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For more information:

Setting your training zones

An introduction to training with power

Power 101

Heart rate variability

Aerobic decoupling, or cardiac drift - the relationship between heart rate and power

Monitoring training intensity

Your training load is determined by a combination of frequency, duration and intensity. Measuring frequency and duration is simple but measuring intensity requires a little more thought....and perhaps a little more technology! However, it is important that intensity is carefully monitored to allow the training load to be controlled in a way that brings you to peak fitness at the right times of the year.

Rating of perceived exertion (RPE)

Borg_RPE_scaleThe simplest and historically the only way to ‘measure’ exercise intensity. Using RPE (sometimes also described as riding on ‘feel’) to monitor exercise intensity simply requires you to decide how hard you think you are working or how hard you found a session. One way to do this might be to use descriptive words such as easy, steady, hard, very hard and maximal to describe your training efforts. This idea was formalised and extended by Gunnar Borg in the 1980’s with the introduction of Borg’s RPE scale (see left).

When assessing your RPE, try to consider the level of exertion for differing systems of your body, making note of your breathing and muscular strain for example. Think about the speed and depth of your breaths, or the degree of ‘burn’ in your legs. Sometimes there is a mismatch between the level of effort when comparing across the systems. This is all valuable information and should be included in your training diary comments.

Heart Rate (HR)

Monitoring of heart rate was first made possible over 200 years ago with the invention of the stethoscope. The next major advance saw the development of the ECG machine in the early 1900s but the equipment was large and suitable only for laboratory use. In the 1980’s things got interesting for the athlete with the invention of the first wireless heart rate monitor. Fast forward thirty years and heart rate monitors are now capable of measuring and storing huge amounts of HR data and also measuring heart rate variability (a measure of the variability in time between heart beats) (Achten, 2003).  See our factsheet on heart rate variability for more information.

Relationship between HR and VO2

In the laboratory, a sports scientist might measure oxygen consumption as a means of assessing the energy requirements of a bout of exercise or the amount of stress the body’s systems are under. Such measurements require bulky and expensive equipment which are totally impractical for the ‘real world’ – there’s a good reason why you don’t see many people racing or training with a mouthpiece to measure VO2! However, oxygen is transported from our lungs to our muscles bound to a molecule called haemoglobin in our bloodstream. Greater oxygen demands in the muscle require more blood to be pumped to the muscles achieved by the heart beating more frequently. By measuring heart rate we are indirectly making an assessment of oxygen consumption.

Factors affecting HR

In most circumstances this use of the relationship between VO2 and HR is fairly robust and a great means of monitoring intensity. Some of the underlying assumptions are occasionally violated and in this case the relationship breaks down. Some of the most common are as follows

Heat – in hot conditions, blood flow is directed to the skin to aid cooling. Just as blood carries oxygen, it also carries heat. Increasing flow to the skin allows (with the help of sweat) this heat to be transferred away from the body. This extra blood flow is in addition to that required by your hard working muscles and so heart rate must increase to meet this demand.

Fatigue – as your individual muscle fibres fatigue, you are forced to recruit increasingly from your fast twitch muscle fibre pool. These fibres are less adapted to aerobic exercise and so require a greater amount of oxygen to develop the same work rate. For more information see the factsheet on ‘The relationship between heart rate and power’. This is not necessarily a bad thing – using heart rate can be thought of as the output of the system whereas power is an input. Heart rate when used in this way is a great way of measuring the stress that a given work rate places on your body during a given session.

Cadence – altering your cycling cadence can affect your heart rate in a similar way to fatigue. Cycling at higher cadences requires faster muscle contractions more suited to fast twitch fibres. Again these are less efficient in their use of oxygen and cause the higher heart rate typical of cycling at high cadence.

