I found it quite tough to keep the pace up today. I think perhaps all the stuff I was doing at the weekend wore me out a little more than I realised. Initially I couldn’t get my 50m reps below 45s, but over the course of the set they dropped into the 44s and I ended on a 43. The 25m reps were all 18s.
Out of random interest, I took my pulse after the end of my 50m set and then again after the end of my 25m set. It is claimed that a rough guide for maximum heart rate is 220 less one’s age, so mid-160s for me. My heart rate after the 50m set was about 140 (could be a little bit either way as I was counting for 15 seconds and then multiplying up), or about 83% of maximum, which I have to admit surprised me given that I’m maintaining that exercise level for almost 30 minutes. Not quite as much of a shock as measuring a rate of 164 after my 25m set though 😀
At around 18s a length plus the rest interval the 25m set doesn’t take very long — I’d actually attempt more reps but for the fact that I run out of time in the session, but I doubt I could maintain maximum heart rate for that long without my head exploding or something, so I suspect the actual case is that my real maximum heart rate is not as low as the rule of thumb suggests.