Wattage? WTF?

Posted: October 14, 2011 in Bike, Equipment
Tags: , ,

Two years ago it was, “Hey Terry, you need to get a cadence reader on your bike”…so I did.  In cycling, cadence refers to the number of revolutions of the crank per minute; roughly speaking, this is the rate at which a cyclist is pedaling/turning the pedals over.  I know, ‘big whoop’, right?

However, once I began focusing on my cadence instead of my speed, per se, I learned how to keep that cadence rate, roughly, around the 95-100 rpm  mark which is also the same approximate speed that my legs turn over while running off the bike.  In this way, I trained my legs to be efficient at managing a particular pace for long periods through the bike-run transition and this knowledge (and conditioning) has served me very well throughout the past two seasons of triathlon.  It was what you might refer to as a ‘break through’ of sorts.

This year, it’s an entirely different ball game as the new spin bikes at our local YMCA have now been equipped with new computers that, besides cadence, also read our power output (measures in watts).  So for whatever reason, our typical spin-slash-Brick class was instantly taken to the next ‘Holy shit!’  level of difficulty and the bar has been raised yet again.  But why is this particular power meter reading so different from anything else I’ve ever trained to?  I’m still pedaling my ass off as hard as I ever was, was I not?

A quick review on Wikipedia gave me this:

Power meters provide instant feedback to the rider about their performance and measure their actual output; heart rate monitors measure the physiological effect of effort and therefore ramp up more slowly.  Thus, an athlete performing “interval” training while using a power meter can instantly see that they are producing 300 watts, for example, instead of waiting for their heart rate to climb to a certain point.  In addition, power meters measure the force that moves the bike forward multiplied by the velocity, which is the desired goal. This has two significant advantages over heart rate monitors: 1) An athlete’s heart rate may remain constant over the training period, yet their power output is declining, which they cannot detect with a heart rate monitor; 2) While an athlete who is not rested or not feeling entirely well may train at their normal heart rate, they are unlikely to be producing their normal power—a heart rate monitor will not reveal this, but a power meter will.  Further, power meters enable riders to experiment with cadence and evaluate its effect relative to speed and heart rate.

That’s all well and good, but I’m still feeling like a blind man at an orgy here so let’s start from the beginning.  For a cyclist, the force is how hard he pushes on the pedals and the speed is how fast he turns the crank (rpm).  The combination of these two things results in the cyclist’s power output, which is usually expressed in watts.

A human being traveling on a bicycle at low to medium speeds of around 10-15 mph (16–24 km/h), using only the power required to walk, is the most energy-efficient means of transport generally available.  Air drag, which increases roughly with the square of speed (“aerodynamic drag force is proportional to the square of the velocity”), requires increasingly higher power outputs relative to speed, power increasing with the cube of speed as power equals force times velocity.  Sure, whatever.  I’m no Einstein but that equates to ‘amount of work’ in this miniature squirrel-sized brain of mine.

Anyway, on a firm, flat, ground, a 70 kg person requires about 30 watts to walk at 5 km/h. That same person on a bicycle, on the same ground, with the same power output, can average 15 km/h, so energy expenditure in terms of kcal/(kg·km) is roughly one-third as much.  Generally used figures are

  • 1.62 kJ/(km∙kg) or 0.28 kcal/(mi∙lb) for cycling,
  • 3.78 kJ/(km∙kg) or 0.653 kcal/(mi∙lb) for walking/running,
  • 16.96 kJ/(km∙kg) or 2.93 kcal/(mi∙lb) for swimming.

Amateur bicycle racers can typically produce 3 watts/kg for more than an hour (e.g., around 210 watts for a 70 kg rider), with top amateurs producing 5 W/kg and elite athletes achieving 6 W/kg for similar lengths of time.  Elite track sprinters are able to attain an instantaneous maximum output of around 2,000 watts, or in excess of 25 W/kg; elite road cyclists may produce 1,600 to 1,700  watts as an instantaneous maximum in their burst to the finish line at the end of a five-hour long road race. Even at moderate speeds, most power is spent in overcoming the aerodynamic drag force, which increases with the square of speed (see above)…and, believe me, this ass of mine creates some significant drag. Thus, the power required to overcome that drag increases with the cube of the speed.

So, to maintain a cadence of 100 rpm  at a resistance level of 12 on the spin bike yesterday or, what should be my recognized comfortable ‘race pace’ on my time trial bike come race season, my exertion was measured at 264  watts.  Well knock me down and call me Susan, was that ever a huge difference in the workout that I’ve become accustomed to!  It felt like my legs were being ripped from their sockets and my hamstring muscles felt like they were being lit up with torches.  That 264 watts then, is the new magic number to focus on this year until it becomes either easier to maintain or significantly increases as a result of my own physical conditioning.

In essence, I’m now able to see and experience what it feels like to maintain a certain measure of required force as it relates to the cadence I am trying to keep, particularly when fatigued.  This will be good mental conditioning as it will be physical and help better transform me into a lean, mean, pedaling machine come Ironman time next September…

…providing I don’t experience a massive coronary meltdown by then that is.

  1. chuck t says:

    No more faking effort at YMCA spinning sessions!

    • Chantal says:

      Seriously..it really sucks that you can’t fake output anymore. Next thing, Bill will get some thinga- magig that will let him monitor everyone’s power output. DAMN!!

  2. Lori Pajtasz says:

    Yikes – I’m now very afraid to return to that class!!!

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