Anatomical Considerations in Bike Fit: Fitting the Machine to the Athlete – Part II

In the first article on anatomical considerations in bike fit, I highlighted the importance of setting up your bike position so that you put your muscles in the most powerful and most comfortable position — at or very slightly beyond each joint’s resting length.

In this article I’m going to delve into the nitty gritty of how to go about doing that, how to measure both your body dimensions and range of motion capabilities to come up with the ideal bike position for someone with your unique body dimensions and flex-ability.

So, how do we measure you up?

I see bike fit as a series of very simple trig problems. An athlete on a bike represents a series of triangles with two elements of the triangle fixed (their body dimensions and the joint angles that they are most efficient at) and one element that is, or at least should be, variable, i.e. the dimensions of your bike frame. The more common, dare I say “traditional” bike fit approach is to fix the “bike frame” side of the triangle and contort your body angles to “make it work.”

As with any trig problem, the first step is quantitatively identifying the known elements in the equation. In this case they are the fixed sides and angles of the triangle, i.e. limb and trunk lengths and range of motion capabilities of the athlete.

Personal Body Dimensions (fixed sides of the triangle)

  • Femur Length: From greater trochanter (bony bit on lateral side of thigh) at bottom of swim suit level to knee joint line
  • Tibia Length: Knee joint line to lateral malleolus on ankle (most bony lateral bit)
  • Foot Length (virtual): Lateral malleolus to top of fifth metatarsal (or wherever cleats are placed)
  • Trunk Length: Greater trochanter of femur to most lateral (acromial) aspect of clavicle (collar bone)
  • Upper Arm Length: Most lateral aspect of clavicle down to elbow joint line
  • Lower Arm Length: Elbow joint line to center of closed fist
  • Hip Width: Horizontal width between most lateral aspects of the hips (iliac spines) from the front
  • Shoulder Width: Horizontal width between most lateral ends of both collar bones (acromial end of clavicle)

Personal Range of Motion Abilites (fixed angles of the triangle)
For these measurements, you’ll need a goniometer (a tool that measures angles). These can be found online in medical supply stores or you can make your own by bolting a long ruler to a protractor.

Open Hip

  1. Standing with a 30-40 degree bend in the knee (I recommend trying 30, 35 and 40 to find best knee/hip combo), put one hand behind your back and extend your back so that there is a defined arch. (Note: Once you find the right knee angle, set up a bench or table at that point so you don’t have to hold it while measuring hip angle.)
  2. Keeping that arch in the back and bend at the knee, bend forward as far as you can from the hip while keeping the back arched. Measure the angle between a line running along the middle of your thigh and a line along the trunk perpendicular to the top of your bike shorts. Keep the arch in your back.

Closed Hip
Lay supine on a flat surface, put a normal arch in your back as above and bring both legs up off the ground (knees toward the chest). Keep bringing the knees up until you feel your back start to flatten or the knee starts to move out (towards the armpit). Stop and measure the angle just prior to this point.

Closed Knee
Standing, slide one foot up the other leg, bending the knee as much as possible without compensation, i.e. no turning in or out of the knee/foot.

Closed Ankle
In the same position, dorsi-flex the ankle, i.e bring the toe towards the shin. Only go as far as you can without compensation (turning out/in/twisting of the foot)

Closed Trunk
Finally, many athletes will be limited in how aggressive they can be in the aero position they adopt by their level of cervical extension, i.e. being able to see up the road. This range can be assessed simply by measuring the vertical difference from position 1 (bent at the hips, straight back) to a second position of bent at the hips with a bent back but still being able to comfortably see ahead.

In Part 3 I’ll show you how to bring all of these numbers together to come up with the perfect bike setup for you.

Categories: Planning

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Alan Couzens

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