Quote Originally Posted by keswicksimon View Post
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Here's a regular condition I've experienced for as long as I've been using a HRM (over 4 years). See image. It happens at any time of day when not being warmed up at all. The attached chart shows my HR when jogging with my wife the other day. Very easy pace, barely out of breath but I feel knackered and don't want to jog any faster. THEN and normally after ten minutes my HR instantly drops off and I feel so much better. This particular run took 15 minutes to drop off. For those that know Great Wood near Keswick, the drop-off to 120 in HR happened as we ran across the old car park before we started jogging up the steep top path towards the mast. Although it's steepish for 300m or so notice the running feels very easy with HR at a level to be expected. My BP is healthy and cholesterol is low. Anybody any ideas?
As Raasay says get to the doctor and get him to check you out...

AF is a possibility, you have it to start with hence weakness then it could be dropping backing to normal rhythm, I'm no doctor though just been through it so seek professional advice, AF is common in runners from 40 onwards