One piece of equipment I have used a lot in my training and I recommend to a lot of my clients is a heart rate monitor (or HRM). I find that a lot of people are ignorant of their actual effort levels when they are training, no matter how long they have been training. The tendency I have seen is that people, when they are supposed to have an easy- or moderate-effort training session (which, let's face it, will comprise the majority of training sessions when training for endurance events), will often push too hard. I'm not sure why, except that it seems most people feel like they are getting a "good workout" only when they are pushing hard enough to feel some pain. Seriously, I don't know why, but the endurance sports world seems to be full of people with masochistic tendencies. As a result, people don't always develop the aspect of their fitness that they think they are developing - maybe they think they are building aerobic endurance when they think they are training in what they think is a moderate aerobic zone, but they are actually training at a higher level that doesn't develop their aerobic endurance; rather, they are probably training a combination of lactate tolerance and anaerobic endurance. Of course, lactate tolerance and anaerobic endurance are, indeed, very trainable. But, you should be able to target different aspects of fitness training accurately, and HRMs can be one fairly inexpensive tool to help you do that.
So, using a heart rate monitor, accompanied by a genuine understanding of your heart rate training zones (see more comments below), can help a lot of people re-calibrate their training and get a lot more out of their sessions.
My attitude towards training with heart rate is also that it can add a very valuable extra dimension, but should not be slavishly followed to the exclusion of other aspects of your training. Even the most expensive, top-of-the-line HRMs can be prone to error, often at exactly the wrong time. Have you ever gotten to the starting line of the race, only to have the "Locating satellites, Please wait" message stay up on the screen for ages and ages, even after the starting pistol is fired?
Or, have you ever started a session out and the HRM is telling you that your heart rate is something ridiculous and totally unbelievable (e.g. it says 182 bpm and you are just starting out on a flat, moderate run)? So, you need to be able to develop other ways of measuring your workout and be able to gauge with confidence your speed or effort levels, alongside training with heart rate.
Another thing to be aware of with HRMs is something called "heart rate lag," which is the time it takes for your HRM to "catch up" when you change your levels of exertion (e.g. when running or riding up a hill). A combination of varying sample rates (how often the HRM receives a signal from the chest strap) and display calculation delays (the time it takes to calculate the raw data received from the chest strap and the GPS, then display all the data on the screen of your watch) means that it can take anything from 15 seconds to 1 minute for your HRM to "settle down" and display accurate HR data. This is another reason why you can't always slavishly follow HR data in your work-outs, especially when doing intervals.
And, finally, you have to remember that your maximum heart rate, as well as your heart rate zones, will be very different for different sports. This is especially important for triathletes to grasp, if you do decide to use heart rate training zones to guide your training. For example, when you are swimming, your body is mostly supported by the water, you are lying down in a horizontal position, and therefore your heart does not need to pump nearly as much as it does when you are, say, standing up and running, in order to achieve a maximum effort. So, your max HR and your training zones will be different when swimming and you should not use the same HR training zones as you would when, say, running.
Of course, very few people at the moment use HR zones in their swim training (it's really hard to check your HRM while swimming), and so they tend to use HR zones mostly for cycling and running. Again, the fact that you are sitting on the bike rather than standing up and supporting the full impact of your body crashing down on a single leg each time you take a step when running, also means that your heart does not need to pump as much when you are cycling as it does when you are running. In an ideal world (like a professional athlete), you would have a fitness test done to test your HR zones in each different sport. However, what most people do is have one fitness test done, and then adjust their target HR training zones for the other sports. For example, most people, if they have a heart rate/fitness test done on a treadmill while running, will then adjust their training zones for the bike by subtracting 7-10 bpm for each zone. It's not 100% guaranteed to be accurate, but it's not a bad way to get started, if you don't want to pay for 2 different fitness tests.
As for the types of heart rate monitors out there these days, there is a huge variety of options, that are roughly divided up the following groups:
Just measuring heart rate, usually with a chest strap. These can be very simple and reliable pieces of equipment and are usually the cheapest. They would typically look like a wristwatch with a few different display modes (perhaps just showing time and date in a "normal mode," and then perhaps showing heart rate and elapsed time in a "training mode"). Some of these HRMs also have some very useful interval timing features and can even store up to 100 laps' worth of stopwatch information (very handy if, for example, you are doing lots of laps in a pool and you want to look at your lap times over the course of the whole session).
The chest strap means that the heart rate measurements are more likely to be accurate (see wrist-based HRM, below), but it also means that it is a little more cumbersome. Some people don't mind wearing the chest strap, some people do.
HRM with GPS. Having GPS features can add a lot to the price (although, there are a few, good GPS-enabled HRMs available for under £100), but it also adds a lot to the functionality. In addition to the timing for your session, you also can get some pretty accurate and useful information about your distances and speeds throughout the session.
Some of the more expensive models in this range also have the ability to download data from each session to your computer and/or smartphone (either via a USB cable or Bluetooth or both). Companies like Garmin have tried to make a real selling point out of the software that they use to show you your data with tables, graphs and maps, making the feedback very detailed. For some people this is useful and fascinating. For some people, this is overkill.
Multi-sport HRMs. These tend to be the most complex, sophisticated models, and the one that triathletes are most interested in. Particularly, when a GPS-enabled HRM says it is capable of taking accurate measurements when swimming, that can be especially useful. You can imagine that, when you wear an HRM on your wrist and you are swimming by swinging both arms around in circles, a lot of models can't get accurate measurements of distance. There have been some big improvements in this area in recent years and a growing number of GPS-enabled HRMs provide accurate swim measurements, but they also tend to be some of the more expensive models.
