What is a power meter and why do I need one?

Question

So I've been looking to become a triathlete soon and I'm hearing all this talk about power meters.

Now, upon googling I found vague information which is why I came here. Exhibit A. That article gives me some sort of idea but it's not good enough, questions that remain unanswered are:

1. Is this another toy to measure data when you train so you know how you've trained?
2. Does it help in any way during a training session?
3. More details… how does it all work? There are heart rate zones and I'm told to run/cycle in a certain zone while training … anything similar for power meters?
4. Lastly and most importantly, I am a beginner/intermediate (ran a few mediocre run and cycle races).
   Will a sports watch (say Garmin Forerunner 920xt with the run HRM) be good enough?

Answer 1

Since you say you're looking to become a triathlete soon it's far too early to be thinking of advanced training aids like power meters.

The first few things to do (not necessarily in this order) are

• join a tri club

• enter a triathlon or two

• join a tri training squad

• observe your (comparative) strengths and weaknesses

• get a well recommended triathlon book.

• use a GPS app for recording and comparing your training and race efforts (eg Strava, mapmyride)

• use a heart rate monitor to help find and track your training levels

It's easy to spend lots of money in the first blush of enthusiasm, only to find later that it was wasted.

Take it a little slower, and make sure you're getting good advice. When talking up a new sport it's easy to go about it the wrong way and cause early injuries, or develop poor habits or techniques that cause long term problems. This applies to all three triathlon legs.

In the case of the cycling leg, you need to get cycling advice from a seasoned triathlete. Triathlon cycling is quite different to normal road racing, especially as the distances increase. You'll be trying to avoid using the muscle groups that provide the drive for the run leg, using a much lower cadence than a roadie would, and using a different bike.

The time to consider a power meter is when you're at the stage where you have a training bike and a race bike, and both are worth more than your car.

Answer 2

They do help with training and racing but they are also very expensive. As you say you are a beginner I imagine increases in fitness/strength (and therefore speed) are going to come fast anyway, even without a power readout to base training around. I would definitely invest in a HRM though and make sure the bike computer you use has cadence as well as speed.

In terms of specific questions, most are covered by the article you linked.

1. Yes, but it can be a useful one. Knowing what is working for you and what isn't can help guide training.
2. Yes, you can plan sessions on power and make sessions repeatable without having differences like weather/route affect the intensity. Intervals can be done to specific ranges of intensity.

3. As for how it works, there are different systems. Some (like the powertap systems) are based in the rear hub and measure the force at the back wheel. Others take their readings from the cranks. Some (eg Garmin vector) take force readings from the pedals. Generally the ones at the crank end of the drivetrain also allow analysis of left/right balance as well as overall power. Some (the cheaper version of the Garmin vector) measure the force on a single side and multiply by two. While cheaper, this isn't as accurate as most people won't be applying power exactly equally through both legs.

   In terms of the actual sessions, I've not used one but I think the basis is the same as for training with HR. Intervals/intensities are based around your max power (IIRC it's generally based on max power for an hour).

4. See above. I wouldn't invest in one at this stage. Train without one for now and you will still see vast improvements. After a while if you find yourself getting more into it or your progress slows, think about it again.

I don't do triathlons so can't comment on the best devices for it but for cycling, I have a Garmin 810 that has everything I need (and supports a power meter if I ever choose to get one). The 510 also has a similar set of features and I've heard good things about them. Having speed, HR and cadence in front of me when training is useful.

Some useful reviews of different systems are on the all-encompassing site of DCRainmaker

Answer 3

From what I've read, adding a power meter betters measuring HR only, for some reasons:

Heat, diet and stress can affect your HR. A low HR might be an indicator that you are in good shape. You can have a high HR and your power output be low

An increase in power implies better performance, but an increase in HR does not necessarily.

So it is good to combine both for serious training, to determine you are getting good:

Low HR and normal or high power compared to previous training would mean good condition. High HR and high power would mean you're still getting better Low HR and low power would mean problems.

So, HRM tells you about the workload your body is sustaining, while power meter tells you about how is your body performing.

Having a record of how your power output evolves through your training season would be more useful than knowing your HR. You can establish some power output you want to reach, and some days you'll get that power with less or more HR.

(Concepts taken from the book The Cyclist's Training Bible by Joe Friel, which mentions a book on the subject: Training and Racing with a Power Meter by H. Allen and A. Coggan)

Answer 4

This is an attempt to provide more detail than the previous answers about why a serious cyclist might want a power meter.

Different cycling events might place demands on different energy systems. Century rides, time trials, triathlons (the cycling bit), and similar events demand steady power output. Road races, criteriums, and cyclocross races often demand multiple efforts at or above one's maximum aerobic capacity. Serious cyclists who want to optimize their capability for a specific event should consider targeted training programs. Targeted training can enable cyclists to make optimal use of their training time, rather than just doing unstructured rides. Targeted training is definitely not necessary to enjoy yourself while cycling, however.

One could use perceived effort or heart rate to target training. After all, to the extent that cyclists had any targeted training in the past, they likely used perceived effort for most of cycling history. However, as discussed in other answers, power is a measure of your body's actual output. Heart rate and perceived exertion are measures of how hard your body is working, akin to the car's tachometer. However, temperature and other environmental changes or illness could have you working much harder to produce the same power output. Moreover, heart rate lags our efforts, and it can be a poor guide for short intervals. Perceived effort is a very rough guide. Speed, or time to complete a segment of known length, is another possible metric, but it involves having segments conducive to the training objective and is influenced by environmental conditions (e.g. wind speed) and traffic. Thus, power is a better guide to targeting one's output level than the other criteria mentioned.

Power, specifically FTP, can also offer an objective assessment of how much one has progressed. Again, best time on a segment of appropriate length could be an alternative measure, but it's influenced by environmental conditions, and it requires a segment of appropriate length.

The obvious downside of structured training using power on your primary bicycle is that you have to buy a power meter and a head unit capable of reading power. These items have come down considerably in cost since the 2000s, but they are objectively expensive. Many serious cyclists may have more than one bicycle. Transferring power meters across bikes takes time and effort, and the meters may not be easily transferrable (e.g. if you have a pedal power meter based on Look Keo road pedals, you may be unwilling to use them in a gravel event and you may be unable to do so in a mountain bike race). Less obviously, one has to commit to a training plan to use power optimally. This requires your own time and effort to track and plan workouts, or it requires an expense to outsource that job to a coach. Most cyclists do not need power to see considerable progress provided they put in the time to ride, and provided they build in adequate recovery time.