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Last week (Thursday to be precise) I went for my usual 10k tempo run at the gym (8.6mph). As per normal, I wore my Garmin and my HRM.
Started off ok, and after about 10 - 15 minutes I noticed that, although my legs were feeling like they were in a tempo run, my heart rate was showing that I was barely pushing past 150bpm (75% of my theoretical max). I pushed on to 3 miles, expecting it to level out at about 165 (as it normally does) but instead it stayed at 150 the entire time.
I put it down to a discrepency on the day and went out on the Friday for a 5 mile run. At no time during that run (I was doing about 7.2 - 7.4) did my heart rate get above 145. My long run on Sunday (12.25 miles in 1hr 38, my fastest by 90 seconds or so) resulted in an average of just 144.
I'm certaintly not complaning about this whatsoever (hell, I can run faster now) but I'm a bit concerned about the harder efforts. I'm worried now that I need to push harder on my tempo runs and speed work to get the aerobic benefit, however if I run too hard then my legs will just burn out. However, if I stay at the speeds I am now then I won't be improving any quicker and will eventually end up chasing my tail.
Does anyone have advice for a confused Duck?