Heart Rate Drift

 

With 2 weeks of solid training under my belt since returning to the Seychelles, I have some interesting data from my recent run sessions.  In contrast to the UK, the weather here has been quite settled, consistently around 31 degrees C and 74% relative humidity.  To illustrate the differences of training in the heat and (lets face it) cold, I'll compare two run sessions of similar effort - one from the UK and one from the Seychelles.

 

14/02/11 1h13m Run - approx 10 degrees C

22/02/11 1h01m Run - 31 degrees C
Firstly, I should mention that although I track all my run data, including heart rate (HR) and speed, I run primarily based on feel.  What you can clearly see from the two graphs above, is that in the heat I spent a greater amount of time at higher HRs.  What you can't see is what this post is really all about - HR drift.  Through analysing the data in WKO, HR drift for the two sessions was as follows:
14/02/11 - 1.51%
22/02/11 - 8.23%
That's clearly a HUGE difference.  But what does it mean?  For running, HR drift is the comparison of HR to pace.  Data from the session is split into the first and second half and for each half average HR is divided by average pace.  To find HR drift (a pace:HR ratio percentage of change), the first half is subtracted from the second half, which is then divided by the first half ([H2-H1] / H1).  For some interesting thoughts on the topic, check out this article by Joe Friel.  

According to Friel, HR (or cardiac) drift is a good indication of aerobic capacity.  If drift is >5%, then you may need to work on your endurance some more.  So, comparing my two runs, they are 3.5% -/+ the recommended 5% - did I lose my endurance in the space of one week?  No. Heat causes HR to drift over time.  There's not only the ambient temperature to worry about, but your bodies core temperature, as well as increased sweat levels.  I've found that 1 litre/hour is just about enough to get me through the longer sessions.  Any less and my performance takes a dive.  
If you're consistently seeing HR drift values over 5%, it may be an indication that rather than focus on threshold sessions, endurance sessions should be your priority.  Over time and with improvement, you should see a decrease in HR drift, which is a good indication that your endurance is improving.  Once you're able to cover the necessary duration with low HR drift, it may be more appropriate to add some faster sessions to the programme.  I say may as for most people, constantly working on endurance will likely reap the greatest rewards - but that's a subject for another post!
Effort doesn't always directly correlate with HR, which is why HR isn't the be all and end all.  Rather than get caught up about seeing higher values, I spend less time checking HR, accepting the inevitable variability associated with it.  That's not to say that it isn't a valuable tool - it is.  But like any tool, it's important to accept its limitations.