Crank Arm Length: What you should know.

Knee angle via video analysis
Knee angle via video analysis
When getting a bike fit or buying a new bike, the crank arm length can often be overlooked. There are general guidelines based on inseam, height, and/or bike size. Such guidelines may look like this:
  • Under 5'10 use 170mm cranks for general (road) racing and training
  • 5'10 to 6'1 use 172.5mm cranks
  • 6'1 plus use 175mm cranks
Or some people like to be a little more scientific by using this formula:

Crank arm length (mm)= inseam (inches) x 5.48

Well, there's a lot of information or misinformation out there on crank arms, cycling efficiency, power output, and knee injuries.

An argument for a longer crank arm is better torque in the pedal stroke especially with climbing and sprinting. An argument for a shorter crank arm is the ability to spin more efficiently and thus is used for cyclists with a high cadence. Another argument for a shorter crank arm is to reduce knee injuries since a longer crank arm will cause greater knee flexion at top dead center, or at the top of the pedal stroke.

Here's my take:  for most of us, we can adapt to a crank arm length one size above and one size below the "suggested" length based on the guidelines above and our cycling mechanics.  However, for anyone with hip, knee, and low back pain, it may be more important to do a little trial and error on selecting the crank arm length.

One key piece to look at that is often overlooked is hip flexibility.  When performing a bike fit and cycling analysis, I am particularly interested in a cyclist's hip mobility, especially in hip flexion or knee to chest and then internal and external rotation as well as hamstring length and lumbar spine flexion.  Furthermore, we check these in all combinations since the cyclist will flex forward more going uphill, lengthening the lumbar spine and hamstrings while bringing the knee to chest.  If a cyclist has reduced hip mobility, especially in flexion and internal rotation, then crank arm length may become a significant factor in reducing the risk of injury and optimizing performance.

Consider someone with a tight hip on the right side.  If the crank arm length is too long, the rider will not have enough hip flexion at top dead center (TDC) and thus will either have the knee veer out laterally on the upstroke (the ever popular figure 8 stroke) or he/she will rock the pelvis to the left and hike up the right side to make more room for the knee coming towards the chest.  Both of these accommodations or compensations can cause back, hip, or knee pain on EITHER leg or back.

Example of Dynamic Functional Analysis
Example of Dynamic Functional Analysis

For this reason, we typically do a flexibility, mobility, stability, and dynamic functional biomechanics examination to fit the bike to you and YOU to the bike.  Fitting you to the bike is as important to adjusting the bike to your body.  Any imbalance found in the functional exam can hinder performance and cause injury, despite the best bike fit.

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