"How do you measure the intensity of your workouts?"

I use the following to determine intensity:
Heart Rate Monitor
Average speed on the bike
Minutes per mile pace on a run
My internal sense of how I feel
Something else:

How discriminating is your internal sense of how Hard/Easy you train?
Very Good: can differentiate 7-10 distinct zones
Good: can differentiate about 5 zones
Not that good: 3 zones - Hard, Easy, and something in between
Pretty bad: I'd be lost without using some sort of measuring equipment/technology

Any comments you'd like to make (elaborations, clarifications, etc).

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The answer . . .

Summary:
I've abstracted and combined your comments into the following general points. (Thanks to those responding -- Justin, Rory, Mimi/James, and anonymous submissions).

  1. Your body will let you know. An internal sense of intensity is always used (understandable, since it's difficult to ignore) and it's often used in conjunction with some other sort of measurement such as mins/mile pace on a run, average mph on the bike, what the HR monitor reads, --- and/or whether or not you vomit on the top of Baldwin hill.
  2. Experience counts. The longer and more experienced one gets with training, the better one gets at matching a perceived exertion level with what one's HR monitor would say. A game mentioned in a comment was to guess your heart rate before checking what the HR monitor reports -- and over time, you can get pretty good at this. The method of pricking the ear-lobe (more on that later) also came up as a scientific way to measure intensity.
  3. A systematic plan is important.Also mentioned in comments was the importance of an annual training plan with periodization -- presumably one in which intensity is systematically worked in.

Details and additional notes...

I've added some additional info/thoughts on this subject that came to mind this past week. Because intensity is difficult to measure accurately (unlike measuring distance or duration), it's best to have more than one measure to rely upon.

  1. HR: Heart Rate. 220-Age is your estimated maximum HR, and recommended intensity training zones are between 65% and 90% of this value. But this may not always be the best measure to use. Consider the following textbook example: One person's AT (see AT below) may occur at 85% of the max HR and another person's AT may happen at 92% of max. If both people are working out at a 90% training zone, then this means one person may already be anaerobic, and the other person is still in their aerobic zone. Very different intensities for the same HR training zone.
  2. Pace: Calculate mins/mile on a run, mph on a bike, yds/min swam, etc. This we know can mean very different intensity levels for the same value. Rory running (loping along?) at a 5:35/mile pace is very different than me running (sprinting?) at a 5:35 /mile pace.
  3. Perceived Effort: One could use a rating of perceived exertion (RPE). This is a scale with 15 levels numbered from 6-20 (roughly matching HR's from 60 to 200), which maps onto 5 intensity-zones ranging from a recovery zone which feels very, very light to a power zone which is very, very hard. (Incidentally, most of you indicated you can easily differentiate about 5 training zones).
  4. Anarobic Threshold (AT) or Lactate Threshold (LT): Physiologically, this is the the body's breaking point during exercise. Above AT, lactic acid accumulates in the muscles thus interfering with energy production and muscular contractions, resulting in fatigue. AT is a "moving target" in that it can vary with a bunch of factors such as body temperature, how tired you are, etc. It can be measured by pricking your ear lobe and subjecting the blood to a blood test (Seriously now, are any of us really going to do this after a work-out?). An alternative and non-invasive way to estimate your AT is by doing a Conconi test (similar to a stress test). Once you know your AT or LT you can then determine appropriate HR training zones in terms of a percentage of your LT, (LTHR).

So here's an idea for an "Intense-Race Contest". We race a fixed distance, and the winner is whoever has the lowest score. A person's score would be the time taken to finish plus the intensity they raced at. Intensity could be measured as the average HR percentage of lactate threshold during the race (Of course, we'd have to figure out a way to do that "ear-lobe-pricking-blood-testing" thing easily). Barney/Justin -- Do I hear you guys betting each other on who's going to win this?