Cycling Biomechanics and Bike Fits

Tour of California 2008
Tour of California 2008
Bicycle Retrofitting is an art, not a science. As a healthcare practitioner, I'm very aware of how flexibility, strength, core stability, and joint restrictions can affect a bike fit, biomechanics on the bike, performance, and injuries. Every week I perform medical-grade bike fits for clients of all abilities, from the commuter to family weekend outings, as well as amateur and elite cyclists. The more and more bike fits I am doing (my bike fits include flexibility, joint range of motion, biomechanics of pedal stroke, strength and stability assessment, and soft tissue assessment), I realize the importance of giving more attention to the BIKER and less on the bike. Our bodies are our most important piece of equipment. Without the maintenance and fine tuning of our bodies, we may be supporting the dysfunctions with a bike retrofit. In other words, with a bike retrofit that causes no pain, we are able to STRENGTHEN our dysfunctions, compensations, and imbalances. I'm making the very big assumption that most of us have bikes that fit our dimensions and have been retrofit by the local bike shop or that we have had a professional bike fit in the past.  In other words, we are well within our "Fit Window". Some of you may be thinking that I'm knocking the bike fit, I am not. I feel it's an extremely important aspect of cycling injury-free and optimizing performance. However, I feel many bike fits ignore the compensations, imbalances, weaknesses, and restrictions the cyclist brings to the bike. PTs and Chiropractors have unique qualifications for bicycle retrofitting:
  • Biomechanical knowledge including posture and the cycling stroke/form
  • Understanding of tissue pathology
  • Understanding of normal muscle balance
  • Ability to identify and correct dysfunctions, restrictions, and weaknesses
  • Periodized exercise training and rehabilitation
  • Injury treatment, management, and planning.
As I gear up to fly to Seattle to receive my next Bike Fit for PTs certification, I am flipping through my previous certification notes and Arnie Baker's "Bike Fit 3rd Edition" ebook. I thought I'd share with you some interesting tidbits from what we know about cycling, biomechanics, and bike fits.  For the Top Ten Things We Know About Pedaling, click here. Cycling Injuries occur from: 1.  Overuse
  • Improper Training
  • Musculoskeletal disorders, dysfunctions, imbalances
  • Skills
  • Poor Bike Fit
2.  Traumatic event Cycling is repetitive:  in one hour, the crank turns 5400 revolutions! Low Back Pain and Climbing The greater the force on the pedal, the more your back has to work.  Think of one of my favorite sayings: "You can't shoot a cannon from a canoe."  Meaning, your back needs to work harder to create a solid foundation for your legs to push off from.  Therefore, it's no coincidence that low back pain sufferers experience more pain on climbs than on flats where they can keep a higher cadence.  If you have had a previous low back injury, are weak in the core, have tight hamstrings, or your muscles possess a lot of scar tissue, you can potentially experience more pain on hard, sustained climbs or sprints. Joint Compression with Cycling In 1988, Hull et al, found that there was a minimization of absolute joint movements or muscular stresses at 95-100 RPMs while at 200 Watts of resistance.  This is a very strong argument for higher cadence pedaling to reduce joint and muscle stress.  Which brings me to saddle height, cadence, and power. Cadence, Saddle Height, Injury, and Power (adapted from Arnie Baker's Bike Fit ebook.) A lower saddle makes it easier to achieve a higher cadence.  The lower the saddle, the less distance the leg has to travel and so more revolutions can be accomplished in a minute.  (This is assuming the crank arm length is the same and the saddle isn't lowered so much that maximum hip flexion hasn't been reached.)  Arnie Baker calculates that the rider should be able to pedal about 7% faster with a the saddle lowered 2 cm.  If you want to learn the math, you may want to buy his ebook.  From reading his Bike Fit ebook, I'm guessing he's a big trigonometry buff. Arnie Baker also found through trigonometry that raising the saddle also increases your knee arc.  In other words, your knee travels further the higher the saddle.  Remember when they used to tell us to raise the saddle if you had knee pain so as to not compress the joint and cause greater knee flexion?  This is a good argument for NOT raising the saddle if you have ANY muscular imbalances, biomechanical dysfunctions, or joint restrictions.  With any of these "pedal faults", your knee will travel further into dysfunction.  Meaning, more repetitive stress in a dysfunctional state.  Most commonly we see hip tightness and weakness causing the "figure 8" pedal stroke.  If the hips are tight, the knee travels laterally at the top of the pedal stroke. If there are muscle imbalances or hamstring restrictions, the knee travels inward at the bottom of the stroke.  Also, raising the saddle can cause more IT Band stress due to a larger arc the distal end of the IT Band has to travel.
