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 or triathlete might want a power meter. In my view, power-based training can definitely help a lot of riders. However, it requires some cognitive effort to use properly. That effort can be your own or outsourced to a coach. It's definitely not required to progress.

Different cycling events place demands on different energy systems. Century rides, time trials, triathlons (the cycling bit), and similar events demand steady sub-threshold power output. Road races, criteriums, and cyclocross races often demand multiple attacks, usually well above threshold. Targeted, structured training using power is more precise at getting you the sort of stimulus you want. For a long-distance triathlon, you may want mainly tempo and sweet spot intervals. Road racers probably mix VO2max, threshold, sweet spot and possibly anaerobic intervals, and you probably heard advice not to add too much tempo (which means don't take what should be a zone 2 ride and make it zone 3). Criterium racers might lean even more to the anaerobic side. Power measurement also offers a way to assess progress, and a way to measure training load.

An alternative is to just add volume, and if you need intensity, you can often get that in group rides or races. Past a certain point (varies by person), if you just add volume, you're likely to hit a ceiling. But this works for a whole lot of riders!

Another alternative to power is to use perceived effort or heart rate to monitor sessions, and overall fatigue to monitor training load. After all, in the past, cyclists did base miles in the off-season and then they race themselves into shape. As discussed in other answers, this isn't as precise. However, it's definitely possible. For VO2max intervals, you go as hard as you can for 5ish minutes - you don't need to be looking at your power meter for that. It's also possible to get a good sense of what your threshold power feels like, and try to hold that.

As an alternative to measuring progress, you can track some segments in Strava. Yes, this is influenced by environmental conditions, but it's still useful. I'd try to find a 4-5 min segment, usually a hill. Those who live near extended climbs that take 20-60 minutes could use one of those. Or else you can find quieter roads and create a time trial course for yourself.

If you have the choice between either a power meter or a smart trainer, the latter is actually worth considering seriously. Smart trainers make it more feasible to ride through the cold months. Or, while some might think of you as boring, they can actually make structured intervals during the outdoor riding season more feasible, because you don't have to take time to drive or ride out to that hill. For your own rides, you'll need to get a sense of what doing zone 2 (long slow distance) feels like and discipline yourself. Either that or you'll be group riding with others, anyway. In summer, heat management indoors is a problem, and you will need adequate cooling. For example, I often have to use my air conditioner and a fan. Or you could get multiple powerful fans.