FAQ for power-based training (version 12.04)
by Charles Howe, with contributions from
members of the Wattage Forum at
Topica.com
CONTENTS
Q:
What is power?
A: To Henry Kissinger it was an aphrodisiac, but for
our purposes, the definition comes from physics, and in particular the science
of dynamics, which is a branch of mechanics.
Power is the rate of doing work or transferring energy, such that power
= work/time, or P = W/Δt. As relates to cycling, it
is measured in international system (SI) units called Watts (W), rather than the
familiar english unit of horsepower that is used as a measure of engine power
(1 horsepower = 746 W). Since work =
force applied through a distance, or W
= F × Δx, these two expressions can be combined and rearranged to express
power as the product of force and speed, i.e., P = F × s, and this may be the best way to think
of it: the speed you can maintain times the total force resisting your forward
motion. Similarly, power can be defined
as pedal force (i.e., torque, which equals = [measured frequency - zero offset]/slope)
× cadence, which means you can increase power by exerting more force on the
pedals at a given cadence, by increasing cadence while exerting the same pedal
force, or by increasing both force and cadence.
Here
are some examples that give an appreciation for units of power:
A 68
kilogram (150 lb) rider traveling on an 8.6 kg (19 lb) bike at 20 mph in on
flat ground in with no wind requires about 177 W.
56.5
W are required to raise a 20 lb dumbbell
25 in. overhead in one second.
Q: Why is
power-based training so important all of a sudden?
A: It’s no more so than it was previously, in fact,
exercise physiologists have used calibrated ergometers for years to impose
precise loads on study subjects. Rather,
the introduction of affordable on-bike power measurement systems (power/speed measuring
device, handlebar-mounted computer, receiver/wiring and computer mounting bracket,
download interface, software) have made it possible to use power in everyday
training as well as racing, then analyze the resulting workout data. This, and their widening use among both
amateur and professional riders has generated considerable “buzz.”
Q: Why should
I train by power?
A: (Eddie Monnier and Andrew Coggan) Because it is the objective measure of
exercise intensity, and as such directly determines physiological and
perceptual responses to exercise, so training by power provides immediate and
quantitative feedback on the intensity of effort. 300 Watts is 300 Watts, no matter how hot,
windy, or hilly it is, or what your heart rate is – though it may “feel” easier
or harder, depending on various conditions.
Three
variables to control in any training program are intensity, duration, and
frequency; of these, the latter two are easy to quantify objectively – duration
is measured in hours, and frequency in sessions per week (the product of the
two is volume). Intensity, on the other
hand, has traditionally been measured by perceived exertion (PE) and/or heart
rate (HR). HR is reliable enough at
lower (i.e., aerobic-only) intensities, but for more race-specific (i.e.,
shorter but more intense) training, it becomes a much less effective proxy for
intensity. Besides being subject to numerous
environmental and physiological variables (e.g., temperature, humidity, hydration
status, altitude, overtraining, lack of sleep, nervousness, and upward “drift”
as exercise progresses), HR responds slowly to workload demands, and thus is a lagging
indicator of effort. That is, it will be
lower than power and during the early part of an effort, and higher
afterward. For example, if you bound up
a few flights of stairs, your heart rate will take a while to reflect the
effort, and will continue to beat at an elevated rate for a while even after
you have stopped climbing steps. The
shorter the duration of an effort, the less useful HR is.
Q: So
power-based training has made perceived exertion and heart rate obsolete?
A: Not quite, but they seem to have been relegated to
a definite second and distant third, respectively! Many still cling to HR an indicator of
overtraining – though declining power for a given PE is the deciding (and often
first) sign of that, too. Nonetheless, there persists some popular, if not
scientific controversy as to the role of HR, with some claiming that it
indicates metabolic intensity, and therefore one should train by HR, while
monitoring power. In fact, just the
reverse is true; particularly during outdoor cycling, metabolic load is more
accurately reflected by power, integrated with PE, the latter being more
reliable than HR and incorporating more physiological variables. Power provides an objective standard by which
effort can be quantified, thereby ‘calibrating’ PE, while PE serves to modulate
power output.
Q: How do I measure
power – I mean, what are some of the various power-measuring systems available?
