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By Ben Wisbey, Sports Scientist, FitSense Australia
Over the past twelve months interest has been growing in the use of predicted EPOC as a means of monitoring training load. Interest in the area has been sparked by new software, developed by a group of Finnish sports scientists, which uses heart rate measures to predict EPOC, respiratory rate, VO2 consumption, and a variety of other physiological measures. This software is now being used by Suunto and FRWD to enhance the analysis of data obtained from their heart rate monitors.
What is EPOC?
Excess post-exercise oxygen consumption (EPOC) is a physiological measure of the oxygen consumed in excess of resting requirement after exercise. It is measured in litres or ml/kg. EPOC reflects the disturbance of the body’s homeostasis brought on by the exercise, and the subsequent recovery demand.
Exact measurement of EPOC is only possible by analysing respiratory gases with laboratory equipment. Practical limitations in conducting this type of testing have led to the development of methods to estimate EPOC based on heart rate. In this article, we will refer to predicted EPOC as pEPOC.
How is pEPOC Calculated?
In order to make a prediction of EPOC, a known maximal heart rate and VO2 max for the user is required. The heart rate and heart rate variability (HRV) measurements during exercise are then used to predict respiratory rate and VO2. Predicted respiratory rate, and VO2 are then used in conjunction with heart rate to predict EPOC.
What could pEPOC be used for?
One of the key challenges in sport science is in determining an optimal training load. Sufficiently strenuous exercise causes a disturbance in the body’s homeostasis. After recovery this results in improved fitness. The challenge lies in determining the optimum balance between training volume, training intensity and recovery. Training that is too easy doesn’t improve fitness, and training that is too hard may lead to overtraining in the long term. It is suggested that by calculating pEPOC, the training load can be quantified. This allows the time needed for recovery to be calculated.
Limitations of pEPOC
There are several limitations to the use of pEPOC as a means of monitoring training response based on heart rate. These limitations are discussed below.
The current predictions of pEPOC are based on formulas for which there is a solid scientific basis. However, in practical trials conducted by FitSense Australia the predicted in-exercise VO2 values based on heart rate were 5-10% out from known laboratory measured VO2 values. Inaccuracies in the VO2 prediction will cause inaccuracies in the prediction of pEPOC measures.
It should be noted however, that if the accuracy of these physiological variables is not 100% when compared to laboratory captured data, they are all still relative in a day to day training environment. This means that a good comparison can be made on an ongoing basis between session loads.
EPOC (and thus pEPOC) is more affected by intensity than volume. EPOC is therefore only a useful measure of monitoring training response to intensity sessions. Figure 1 highlights the rapid increases in EPOC at high VO2 intensities, as well as the minimal rise in EPOC associated with low VO2 values.
Figure 1. The increase in EPOC over time at varying exercise intensities.
The pEPOC value taken during a session is the peak pEPOC reached. This effectively means that repeated efforts with a long recovery will not result in an increased pEPOC. This is highlighted in Figure 2 which shows a 10km effort followed by a long low intensity recovery, and then an intense 5km effort. Peak pEPOC was measured at the end of the 10km effort and despite another elevated pEPOC level at the end of the 5km effort, peak pEPOC did not increase. If pEPOC was an accurate measure of training load, then theoretically there would have been no physiological value in undertaking the 5km effort.
Figure 2. pEPOC and heart rate graph from a running session with a 10km effort, long recovery jog, and then a 5km effort.
Using pEPOC as a Training Tool
The sports scientists at FitSense Australia have trialled the use of pEPOC as a measure of training response over the past 12 months. It was found to be a useful measure of intensity-based training load for endurance sports and team sports, although some limitations do exist as discussed.
Outlined in Figure 2 is a screenshot of the data presented after a running session. From this information we can see a consistently high, but variable VO2 (~86-93% of max, see second line from bottom in figure 2) during the 20 x 1 minute efforts. An easy walk period is also evident during the short recovery after the 10th effort. Steady state VO2 was not reached in the 1st or 11th effort (after the walk) but all subsequent efforts resulted in steady state VO2 being achieved due to the short recovery. We can also see recovery being reduced between the later efforts, although goal VO2 was still being achieved.
Figure 2. Session file showing HR, pEPOC, VO2, and altitude.
Looking at pEPOC in figure 2, we can see quite a rapid increase during the series of efforts. Due to the short recoveries, pEPOC does not have the opportunity to decrease during this period. The EPOC of 225 ml/kg reached at peak indicates that this session was quite intense given that there was only 20 minutes of hard running.
Getting Started with pEPOC
Record your pEPOC over a number of intensity sessions (intense efforts with a short recovery) in order to develop a personal benchmark. Once a benchmark has been established, you can begin to manipulate workload and recovery. The following scenarios can be used as a guide:
An intensity session gives a lower pEPOC than normal, while your rating of perceived exertion remained high: This indicates you were fatigued going into the session- look at your recovery, nutrition, hydration, stress and health status in the days prior to the session as there may be problems in these areas.
An intensity session gives a lower pEPOC than normal, accompanied by a lower rating of perceived exertion. This indicates that the session intensity may need increasing to continue to achieve adaptations. Increase intensity by manipulating the number of reps, or the recovery period, or the length of the rep.
You increased the intensity of your intensity session, and have recorded a higher pEPOC. This indicates your training is on the right track- make sure you pay attention to your recovery however.
The use of predicted EPOC is definitely a useful tool for monitoring the intensity load of endurance based training sessions. While more work needs to be done to determine its scientific accuracy, in a day to day training environment, the predicted EPOC is a useful way of assessing training demand and maximising intensity training session adaptations.
Ben can be contacted at bwisbey@fitsense.com.au.
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