There are really two ways in which your power meter can be used.
- Setting the training intensity during interval training. Most cyclists who are introduced to power meters fall under the mistaken impression that the most effective use for their power meter is in setting the intensity. However, this might not always be beneficial (which we will highlight below).
- Collecting data for analysis after your training. Cyclists love data and if we can measure it, we probably will. However, data is only really valuable if it is correctly interpreted. Power output helps cyclists and their coaches monitor training load and, most importantly, progression. Using the power meter to collect data for later analysis is the more effective strategy.
Using the power meter to set training intensity:
Before the advent of power meters, most elite cyclists used heart rate as a measure of training intensity. Heart rate in a laboratory setting is almost always linearly related to power. i.e. as power output increases, so does heart rate and the rate of increase stays consistent. It was therefore a useful way to set specific training zones based on a laboratory test done at the beginning of the season in a performance laboratory.
However, out in the field (on the road or trail) there are many factors that affect the relationship between heart rate and power. These include dehydration, temperature, altitude, fatigue, caffeine intake, stress and body position. Together these factors can change heart rate by up to 25 beats per minute for the same power output.
Power meters measure the amount of work done while cycling. Power output, measured in watts, is the product of force and angular velocity. Power is not influenced by environmental conditions, fatigue or any other factors, which makes it a less variable measure of intensity than heart rate or rating of perceived exertion.
Does that mean you should only use power to prescribe training intensity?
Although power is a very objective and reliable measure of intensity, doing intervals based on power may not be the most effective strategy. A study conducted by Dr Jeroen Swart at the Sports Science Institute of South Africa examined improvements when training by power or heart rate. They took 21 elite male cyclists and trained them using either power or heart rate prescribed interval sessions. Before and after a 4 week training period, the athletes completed a VO2 max and peak power output test as well as a 40km time trial. To ensure that they had all performed exercise at the same intensity, the average training power outputs and heart rates from all the training session for each group were compared. They were identical. When the performance tests were compared, the peak power output tests showed that the heart rate group had improved by over 5% while the power group had improved by only 3.7%. Analysis determined that the heart rate based intervals were 60% more likely to result in improvement than the power based intervals.
If both groups trained at the same average intensity, how can that be? Well, another often cited deficiency of heart rate monitors is the lag between the increase in intensity of the exercise and the increase in heart rate. This can often be as long as 30 seconds. As a result, the heart rate group had performed intervals where the initial power outputs were very high in an attempt to get the heart rate up to the target. Later in each interval, the power outputs dropped off significantly, ending up much lower than that of the power group. The power group, as expected, churned along at an even intensity for each interval. The hypothesis is that this initial surge in the heart rate group could have been responsible for the extra training effect of using heart rate.
That said, using power to prescribe training intensity can allow the athlete or coach to progressively increase the target intensity and when this is done appropriately, it can force greater improvements in performance. A power meter can keep you honest during your training and prevent you from soft pedalling during your intervals. If you see your power output starting to drop towards the end of your interval, you are more likely to try and put in a little more effort to keep it at the target wattage.
At Science to Sport we use both heart rate and power to prescribe training, depending on the specific session. Setting the right intensity requires analysis of the training data to ensure progression and avoid excessive fatigue.
Analysis of your training data:
Power meters are great for race analysis too. If you are working with a coach or perform all your analysis yourself, race data may allow you to determine what went wrong during your race. Did you go too hard too early? Did you make too many surges early on in the race that you paid for later? How did you pace yourself during the race?
In addition, knowledge of the amount of work done (in kilojoules) during your training can allow you to fine tune your nutrition to ensure that your energy intake is matching your energy expenditure.
How do you monitor training load with a power meter?
Once you are recording all your training sessions with a power meter, you are able to plot an accurate Performance Management Chart (PMC). There are a number of different applications that enable you to plot a PMC; such as TrainingPeaks, Golden Cheetah and others. The variables plotted on a PMC include your chronic training load (CTL), acute training load (ATL) and training stress balance (TSB). These are defined in the glossary below.
The PMC will give you a snapshot of your fitness (CTL) and how fatigued you are likely to be (TSB). How high a CTL to aim for is dependent on many factors such as your training history, age, work related stress and others. A top professional road rider might aim for a CTL of 100-130 while your mid 40’s exec / weekend warrior will be best off with a lower value such as 65 or 70.
How can I ensure that my training load is sufficient?
Analysing individual session data will be able to assist you in establishing if your current training load is sufficient to produce optimal gains. Analysing training data will allow you to assess progression. Are you managing to produce a higher average power output for the same interval session? If not, why not? Are you training too hard and not recovering? Do you need to train harder? That’s where an expert coach will come in. They have years of experience and often first hand knowledge through their own racing experiences to guide your training appropriately.
As coaches we use a number of metrics to monitor external training load (the stress applied to the body), but also use other measures of stress to ensure that we don’t miss anything. An example is the Lambert and Lamberts Submaximal Cycling Test (LSCT). This and other tests allow us to measure the internal load (i.e. how you are responding to the load).
In the meantime, avoid becoming obsessed with the numbers and remember to enjoy your riding as well.
Acute Training Load (ATL) is a 7 day rolling average of your TSS scores. The ATL can be loosely regarded as a measure of fatigue.
Chronic Training Load (CTL) is a 42 day rolling average of your TSS scores and provides an objective measure of fitness.
Functional Threshold Power (FTP) is the maximal average power output that you can sustain for an hour. This is the value that the PMC uses to judge the intensity and load of any training session. We at Science to Sport establish your FTP from physiological testing we perform in our laboratory, alternatively, a common method of measuring FTP is to do a 20min maximal effort and to multiply the average power of this effort by 0.95.
Training Stress Balance (TSB) is the difference between CTL and ATL and provides an indication of ‘freshness’. Negative TSB values indicate that some acute fatigue may be present.
Training Stress Score (TSS) is a score assigned to each training session to quantify the stress of the session. Riding at your FTP for one hour will produce a TSS of 100 points. The algorithms built into the PMC will assign an exponentially higher TSS for efforts that are above your FTP.