Dynamic Bike Fit with the CompuTrainer’s SpinScan
Takes the Guesswork out of Positioning
By Sean Drake
Proper bike
fit is a marriage
between the
human body, which
is somewhat
adaptable, and a
machine that
is somewhat adjustable.
Dr.
Andy Pruitt, EdD
Do you think I should lower
my aero bars? If I switch to a pedal system with more float to save my knees,
will I compromise my power? How much should I move my saddle forward to
optimize my aero position? It seems that no matter what the question, the answer
usually goes like this, "Well, that depends. Why don’t you try it and see
how it feels?" Herein lies the dilemma. Most of us have probably
never been in that optimal position to begin with, or haven’t been lucky enough
to have logged the 150,000 miles of saddle time necessary to actually know what
it is supposed to feel like. Some gurus claim that they can pick out the
idiosynchrocies of one’s bike fit with just a glance, but not all of us have
our own personal guru to seek out every time we want to change our bike
position. Dr. Andy Pruitt, EdD sums up proper bike fit as " a marriage
between the human body, which is somewhat adaptable, and a machine that is
somewhat adjustable" .1 Dr. Ed Burke, PhD goes on to add,
" In fitting the bicycle to the cyclist, one needs to adjust the bicycle
to the cyclist so that the cyclists has to adapt as little as possible ". 1
A dynamic bike fit utilizing the SpinScan pedal stroke analyzer of the CompuTrainer
by RacerMate (Seattle, WA) is an excellent way to take the subjectivity and
guesswork out of the bike fit process.
A dynamic bike fit is an
iterative process whereby the graphic video biofeedback provided by the SpinScan
is used to evaluate the impact of small adjustments to bike position. The
SpinScan feature provides a multi-color torque graph, which represents one full
360-degree pedal revolution divided into 15-degree segments. It will identify
"flat" or "dead" spots in the pedal stroke where optimal
power is not being transferred to the drive train. This feature is given a
quantifiable measure with the SpinScan Number, which is the average
torque divided by the maximum torque times 100. If our muscles were able
to produce equal torque through the full 360-degree pedal revolution, the spin
scan number would be 100. While this is physically impossible, a number between
70 and 90 is considered to be a very efficient transfer of power.
The high "peaks" and low "valleys", or
flat spots, in the Spin Scan at the top represent a choppy, pedal mashing style
of cycling that does not maximize power distribution. The Spin Scan on the
bottom, with the flat and even bargraph, shows a more optimal pedaling
mechanics as reflected in the SpinScan Number of 75.
Dr. Tim Sandell, MD, of the
Pike’s Peak Physical Medicine Clinic in Colorado Springs states that the
balance of power distribution between the left and right leg as a percentage of
100 can identify strength imbalance as well as bone structure (leg length
discrepancies) or soft tissue (flexibility) limitations. Once identified, a
qualified professional such as a sports medicine physician, biomechanist,
chiropractor, or massage therapist can address these issues.
This is the Polar
Spin Scan display. This cyclist’s non- symmetrical pedaling shows up as an
"unbalanced" peanut as well as a 35/64 split between the left and
right leg respectively.
This torque information,
along with power and heart rate data, allow the user to make objective
evaluations of the effects of lowering aero bars or moving the saddle forward
into an extreme position.
The Coaching Software feature of the CompuTrainer
provides a graphic data plot of power, heartrate, velocity, cadence, and
spinscan in relation to each other in much the same manner as the SRM
powermeter. This data is critical to making objective evaluations of equipment
selection and bike position.
A dynamic bike fit was the
perfect way to solve an equipment choice facing a professional duathlete
preparing for the 1995 the ITU World Duathlon Championships in Cancun, Mexico.
The course was known to be relatively flat, but the wind conditions off the
ocean were questionable. The equipment dilemma that we couldn’t decide on was
whether to use a standard shoe/pedal combination or platform pedals for a quick
transition? Would the time saved in a quick transition be negated by a lose in
power transfer? It was decided to answer the question in training where a
critical mistake would not cost a spot on the podium. A short time trial course
was set up on the CompuTrainer, and we ran several trials over various grades
and wind conditions to evaluate the effects of the different pedal systems on
power output at a given heartrate. In the end, a critical performance question
had been answered without risking a race result and gave this athlete
confidence in her equipment going into the competition.
