Are you like my old Ford Cortina (her name was Connie)? A dark poo-brown beast of a vehicle that had a dodgy automatic transmission and basically had just two gears - 'on' or 'off'. She also had an elusive third gear where she would purr (and shake a little) on the motorway doing an almost comfortable 85km/hr. Almost.
She was a sweet, sweet first car, but she was hardly what I would describe as efficient or reliable.
I wish I still owned my Cortina for my wedding day.
Lots of runners are like my dear Connie. You mean well, but you only have two gears - on or off. You are either running (at your "comfortable pace") or you're not. You also have that elusive third gear where you are working hard which, if you can hit it on any given day, you run at that speed as long as your radiator holds out. Fun? Sure. But it's not sustainable (running or driving).
*Most* runners run too hard too often. And when I say too hard, I'm more often than not also including your 'on' gear in that category - a lot of runners have a "comfortable pace" that is too fast for optimal training load and endurance development. Add to that your elusive fast gear, and you're running too fast and/or hard way more regularly than you should.
Training is all about balancing a majority of easy running with specific higher intensity and/or volume sessions. To put this in really basic terms; on a scale of 1-5 (1 being easy, 5 being max. effort) you should be spending most of your time at 1-2, with a small portion of your time at 4-5. The 3 on the scale is your pesky 'comfortable pace', which usually is too fast to be an easy day, and too slow to be a valuable hard day.
You: "Gosh darn! Find me a mechanic that can give me a new gearbox and do away with my basic dual speed transmission"
I've got one for you - your heart.
The heart is a perfect training partner. If you monitor your heart rate, it will give you feedback on whether you are sitting at a 1 or 2, whether you are in the plateau zone (3), or whether you are reaching correct effort on a hard day and hitting 4 or 5.
If you combine monitoring your heart rate with a clear purpose for your run (Is this a hard or easy day? What am I trying to achieve today with this run?) you will know exactly how much to push or ease back at any time. Easy day? Stick to the HR zone that corresponds to your 2. Hard day? Get your HR up and hold it in your zone 4.**
[** Notice the word 'your' - each runner needs to individualise their zones]
There's something else special about using your heart-rate for training intensity moderation, and that's the fact that your good ol' ticker automatically reflects other stresses affecting your ability to push hard or go easy on any given day.
Had a crappy night of sleep? Your heart rate will often sit higher for a given pace or perceived effort, a reflection that the low-quality sleep has stressed your body. The outcome is that by training to heart rate you are forced to run slower (to keep your already elevated HR within your training zone), which places less running stress on your system - you are reducing your running stress to compensate for the additional stress from your poor sleep. Make sense? You're basically filling a water bottle with stress, and if you've already half-filled it with stress from a restless sleep you have less space left to fill it with running stress.
The same thing happens (an external stress elevating your HR) for numerous reasons - poor diet, a hot day, inadequate hydration (and usually I'm talking throughout the day, not just during your run), etc, etc. Your heart intuitively does the work in calculating the impact of all your daily external stress, and then gives you the answer as to how hard you can/should push at any given time. Neat huh?
There's a counter-argument that does the rounds that HR training isn't a reliable driver of training intensity because of this intrinsic variability, but in my opinion that's precisely the reason it is so useful. Those who argue against HR training generally have it stuck in their head that HR zones should correlate closely with your perceived effort and/or pace. For example:
--
Runner A has a Zone 2 of 125-135bpm, and generally their pace in the middle of this zone is 5min/km. Today though, Runner A has a mild head cold and hits Zone 2 (125-135bpm) at 5:15min/km (i.e slower running for the same HR).
"I normally run faster that this to hit zone 2 - so I'm going to run in Zone 3 today to make sure I get up to 5min/km".
--
In my opinion, this thinking is completely incorrect. The heart is telling Runner A that their mild cold has stressed their system, and that for today's run they should be running slower. It's being super clear on that point. To push into Zone 3 is too much stress for that run, and your heart is trying to tell you to moderate your effort to compensate for the existing stress. To go against this is ignoring the very reason HR training is so effective. If you're not comfortable with this, HR training probably isn't for you.
HR is just as effective for your hard days too, as it also gives you feedback as to when you are running too easy. Supposed to be in Zone 4, but chugging along in Zone 3? Run faster.
Remember - the middle ground is dangerous; most people run too hard on their easy days, and too easy on their hard days.
And that's why I sold my beloved Cortina.
-- This post isn't about the specifics of heart-rate training - it's particularly individual and there are plenty of books on the topic that do a better job of explaining the detail than I could in a blog post.