Wrist-based HRM (aka Strapless HRMs). This is the newest area of growth in the HRM industry. By measuring your heart rate at the wrist, these types of HRMs don't need a chest strap, so they can be much less cumbersome and easy to wear. Sadly, they also tend to be much less accurate than the HRMs with chest straps. My personal experience is that even the most highly rated of this range will NOT give you reliable HR data, and the wrist-based models still have a ways to go before you can use their HR data as serious, targeted, reliable training tools.
However, it is also worth noting that these HRMs also tend to have tons of neat additional features, with pretty accurate GPS measurements, all kinds of interval settings, and so on. If you are doing sessions (e.g. hill reps) where HR data is not necessarily your primary focus, then these models can be useful.
Heart Rate training zones
If you are planning on training according to heart rate, then I would strongly urge you to get some kind of a fitness test (like a VO2max test or a Lactate Threshold Test). Otherwise, the recommendations for your heart rate training zones are likely to be wildly inaccurate and, therefore, of little value.
There are lots of places where people tell you that you can do various fitness tests on your own, such as FTPTs (Functional Threshold Power Tests) or CPTs (Critical Power Tests), that usually involve a 30-minute near-maximal effort with measurements of your heart rate beginning after 10 minutes. But, most of these people recommending these tests will also tell you that they will initially be pretty inaccurate, and will only get more accurate as you get more experienced as to how to pace yourself for a 30-minute near-maximal effort test. Most inexperienced athletes go way too fast in the first 10 minutes and then run out of steam about halfway through the session, and so their results are very inconclusive as to what their real Functional Threshold or Critical Power level is. So, sure, if you want to save some money, go ahead and conduct inaccurate tests and use inaccurate data for several months until you get it right. But, I would recommend you plunk down some money (usually around £100-150) to get a proper test done in a lab that will give you some useful data you can follow for many months to come.
And, whatever you do, please do NOT use the "220-minus-your-age" calculations that are recommended by so many equipment manufacturers (and even a shocking number of fitness web sites and fitness professionals). For example, here is what my recommended heart rate training zones (measured in bpm or beats per minute) would be if I followed those calculations:
Zone 1, easy, Active Recovery Zone, for warm-up and cool-down (60-70% of max): 101-118 bpm
Zone 2, moderate, Aerobic Endurance Zone, for long distances (70-80% of max): 118-135 bpm
Zone 3, moderate-hard, Tempo Zone, for tougher efforts like hills (80-90% max): 135-152 bpm
Zone 4, hard, Anaerobic Zone, for really hard, short distances (90+%): 152+ bpm
Max heart rate, calculated by taking 220 and subtracting my age (which is 51): 169
Now, here are results of my last HR testing, done on a treadmill:
Zone 1: 115-138 bpm - this is my Active Recovery Zone, where you can see on the other chart, they suggest my heart rate should be here for long-distance efforts at Zone 2...
Zone 2: 139-153 bpm - this is my Aerobic Endurance Zone, where you can see on the other chart, they say this is where my heart rate should be on tougher efforts like hills...?
Zone 3: 154-167 bpm - this is my Tempo Zone, which is actually a zone that most inexperienced athletes want to avoid, as it is too tough for building Aerobic Endurance and not tough enough for building muscular endurance or power on hills and strong efforts,and so it is important to understand exactly where this training zone is...
Zone 4: 168-175 bpm - this is my Threshold Zone, between Lactate Threshold and Anaerobic Threshold, and it is where most muscular endurance and power training occurs (I always find it curious that a lot of equipment manufacturers don't even refer to this training zone)
Zone 5: 176-188 bpm - this is my Anaerobic Endurance Zone, where sprint training and interval training is done
Max heart rate: 188 bpm
It is worth noting that there are also widely varying recommendations when it comes to HR training zones. Some coaches (like the very famous Joe Friel, author of The Triathlete's Training Bible) recommend more training zones, like 6 or even 8 of them, to get more specific focus in what you are doing when targeting a specific zone. I personally like to keep a little more simplified and I just stick with the 5 training zones described above.
As you can see, if I followed the first set of calculations, I would miss out entirely on the top two training zones in my training plans, and I probably would never develop any lactate tolerance or anaerobic endurance. I also might not get the fullest range of aerobic endurance performance benefits that I know I can get when I follow the second set of numbers in my training. For example, I know that I have run a marathon in my Zone 2, Aerobic Endurance Zone, at about 8 minutes per mile without going much above 153 beats per minute for most of the race, finishing in 3 hours and 28 minutes. If I followed the first, generic set of calculations, training and racing in the Zone 2 that they recommend, I reckon I would have done that same marathon in about 4 hours. I think that is a huge difference, I don't know about you.
That's why you don't take generic advice on heart rate training zones, if you plan to train with a heart rate monitor this year. And that's why I say that sometimes spending a little money on some professional support services can be the best use of your money. For example, instead of getting a top-of-the-line, super-flashy HRM that costs £300-400, why don't you get a good one, a decent reliable one, perhaps with fewer (arguably not-all-that-necessary) features for £150-200, and spend around £150 on a VO2max test in a lab somewhere, so you can really make good use of the HRM and the concept of targeted HR training zones? Maybe you, too, can shave 30 minutes off your next marathon, like I did, just by training with some proper data.