There are two basic kinds of training load: external and internal.
External training load is a totally objective measure of how much work you did: trained 10K at 5 or 6 minute pace, ran 5 km at 80% of my Max HR or whatever your training maybe.
Internal training load represents how hard you, as an individual, had to work to complete the session. That might be represented by your average heart rate during the workout, or simply by your subjective assessment of the effort you put in and how tired you are.
Both types of metric are important, but it’s the internal load, the actual level of physiological stress your body experiences during a training sessions that determines how you’ll respond and whether you’ll get fitter or overtrained.
The fundamental question about training, from which all other debates and decisions arise are –
How hard was that workout you just finished? There are many ways of answering that question, using heart-rate monitors or your GPS watches measuring speed/pace and gut feelings. One way or another, you have to have a sense of how hard you’re working, so that you can make decisions about how to distribute your training efforts each session and week and how to gradually increase your training load from month to month, and how to recover before races.
The other internal load measure is very simple, during each interval / Training Zone and shortly after the workout is over, you ask yourself the question “How hard was interval or session? ” on a scale from 1 to 10, this is called the RPE scale (Rate of perceived Exertion) and it’s free and never runs out of batteries.
In other words, if you’re measuring RPE and doing it well, this suggests that you’re getting as much information about your training effort as you’d get from a bunch of sophisticated analysis.
I’ll admit that I like that message and I use this method more then I use any other, the idea that our internal computers are as sensitively tuned to the subtleties of effort as any external gadget can be. I think it’s important not to completely be dependent on your pacing decisions to technology, and to have a finely tuned sense of how hard you’re working (as distinct from how fast you’re going). But it’s also important not to flip too far to the other side and discount the importance of external data, for a few different reasons.
In other words, it’s not about which metric is better. Instead, it’s the relationship between internal and external metrics that tells you whether you’re getting fitter or overly fatigued.
In training, it’s meaningless to know that your internal load is going up or down if you don’t also know whether you’re going faster or slower.
In racing, it’s useless or even counterproductive to know what pace you are paddling at or “should” be able to maintain if you don’t also consider how you’re feeling that day or the conditions your paddling in, currents, winds and tides etc are the determining factor of your board speed.
So the message here isn’t that you should ditch your GPS or Heart Rate Monitors It’s that you should also trust how you feel (RPE) and trust in it.