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As a ride goes on, it takes more and more effort to maintain speed. This goes for everyone. The Cyclist's Training Bible, calls this cardiac drift: a rider's heart rate tends to gradually rise as he/she maintains the same power output. We measure this using a ratio of power to heart rate. (Watts to beats per minute, or W/bpm.)

Basically, the cyclist goes for an easy, steady ride, then compares the average W/bpm ratio for the first half to that from the second half. For riders with well-developed base aerobic metabolisms, the second ratio will be within 5% of the first.

Curious, I dug into today's easy lunchtime ride.

I should say at the outset here that I don't own a power meter, and that all the numbers I'm using for power were estimated by Strava based on my GPS data.

I trimmed out the warm up, and the downhill finish, and the part in the middle where I stopped to pick up my lunch. That left me with two rough "halves."

	1st Half	2nd Half
Avg. Power	149	125
Avg. HR	141	131
Ratio	1.056	0.954

My W/bpm ratio dropped about 9.7% from the first half to the second. Which tells me... not a lot, actually. Is the drop due to a lower heart rate? Is it because this is my third ride in three days and I'm fatigued? I need more context.

So I looked at yesterday's ride. Since it was easy right from the start, and I kept a steady pace, I was able to simply grab the first and second halves, without munging the data at all.

	1st Half	2nd Half
Avg. Power	147	152
Avg. HR	137	141
Ratio	1.0729	1.0780

The cardiac drift comes out to something like 0.47%. That's half a percent. About as close to perfect as anyone cares to get. (Also note that my average heart rate for both halves was right around 140 bpm.)

So what?

I think it means that my aerobic endurance over 60 minutes is solid (provided I'm fresh). But I know that I feel like I fade after an hour or so.

I did a ride last week that was long for me: 30 miles or so, and it took about 2 hours. So I dug into the data for that.

As expected, I faded hard in the second hour, with my W/bpm ratio falling more than 17%. I wanted more detail, so I broke out this ride into half-hour chunks.

	1st 30 min	2nd 30 min	3rd 30 min	4th 30 min
Avg. Power	141	151	119	70
Avg. HR	138	136	133	119
Ratio	1.02	1.11	0.89	0.59

I'm not at all sure I can draw any conclusions from this. The numbers from the first 90 minutes seem reasonable: steady start, strong effort through the bottom of the hour (when I got into the drops and had a long open road to fly down), then fatigue sets in. But I just plummet after that, which doesn't match how I felt at all.

It could be that I'm most efficent at around 140 bpm, so when my HR dropped to around 120, my power output kind of crashed. Or it could be that the short, sharp climb I did just before heading home really did cook me, but I never realized it because I was cruising downhill toward home and not really trying very hard.

And it would make sense if my W/bpm ration surged in the second 30 minute period, as my HR increased to around 150 bpm and (maybe) I started dipping into my anaerobic energy supplies.

What is totally clear is that I need to learn more, and I need to do a two-hour ride at a steady 140 bpm and see what turns up.