How to Determine your Optimal Crank Arm Length

In order to help you understand how important crank arm length is, having the wrong crank length instantly shifted my performance from consistently being one of the top three road racers in the state to being a mid-pack finisher.  In fact, I also almost got dropped out of a race for the first time in two years of racing.  Check out my uneventful 2014 results to see more.

I initially thought my lack of performance was due to deconditioning from the crash (I also didn't want to be THAT guy who blamed the bike), but it turned out that by simply optimizing my crank length, my heart rate consistently operates about 20 bpm lower than before, and I'm riding A LOT faster.  This is not an exaggeration.  Below is a workout comparison which proves this.  For this reason, please follow my advice:

If you have not yet determined that your crank length is optimal, confirm this BEFORE considering to race or buy a new bike/ component- you are potentially at a SEVERE disadvantage.

Since crank length directly affects cadence and cycling economy, the stronger you get, the more the negative effects are amplified.  I experienced this firsthand because despite winning the first race of the season, I performed worse as the season progressed.

Cycling economy is increasingly becoming regarded by researchers to be more important than VO2max and Lactic Threshold because good cycling economy can compensate for a low VO2max AND Lactic Threshold.

HOW MUCH DOES CRANK LENGTH AFFECT PERFORMANCE? 

For the same effort, my heart rate consistently operates ~20 beats per minute lower.  This equates to approximately two heart rate zones.  For a speed that used to require my maximal aerobic capacity, it now only requires a submaximal effort to sustain.  In terms of perceived exertion, it was the difference of being completely out of breath to being able to talk comfortably.

To visualize what a ~20 bpm improvement looks like, check out the workout comparison below.  If you follow me on Strava (https://app.strava.com/athletes/444535), you'll notice that I always log wind speed, wind direction and temperature.  This allows me to compare two rides and consider differences in environmental conditions.  The first link/ workout was performed on a 172.5mm crank.  The second link/ workout was performed on a 165mm crank.  The image below is an overlay of the two graphs.  Based only on these two workouts, my heart rate averaged 17 beats per minute lower with the shorter crank and peaked 11 beats per minute lower. 

While the improvement in heart rate is impressive, heart rate doesn't paint the entire picture.  The wind conditions on the shorter crank (second link) were a lot worse.  I rarely had a tailwind due to how the houses channeled the wind and the gusts made it impossible to hold a consistent speed.  Unexpectedly, my average speed in this ~40 mile segment was faster than the average speed sustained by all of the group rides posted on Strava that day.  Since these two workouts were separated by less than two months, training effect could not explain this significant improvement in performance.  The major difference I experienced on the shorter crank was the ability to ride consistently lower (more aerodynamically) and comfortably pedal at a wider range of cadences to make adjusting to various wind directions A LOT easier to accomplish.

Workout #1 (172.5mm) - Solo, Steady State Aerobic, 88deg, NNW 7mph http://www.strava.com/activities/186214760/analysis/1627/8865

Workout #2 (165mm) - Solo, Challenging ride due to unpredicatable wind conditions, Steady State Aerobic, 86->79% humidity, 63deg, WSW 14mph (start), W 14-16mph (11:15am-11:35am, 12:55pm-1:15pm), WNW 16mph, 24-29mph gusts
http://www.strava.com/activities/202810561/analysis/1383/8676

172.5mm crank length (thin line) versus 165mm (thick line) crank length.
On the 165mm crank length, my heart rate averaged 17 bpm lower and
peaked 11 bpm lower.

HOW NOT TO DETERMINE OPTIMAL CRANK LENGTH

All of these methods have been proven by multiple studies to be an inaccurate method at determining crank length:

1. Inseam
2. Femur to tibia ratio
3. Height
4. Leg length
5. All crank arm length calculators

While the listed concepts above all appear to make sense, none of these methods account for the most important variables- mobility and flexibility.  Unlike the elbow which has a clear skeletal limit, the hip, knee and ankle are more likely to be limited by musculotendinous tightness and not the bone.  For example, a rider with a long tibia and flexible ankles won't experience the same thigh to trunk contact as a rider with a long tibia and inflexible ankles.  For this reason, measuring skeletal landmarks will never be accurate because that's not the only limiting factor to a cyclist's range of motion.

HOW TO IDENTIFY THE WRONG CRANK LENGTH

Two clear symptoms of having the wrong crank length are hip to trunk impact and cadence.  

In an aero position with the hands on the drops, a long crank will cause the thigh to hit the trunk at the upstroke.  As a result, the pelvis will rotate which leads many bike fitters to assume that it's a saddle height issue when in fact, it's a crank length issue.  I also want to mention that if the pelvis is rotating, the lumbar spine is also rotating which can easily lead to lower back pain.  The optimal crank length is one that allows the trunk to be parallel to the ground without constriction at the hip; this optimizes both aerodynamics and power production, respectively.

In terms of cadence, the crank length you choose should be one that optimizes venous return.  In order to optimize venous return, studies have found that cadence should either meet or exceed 90 revolutions per minute.  Optimizing venous return is extremely important because it allows the heart to work less hard, especially during and after hard efforts.  When cadence is too low or zero (coasting) the heart has to contract harder or increase its rate of contraction (heart rate) to push the blood through the veins.  In my case, my natural cadence on the long cranks were mid to low 80 rpm and as a result, my heart rate would take longer than usual to recover after hard efforts.

Just in case you were curious about whether neuromuscular drills can help to increase cadence despite having a long crank, it doesn't work because I tried it.  Over the course of a year, I regularly performed neuromuscular drills to increase my cadence and saw no meaningful results.  A study found that various crank lengths limit cadence due to increased requirements in hip, knee and ankle range of motion to complete a pedal revolution.  This explains why my efforts to increase cadence failed- the physics would have never allowed it to happen.

HOW TO DETERMINE OPTIMAL CRANK LENGTH: MY METHOD

While it's easy to identify a crank that's too long, there is currently no accurate method of determining the optimal crank length other than trial and error.  I'm proud to say that I have found a simple noninvasive solution to determining optimal crank length without wearing a mask and testing VO2 at various crank lengths.  It incorporates mobility, flexibility, posture, pelvic stability and balance to clearly identify the optimal crank length.  I will be presenting my crank arm length test to a few research groups to confirm its validity.  I plan to share my crank arm length test publicly once I get the results.  Based on the clients/ athletes and family members I have tested, this test has been very accurate at guiding them to a crank length that optimizes their performance.  If you are interested in scheduling a bike fitting via one-on-one private session or video conference, please contact me at EatSleepTrainSmart@gmail.com.