Other stresses - work, relationships and illness are all examples of 'lifestyle stresses' that can have a marked impact on your heart rate. The body's response to such stress is complicated and individual but the hormones released as a result can impact on the sympathetic and parasympathetic branches of the nervous system that are responsible for controlling heart rate. For more information on this concept and how we can measure the effect on the nervous system, check out our factsheet on Heart rate variability (HRV) or Dan's blogpost on his PhD studies into HRV. The details do not need to be fully understood by you as an athlete, what is important is to understand that these factors can have a large impact on your heart rate response and try to recognise when this is the case. The following are some of the pros and cons to using a heart rate monitor:

Heart_rate_pros_and_cons

Power

The next revolution in monitoring training came with the availability of portable devices to measure the power output developed by the rider. This development was a fairly major in terms of measuring a cyclist’s work rate – power, measured in watts, is an absolute measure unaffected by weather conditions and other external influences. 200W is 200W, whether riding up hill, downhill, into the wind, for an elite rider, a novice and any imaginable situation.

What is power?

Power is defined as the total amount of work done divided by the time taken to do that work. Equivalently power can be expressed as force multiplied by velocity or when moving in a circle (think pedalling!) as torque multiplied by angular velocity. In simple terms, power is the product of how hard you push and how fast you spin. The equation below explains the relationship between power and speed. The details are unimportant; the main thing to understand is that power is what makes your bicycle go forwards – all else being equal more power equals more speed.

Power_equation_of_motion

Types of power meter

PowerTap_SLC_sizedThere are a number of systems on the market for measuring power, each with their own pros and cons. The first thing to consider is how the system ‘measures’ power. In terms of accuracy and reliability, those that rely on strain gauges for calculating power output are preferable. Other systems (such as the Polar and iBike systems) do not make direct measurements instead measuring other variables, such as chain vibration and air speed, and infer power output based on a number of assumptions. A number of reports have shown favourable comparisons with direct measures of power but there is a greater potential for error when the underlying assumptions are violated.

Also worthy of consideration is where the power measurement takes place. Broadly speaking the currently available systems can be divided into those that are crank based (e.g. SRM, Quarq) and those that are hub based (e.g. Powertap). Crank based systems have the advantage that training and race wheels can be swapped at will, whereas hub based systems are restricted to the wheel built around the hub. On the other hand, hub based systems are very easy to swap between bikes, although the majority of crank based systems are also fairly easy to transfer for athletes with basic understanding of bike maintenance. For more information on the individual systems check the manufacturers’ websites.

The best choice of power meter for an athlete depends on their individual needs and budget so it is important to weigh up the pros and cons of each system and/or consult with your coach before parting with your hard-earned cash!

Power_meter_pros_and_cons

The Ultimate Package

In conclusion, the ideal scenario for monitoring your training and racing is to use your power and heart rate monitor alongside your rating of perceived exertion. Each in turn provides valuable information when assessing your training intensity but by examining them all in relation to each other you get a great picture of the overall stress of the session – you get to see the ‘bigger picture’. Learning the strengths and weaknesses of each system will help you to focus on the critical information in any scenario.

Remember:
  • Power is the input – a measure of the strain you are applying to your bodies systems
  • Heart rate is the output – how much stress you are under to meet that work rate
  • RPE gives an overall picture of the intensity of the session in the context of your current condition, taking into account fatigue, motivation and all of the other external factors affecting your performance.

Tips for monitoring intensity

Steady workouts in zones 2 and 3 are ideal for using your heart rate monitor. The slight delay in your heart rate response to changes in intensity are actually a benefit in this situation, helping you to maintain a steady effort. Guide your workouts of this sort using HR before analysing the power output this returns

Short intervals are best guided using your power meter – the delay in heart rate responding to a given work rate means it is not the best metric for monitoring intensity for intervals < 10 minutes. Using your power meter for these workouts will ensure you hit the right intensity from the very first pedal stroke.

Don’t become a slave to your power meter when racing – remember that your power meter is just a tool to help monitor your effort, it should never restrict or limit your performance. A vital skill is learning to pace an effort based on ‘feel’. Use your power meter to help guide your pacing but still listen to feedback from your body to get the most out of your performance.