well you adapt to training and how fatigued you are at any point in time. Analysing your training load alone may suggest that you should be fine to complete the days prescribed training session. However other factors in your life may mean otherwise. An athlete needs to consider ‘total stress’.

Changes in your power:HR relationship can help to measure this fatigue. To confuse issues, for a given power, heart rate may be higher, or vice versa, lower than normal when tired. When the body is under stress, a number of hormones (such as cortisol) are elevated and can affect your heart rate response. Take a look at your power:HR relationship in the warm up to your training session. Is it within a normal range for what you would expect? If not consider other factors that might influence your heart rate – is the temperature particularly hot/cold? How do you feel? If you are sore, lethargic or the power target requires a greater effort than expected AND your power:HR is different then take it as a sign that you are inadequately recovered and consider whether it might be wise to abandon the training session or just have an easy recovery spin. Ultimately the aim of training is to stress your body to adapt and become stronger but there comes a point where extra training will just make your more tired rather than more fit. Use power:HR as a tool to help identify this point and prevent the need for an extended period of recovery.

Summary

For more information check out the following links

Fitness versus endurance

The importance of base training

Training in zone 2

Training in zone 3

While it is true that heart rate is affected by many external factors that limit its effectiveness in monitoring training intensity, when combined with the use of a power meter it becomes a very powerful tool. Used with other information such as your rating of perceived exertion (RPE) for each session it can provide valuable information on how well your endurance is developed, the first signs of improvement in your training zones and an indicator of your fatigue levels on a day to day basis.

None of this is possible unless you remember to wear your heart rate strap for EVERY session and make detailed comments in your training diary on how the session felt!