Experience has shown that
after an initial pedal stroke evaluation with the SpinScan, most people can
improve their pedaling efficiency by as much as 20-25% in their first session
alone simply by making improvements to bike position. Once in the optimal
position, additional gains of 10-15% are possible using the feedback from
SpinScan while riding courses and doing drills that address the weak areas of
the pedal stroke. For example, by simply concentrating on "wiping gum off
the bottom of your shoe" at bottom dead center will result in an increase
in torque. Does this improvement in pedaling mechanics actually translate into
improved performance on the road where it counts? Actually, the scientific
literature that exists, although limited, suggests that this may not be the
case. Dr. Ed Coyle and colleagues at the University of Texas found that it was
maximal torque during the down stroke and not the "smoothness" or
technique of the pedal stroke that was one factor which differentiated a group
of elite cyclists from their sub-elite peers .2 Still, Dr. Jeff
Broker, biomechanist at the Sport Science and Technology Lab at the US Olympic
Training Center, has a contrary opinion. The developer of a highly
sophisticated set of force pedals that evaluate force distribution during
pedaling similar to the SpinScan feature of the
CompuTrainer, Broker
believes that by distributing the force more evenly throughout the entire pedal
stroke, and thus distributing the power demands to a greater muscle mass, a
cyclist is able to conserve the major cycling muscles for the latter stages of
a race when the demands are greatest. He goes on to explain that this even
power distribution is especially important to off-road cyclists, as the more
even torque distribution provides better traction and prevents tire slippage in
loose terrain. This is certainly an area where more information is needed, but
in the mean time, a statement made by Dr. Jay T. Kearney, PhD at the 1996 USOC
Sport Science & Technology Quadrennial Conference summed it up best when he
pointed out that, on average, a performance difference of one-half of one
percent is all that separated a gold medal performance from not even medaling
at the 1996 Olympic Games.
Serotta, makers of the Fit
Kit System are about to release their newest version of the Fit Bike that will
incorporate the SpinScan capability of the CompuTrainer. This combination will
certainly take the concept of fitting the bike to the person to a new level!
So, once you’ve dialed in
that saddle position, set your cleats to just the right angle, used some
orthodics to address that leg length discrepancy that you never knew you had,
set your ergo stem as low as it can go, and worked up to 5 minute single leg isolations
at 150 watts; you’re still not ready to retire your Spin Scan. The CompuTrainer
PC1 software allows you to ride a course, chosen from their website library
such as the Ironman Australia or Escape Alcatraz; or one that you’ve designed
yourself, and still get SpinScan feedback from a small window on the screen.
With the PC1 software, you can evaluate the effects of
terrain and wind on your new position under realistic race conditions on a
course of your choosing or design.
Even if you have perfected
your pedal stroke on the flats, you can now evaluate the effects of a 5% grade,
or determine if you lose good pedaling mechanics when you drop down into your
aero bars. How many of us haven’t wondered what the effect of standing up to
pedal on a climb actually has on our power output? With feedback from the
SpinScan feature of the CompuTrainer, you can take the guesswork out of bike
fit, positioning, and even race strategy.
References
- Burke, E.R., and Pruitt, A.L. Body positioning for cycling. High-Tech Cycling. Human Kinetics: Champaign, IL. 1996.
- Coyle , E.F., Feltner, M.E., Kautz, S.A., Hamilton, M.T., Montain, S.J., Baylor, A.M., Abraham, L.D., and Petrek, G.W. Physiological and biomechanical factors associated with elite endurance cycling performance. Medicine and Science in Sports and Exercise. 23:93-107. 1991.
Sean Drake is an
exercise physiologist who has worked with the US National Triathlon Teams and
as the Scouting Coach with USA Cycling.