Heart rate monitors are notoriously unreliable. You can get on identical machines and have your heart rate different by up to 30% or more. It depends on so many factors including the amount of sweat on the sensors, the amount of sweat on you, the temperature, static in the machine etc.
I would recommend getting a chest strap for a start and also wearing cotton running tops (not as a rule as they soak up the sweat and rub but for the purposes of heart rates as the technical tops generate static which messes with the HR readings) to get a more consistent idea of your HR. Try to wear the same t-shirt too as it can different with different tops.
It sounds mad but I have been experimenting with this for years after becoming concerned about the recorded profile of my heart rate when running. I can also spike the HR on the watch by running past one particular electricity substation yet run past other identical looking one and nothing happens.
I've been playing around with the 5:2 diet and also with trying to improve my ability to burn fat as opposed to glycogen on my longer runs.
I've noticed that when I've not eaten for 16 - 20 hours (I say not eaten, maybe 5 almonds or something but that's it) when I run my HR seems about 10 bpm higher than when I'm fully loaded with glycogen for the same pace (zone 2/3 so not pushing it). Oddly, my perception of the effort is that it feels fairly easy, certainly a lot easier than my watch is telling me.
I'm guessing that my body finds it a bit harder to metablise fat when I am running and is having to work harder to do so which sort of makes sense but I am curious as to why my perception of the effort is not in line with the HR readings.
I'm properly hydrated all the time so it's not that.
Just wondering whether anybody else has played around with and what their observations were. What were the results....do you think you managed to train your body to burn fat more efficiently?
tony2476 - I'm trying to work out my pacing for this and am interested in the fact that the route didn't make a massive difference to your marathon time.
I'm a 38 minute 10k / 1.25 half marathon runner. I did the Leith Hill Half in 1.41 but wasn't in the shape I am in now. I was thinking about aiming for 10 minute miles on legs 1 and 3 and 8 minute miles on legs 2 and 4. Maybe I should reconsider that....
Tricky business. Looking forward to it though. Am definitely going to be running to enjoy and not busting a gut to get a "fast" time.