A: Here are the four bicycle-based systems presently
available, with links to each manufacturer’s site:
Ergomo Sport (a torque-measuring bottom bracket available
in Campagnolo square-taper or Shimano OctaLink): http://www.ergomo-usa.com and www.ergomo.de/eng_main.html
Polar
S-720i or S-710i (uses a chain vibration sensor that mounts on the right
chainstay): http://www.polarusa.com/consumer/productfinder/productfinder.asp
or http://www.polar.fi/power_output. User’s manuals are on-line at http://tinyurl.com/xyny
and http://tinyurl.com/xys4
PowerTap
(a torque-measuring hub that you
build into a wheel): http://www.power-tap.com/
SRM Powercrank (a torque-measuring
crank that replaces your present model): http://srm.de
and http://www.thebikeage.com
Note: contrary to claims, Ciclosport models do not actually measure power, rather,
they only give a rough estimate based on speed, total mass (rider/equipment),
and road grade, which may be accurate on steeper grades, but is useless on flat
terrain, particularly in group rides or if any wind is present.
Finally,
you don’t need a high-tech gizmosystem to figure your power. For instance, you can use a hill with a
steady grade of ~7% or more by timing yourself over a measured portion of it, and
then calculate power quite accurately (so long as air was sufficiently calm) using
http://analyticcycling.com. You can even get a consistent estimate
running up a constant grade or a flight of continuous steps, such as in a
stadium:
Q: How do power-measuring
devices work?
A: (Garth Rees and Charles
Howe) The various on-bike systems
measure force the applied either at the crank (SRM), the rear hub (PowerTap), crank
spindle (Ergomo), or chain (Polar). (Note:
a patent was granted to Shimano in November 2003 for a torque-measuring bottom bracket, U.
S. Patent 6,644,135.) The former two use strain gauges,
which are fine polymer sheet, with ultra-fine wire or foil sandwiched in it,
and the electrical conductivity of the metal changes as they are twisted or
deformed when force is applied, due to the secure bonding to the material under
test (the energy absorbed by the strain gauge is so close to nil that it can be
neglected in any loss equations). Strain
gauges are fragile when not bonded, and typically no bigger than your small
fingernail, often 2 × 4 mm or smaller, depending on application. They may be in single, half-rosette (2
gauges, 90° offset), or full rosette (4 gauges, all at 90° offset, i.e., 2 opposed
half-rosettes) configuration, with the last having the best accuracy of all,
since it compensates best for the strains in the two major axes, resulting in good
self-cancellation of any errors in the two devices. The difference in accuracy from half to full
rosette is not as great as is the implementation cost. Here are pictures of the PowerTap hub
mechanism (U.
S. Patent 6,418,797) from cyclingnews.com
and bike.com
, showing the strain
gauges in a full-rosette arrangement:
The
strain gauges measure torque inside the hub, then this data is transmitted, along
with wheel speed, to a seatstay-mounted receiver via digital radio frequency
(RF) waves, and then by wire to a handlebar-mounted computer with a 16-bit
microprocessor, where they are used to calculate instantaneous power, road speed,
cadence, etc.
Similarly,
the SRM senses torque exerted at the crankset, then multiplies it by crank rpm (cadence),
measured with a crank magnet and sensor, to give power.
Both
Polar models measure chain tension via a chainstay-mounted sensor that detects
vibrational frequency; just like a guitar string, a chain vibrates faster as
its tension goes up. This is translated
into an amount of force, which is then multiplied by chain speed, as measured by
an optical sensor mounted on the rear derailleur, thereby giving power output:
power (W) = chain tension (N) x chain velocity (m/s).
Finally,
the Ergomo Sport uses a bottom bracket with a photointerruptor circuit actuated
by two “combs,” or flat discs mounted on the bottom bracket spindle, each having
numerous slots spaced evenly in a radial fashion. Two optical sensors measure changes in the
alignment of the slots, which is determined by how much the spindle twists, and
hence how much torque is being exerted.
This value is then multiplied by crank rpm (cadence), which is measured
by the bottom bracket unit, thereby yielding a value for power.
Q: Where can I buy a
power-measuring system?
A: Check with your local bicycle or triathlete shop,
or the manufacturers’ web sites for dealer listings. You may also find the products in cycling
catalogs and/or on the web, and many coaches are also dealers for the several
systems.