Example of one of our biomechanical analysis tests.  Notice the left hip translates too far to the left (right picture).  This is an example of a biomechanical dysfunction.
Example of one of our biomechanical analysis tests. Notice the left hip translates too far to the left (right picture). This is an example of a biomechanical dysfunction.
But what about economy and power? Arnie Baker uses the example of a weight lifter and the squat.  A weight lifter (or any of us for that matter) can lift more weight if we only squat 1/4 of the way down compared to squatting half way or into a full squat.  Or for a better analogy.  We easily walk up one step at a time.  Two steps become more difficult due to an increase of hip flexion.  The more the hip is flexed, the less power we have pushing down.  Our muscles work better when their length is in the midrange, not at the extremes. However, we cannot just stop with the saddle height to improve economy and power.  Trunk flexion plays an important roll in how much the hip is flexed.  If you cycle with an anteriorly rotated pelvis, you essentially close the hip angle and thus, reduce economy and power.  Most racers have a more rounded back due to their pelvis tilting more posteriorly.  This opens up the hip angle.  Try this:  Stand on both feet.  Life one leg with the knee bent about 90 degrees as high as you comfortably can without rounding the back and forcing the hip flexion.  Most of us can get about 90 degrees of hip flexion.  Lie on your back and raise your knee towards your chest, you can pull on your knee to get more flexion of the hip.  You may now be able to get to 135 degrees which is normal for an uninjured, non-arthritic hip.  It's harder standing b/c of gravity. Your muscles have to work hard to bring your knee towards your chest.  Now do the same thing standing, but flex your trunk forward about 50 - 60 degrees.  Your hip flexion may be less now.  You can also measure it each time by how high your foot raises off the ground.  The more upright you are, the greater the hip flexion, the less work your muscles need to do.
Racer hip flexion angle is 78 degrees allowing for increased power and economy.  Notice his pelvis is rocked backwards a bit.
Racer hip flexion angle is 78 degrees allowing for increased power and economy. Notice his pelvis is rocked backwards a bit.
Notice this racer's hip angle, which is similar to the other cyclists below.
Notice this racer's hip angle, which is similar to the other cyclists below.
Another racer's hip angle around 50 degrees.
Another racer's hip angle around 50 degrees.
Another racer's hip angle around 73 degrees allowing for increased power and economy
Another racer's hip angle around 73 degrees allowing for increased power and economy
Notice her hip angle is about the same as the racers' hip angles.
Notice her hip angle is about the same as the racers' hip angles.
Notice her hip flexion is less than the racers' hip flexions causing a reduction in economy and power.  Her anteriorly tilted pelvis can also cause the hamstrings to become more taut which will in turn affect her bike fit and saddle height.  This is a strong argument for rehab to teach her body to obtain a more neutral pelvis on the bike, allowing for greater power, economy, and reduced risk of injury.
Notice her hip flexion is less than the racers' hip flexions causing a reduction in economy and power. Her anteriorly tilted pelvis can also cause the hamstrings to become more taut which will in turn affect her bike fit and saddle height. This is a strong argument for rehab to teach her body to obtain a more neutral pelvis on the bike, allowing for greater power, economy, and reduced risk of injury.
Conclusion I chose just a few topics to blog about here, but I hope you understand that there is more to bike fitting than measuring angles and using plumb bobs.  Fortunately, our bodies are adaptable and dynamic.  Unfortunately, we bring to the bike a lot of baggage:  old injuries, compensations, desk job postures, skill levels, lack of time in the saddle, etc. Hopefully this is a stronger argument to have your body checked to fit the bike, not just the bike checked to fit you. Dr. Greaux is a sports medicine practitioner specializing in video analysis, biomechanics, gait and cycling analysis, and bike fits.  To learn more about Dr. Greaux, please click here.  To schedule an appointment, please click here.
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