Dynamic Bike Fit with the CompuTrainer’s SpinScan™ Takes the Guesswork out of Positioning — by Sean Drake
 The high “peaks” and low “valleys,” or flat spots, in the SpinScan at the top represent a choppy, pedal-mashing style of cycling that does not maximize power distribution. The SpinScan on the bottom, with the flat and even bar graph, shows a more optimal pedaling mechanics as reflected in theSpinScan Number of 75. |
Do you think I should lower my aerobars? 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 idiosyncrasies of one’s bike fit with just a glance, but not all of use have our own personal guru to seek out every time we want to change our bike position. Dr. Andy Pruitt, EdD, sums up proper fit as “a marriage between the human body, which is somewhat adaptable, and a machine that is somewhat adjustable.” 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 cyclist has to adapt as little as possible.” 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.
This is the Polar SpinScan 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. |
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 the 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 peal revolution, the SpinScan number would be 100. While this is the physically impossible, a number between 70 and 90 is considered to be very efficient transfer of power.
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.
 The Coaching Software feature of the CompuTrainer provides a graphic data plot of power, heart rate, 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. |
The torque information, along with power and heart rate data, allows the user to make objective evaluations of the effects of lowering aero bars or moving the saddle forward into an extreme position.
A dynamic bike fit was the perfect way to solve an equipment choice facing a professional duathlete preparing for the 1995 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 loss 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 win conditions too evaluate the effects of the different pedal systems on power output at a given heart rate. 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 the 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.
Still, Dr. Jeff Broker, biomechanist for 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 meantime, 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 from not even medaling at the 1996 Olympic Games.
Sean Drake is an exercise physiologist who has worked with the US National Triathlon Teams and as the Scouting Coach with USA Cycling. For more information on CompuTrainier products or toe order RacerMate’s Dynamic BikeFit video, call 206-524-7392 or visit their website at www.computrainer.com.
