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If you've decided to engage in the popular practice of performing a race or workout at a particular heart rate (or fixed percentage of max heart rate), you've eliminated a big problem: you won't have to worry about estimating your actual cycling or running speed during your exertion or try to judge the overall quality of your effort by how you feel

Your effort - at least your cardiovascular effort - will be precisely defined by the reading on the face of your heart-rate receiver. If you have an upscale monitor, any excessive deviation from your desired pulse will trigger warning whoops from your receiver; with a low-end model, you'll simply need to glance at your receiver every minute or so to find out if you're doing the right thing. You'll be able to cruise through your whole race or workout at the exact heart rate you want, without worries about your actual velocity

Using a monitor can be pretty relaxing; during workouts, you can focus intently on your running form and how you feel, listen for the occasional communiques from your receiver, and just let the miles roll. With a monitor, there's no need to be concerned about whether you're exceeding a level of cardiovascular effort which you know you can handle. However, in spite of this ease, precision, and comfort, if you use a monitor to measure the intensity of your workout or race, you're probably headed for trouble

'Cardiac drift'
The trouble will come in a variety of ways, but a key source of difficulty will be something called 'cardiac drift'. This phrase simply refers to your heart's perverse tendency to avoid a constant rate of functioning. More specifically, cardiac drift means that your heart rate tends to rise slowly but steadily as you exercise, even when you're cruising along at a constant pace. And the magnitude of this drift is usually more than just a pesky beat or two: heart rates can rise by as much as 20 beats per minute during constant-velocity efforts lasting less than 30 minutes!

There's no need to worry about why cardiac drift occurs, although staying well hydrated before and during your effort can partially control - but not eliminate - your heart's tendency to beat faster and faster (if your exertion is going to last for about 40 to 45 minutes or more and you're going to be sweating fairly profusely, you should try to thwart drift by drinking 12 ounces of fluid before you start and taking in three to four swallows of liquid every 10 minutes thereafter)

If you monitor your efforts by using heart rate, you do need to consider what effect drift will have on your exertion. Basically, if you're locked into a particular heart rate for a race or long workout, drift will force you to run slower and slower as the effort proceeds, even though you have the ability to maintain your even pace. For example, if you're running, you might be cruising along fairly comfortably at seven-minute pace and a heart rate of 160, until drift sends your ticker up to 166. If you're too in love with your heart rate, you would ease off on your pace until you simmer your cardiac rate down to 160, and you would suddenly find yourself at 7:15 tempo, instead of the seven-minute effort which you actually could handle. In a race, that would leave you with a disappointing time; in a workout, you would spend less time practising your goal pace - and therefore develop less efficiency at that pace. Most of the time, it's better to just let heart rate rise slowly and steadily during your effort (as long as you're still feeling okay). Let fatigue - not the gadget on your wrist - be your guide to what you can do

A range of paces
Of course, another problem is that a specific heart rate - the one a coach has recommended for a race or workouts, or the one you've decided to use based on a recommendation in a newsletter or book - is going to produce a variety of different cycling speeds or running paces during your training. That's because heart rate is quite sensitive to environmental conditions - and your psychological state. Generally, your heart rate is going to be higher than usual when the weather is hotter or more humid - or when you're more tense and irritable

To see what can actually happen, let's say that you're a runner and you want to develop the ability to run a half-marathon at 90 per cent of your max heart rate - a laudable goal. And let's say that - in deference to the specificity of training principle - you've decided to run a variety of different workouts at that specific intensity. That sounds good in theory!
The first time out, on a fairly hot and humid day, you run for an hour at your desired heart rate - 90 per cent of maximal. Your average running pace for the whole workout turns out to be seven minutes per mile

The next time you train at 90 per cent, it's a perfect day for running - cool and dry. You zip along for an hour again at 90 per cent of max, but when you get through, you discover a startling fact: your pace was 6:45 per mile!
The third time out, it's hot and humid and windy. You're still stuck like glue on 90 per cent of max heart rate, though, and so your hour passes at a comparatively lethargic pace of 7:20 per mile (remember that when it's hot and humid, heart rate rises more quickly than usual, bringing you to a specific rate at a slower running pace; running against wind compounds the problem)

On your fourth encounter with 90 per cent of max heart rate, weather conditions are fine again, but you've just had a fight with your spouse. You're tense and excitable, sending your heart rate to higher-than-usual levels. So, you reach 90 per cent of max too easily. In fact, at 90 per cent, your running pace is only 7:30 per mile

Suddenly it's race day, and by golly you're pretty sure you can handle the half-marathon at 90 per cent of max heart rate. But when you finish the race, are people going to ask you, 'Hey, what heart rate did you have out there?' Or will they ask you about your time? And are you going to care more about your heart rate or your actual finishing time?

