I think as endurance athletes we’re hardwired to quantify, ad infinitum, our training and results and symptoms and improvements and deficits until we perceive some higher understanding of self. It’s ironic, given how individual these pursuits ultimately are that we subscribe to many different “standards” of measuring the status of our fitness on any given day, instead of relying solely on symptoms and perceptions. These two contrasting philosophies are in fact hotly debated in the coaching worl, but I remain of the mind that using different metrics and understanding how they relate to what's happening in your body continues to thrive over simple guesswork. Nothing will replace your own sense of self and awareness of fatigue and exertion, but having quantitative validations and other data can supplement and reinforce.
I used heart rate monitors all through my training as a ski racer and continue to now as a mountain runner. Because of the demands of terrain, pace is largely erroneous when trying to ascertain or measure your output in training; heart rate is a good alternative. Another form I'm experimenting with right now is measuring power. A few weeks back I got a Stryd power meter - the device works like a heart rate monitor in that it attaches to a chest strap and synchs with your existing wristwatch interface (I use a Suunto Ambit 3). The readout will then present actual power and average power, along with other standard variables such as heart rate, etc. There's a tremendous amount to be determined from this application but I'm just scratching the surface right now. I heard about running power meters from my old coach Scott Johnston, who himself is coaching several ultrarunners with the help of power meters and found them to be really successful.
How beneficial is measuring power? I'm still answering that question for myself. Since I've always been on the heart rate train, I'm approaching it with the same mindset, i.e. can i pinpoint power zones which identify appropriate effort ranges for me to work in? Experts in the running power field such as Jim Vance and Allen Lim say yes, and what's more, the analysis portion can be really robust. TrainingPeaks, the software we use for our coaching clients, is based around taking your workout metrics and plotting them on a long-term chart which then calculates (using robust algorithms) your overall state of fitness and fatigue. This chart, known as the PMC (performance management chart) is best derived via power measures. The program was developed largely by and for cyclists who in the last decade have benefited from power meters so this makes sense; now runners can start to draw the same benefits themselves.
For the last two weeks I've just been gathering data during my workouts and not trying much to analyze it. I've done a couple hard sessions with the power meter; one 5km twisty-turning road race, and several interval/threshold workouts. I'm starting to get a picture of what my sustainable "threshold" or maximal steady-state power is like, and hope that in time I can use that to guide future training.
As an example, here's a display showing the power output from my workout yesterday, a 3x10min uphill tempo run on Tiger Mountain:
In reading the graph above, I can see that I was averaging a little under 500 watts for each 10min effort. However by the third one that average was dropping a bit, down into the 470s. When we talk about hard training we often refer indirectly to "maintaining power", but until now that's been an approximation. Calculating it (power = force x velocity) requires that you can actual determine WHAT your force is; Stryd apparently does this and completes the power calculation by using several accelerometers and a complex algorithm. And here's where I see the benefits occurring.
Power offers some great advantages which heart rate doesn't; namely, you can use power output to measure technique efficiency in your running form. The way to do this is to hold velocity constant. I've been using hill sprints for this purpose; find a section of terrain which is steep enough and lasts for about 10 seconds of hard uphill running. Do one repeat, time the effort for a given distance and see what your power reaches at the end. Now go back down (rest for 2-3 minutes) and do the effort again, this time focusing on relaxing or improving some part of your stride. See if you can hit the same time with a lower peak power. If you can, this means you created efficiency in your stride by reducing force at the same velocity, resulting in a lower metabolic cost.
There's obviously a lot more to be gained from the power measurement arena and I'm only scratching the surface, but I think the salient point to take away is that there is no one approach to monitoring training, no dogmatic way of determining effort or performance. Gather data, determine how useful it is, make an interpretation and then balance it with your own (and your coach's) subjective observations.
And sometimes, just tear the watch off and run like hell up a mountain.