Heart versus legs
The point is that if you have even an ounce of competitive spirit, you're going to be more concerned about your overall performance time than the rate at which your heart was flapping during the race. Paradoxically, though, you've been training to run the race with a particular heart rate - not in a particular time. You're at the mercy of your heart - and that expensive strap you've got around your chest. Wouldn't it make more sense to choose a sensible goal pace for your half-marathon (say about 10 to 15 seconds slower per mile than 10K velocity), a pace which will bring you to the finish line in the time that you want, and then learn to handle that pace under a variety of different conditions during training? Practising that pace will give you the precise neuromuscular coordination and the exact leg-muscle functioning that you'll want on race day. Who cares if your heart strays above some pre-defined rate of ticking? Believe me, it will be none the worse for wear on the following day

Basically, you have to make a decision about your training. You know that environmental conditions and your psychological state are going to vary on different workout days. Higher temperatures and humidity will send your heart rate up, as will tension and anxiety; cool weather and calmness will bring it down. You can stick with a specific heart rate - and therefore let actual running pace wander all over the map. Or you can stick with a specific pace - and let heart rate vary enormously. Which is better?
Obviously, sticking with a pace and letting heart rate vary is preferable. As mentioned, attaching yourself to a pace teaches your leg muscles and nervous system to function more effectively at that goal speed. The more you practise the pace, the better will be their coordination and efficiency - at that pace. The less you practise the pace, the lower will be coordination and economy

In contrast, the heart's coordination and economy do not vary. The heart doesn't need to practise beating away at 90 per cent of maximal to get good at it; it already has that down pat. It's just as efficient at 90 per cent of max as it is at 87 per cent of max - or at 93 per cent

Don't worry about your heart getting tired
Basically, your heart is pretty much along for the ride. It will do what your leg muscles tell it to do (within limits, of course; the legs can't tell the heart to beat faster than max heart rate, for example). If your heart's been whacking away at 93 per cent of max for a good deal of time, it will never shout down to the leg muscles, 'Hey chaps! You've been pedalling (or scampering) for long enough. I'm getting tired, so will you please slow down?'
In fact, your legs will become fatigued far faster than your heart does. The heart will slow down if the leg muscles slow down, not the other way around. That's why the focus of your training should be on your leg muscles - that is, on the pace created by the leg muscles. Your goal should be to develop greater fatigue resistance in those leg muscles at your desired running paces or cycling speeds. You don't have to worry about the heart getting fatigued: that old fellow can pound away at high rates for long periods of time. Your leg muscles are your weak link

And yet training based on heart rate makes the heart dominant and the leg muscles subordinate - just the opposite of what should occur! If you really want to run a race at a goal pace, practise that pace, not a heart rate. You can let your heart rates roll around a bit

After all, the heart is an imperfect indicator of what's happening to your leg muscles. An increase in heart rate might indicate increased stress in your leg muscles, or it might just represent tension, drift, or the fact that a little more blood has settled in the skin on a hot day. Don't enshrine an imperfect indicator as the absolute dictator of your training and racing

What is threshold heart rate?
While we're on the topic of heart rates, we should point out that there's an incredible amount of information floating around about how to train with a heart monitor, but - unfortunately - a lot of it is worthless. For example, you might read or hear that the best training intensity for raising lactate threshold - the key indicator of running performance - is 82 to 88 per cent of max heart rate. If someone tells you that, you've learned something important: never trust what they tell you about your training

That's because - first - there's no scientific evidence that this is true. In fact, the available research says that - for runners - running at 10-K pace (which is often around 90 to 93 per cent of max heart rate) is the most time-efficient way to boost threshold - and the method that produces the biggest increases. Second, while it's true that training at your threshold can probably raise it pretty well, too (that's why 'tempo workouts' are so darned good for you), threshold heart rate varies considerably between individuals. For example, for some athletes threshold occurs at 65 per cent of max heart rate. In others, it's at 75 per cent. Experienced, competitive athletes often check in at 85 to 88 per cent or so, and some of the elite Kenyan runners don't reach threshold until they get to 92 to 94 per cent of max

The bottom line? To lift threshold, you're better off forgetting about heart rate and training at 10-K pace or a little slower (if you're a runner, 10-minute intervals at 10-K pace represent a particularly good way to train). If you're a cyclist, swimmer, or cross-country skier, it's wise to carry out 10-minute intervals at an intensity you could sustain for no more than 30 to 35 minutes or so

Some coaches get really high-tech and measure heart rates at various blood-lactate levels. They then define workouts as 'easy' or 'aerobic' if they're at a heart rate below the rate which produces a lactate concentration of 2 mmol/L, and they say that 'threshold' workouts are at the heart rate which lifts lactate to 4 mmol/L, while 'hard' efforts are at heart rates associated with lactate above 4 mmol. Their role is then to merely find the right balance of easy, threshold, and hard efforts

That's great, except for three little things: (1) If training is going well, the paces associated with 2 and 4 mmol/L of lactate will increase steadily over time, so finger pricking for lactate detection will have to take place on a regular basis. (2) Threshold doesn't always occur at 4 mmol/L. Some athletes reach threshold at 2 mmol, while others don't hit it until 7. For those individuals, training at 4 may be too heavy or too light to be a real threshold session. (3) Heart rate varies according to environment and mood, but threshold does NOT follow heart rate up and down. Threshold is a function of how hard the muscles are working - not how fast the heart is beating!

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