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Caleb Ewan's Sprint Position - Revealed through Kinesiology

Crank Arm Length: Everything You Need To Know

Everything you need to know about crank length in a flow chart!
Crank length is a complicated topic because determining optimal crank length involves going to a lab and measuring expired gasses, performing muscle biopsies and evaluating blood samples to test lactic concentration.  Although lactic testing only requires a finger prick, muscle biopsies require a needle resembling the size of a pencil.  Below is a picture for anyone who hasn't seen one before.  Ouch!
Muscle biopsy needle
Although the lab method is the gold standard, I would like to propose a new method that's less invasive/ painful and involves a power meter, heart rate monitor, motor control tests and ratings of perceived exertion.  In testing myself and my clients, I have used a CompuTrainer and a Kurt Kinetic Smart Road Trainer with equal success.

It's common for sprinters to experience a 200 watt improvement solely from optimizing crank length, and I want to pass these benefits to the masses.  I hope that after reading this post, you'll have a better understanding of how crank length affects both physics and physiology to better determine what crank is needed to perform your best.

It's the only component on the bike that directly affects the only two ways you can produce power- the amount of torque you can generate and the rate that it can happen.
Power = Torque (pedal pressure) x RPM (cadence)
The more torque you have, the faster you'll hit your peak power.  In other words, you'll accelerate to your top speed faster, but due to the low cadence, it will be more difficult to maintain the power to stay there.  The more RPM you have, the easier it will be to maintain high power, but this is at the expense of a fast acceleration (low torque).

Using a crank length that requires too much torque or rpm leads to unique mechanical challenges that are only considered disadvantages IF the body isn't capable of efficiently meeting those demands.  If a rider's physiology is capable of pushing a low cadence efficiently, then a long crank is more optimal- the opposite is also true, and this reinforces the fact that individual variances cannot be ignored.  In the realm of crank length optimization, Physics and Physiology can appear to contradict each other which only adds to the complexity of determining optimal crank length.  Clearly there's more to crank length than simply using a sizing chart method to find optimal crank length.

In comparison to the gold standard of determining optimal crank length, anthropometric methods only partially address the limitations between crank length and range of motion.  It fails to account for the following variables that crank length can also influence:
  • Muscle fiber adaptations
  • Aerobic efficiency
  • Respiratory thresholds
  • Lactic accumulation
  • Venous return
  • Perceived exertion
  • Maximal power output
  • Heart rate
Without accounting for these variables, it's no surprise that numerous studies have found no correlation between leg length, inseam length and height to optimal crank length.  In terms of evaluating range of motion, it misses the other structures that are more likely to restrict range of motion:
  • Muscle
  • Tendon
  • Ligament
  • Tissue
  • Fascia
  • Excess fat
A better correlation may be found if all of these components are also considered.

Interestingly, unlike the skeleton, all of the variables listed above can be enhanced through corrective exercise.  Many times, common problems related to crank length (thigh to chest contact, aero position discomfort, low back pain, knee pain, etc.) can be completely eliminated with corrective exercise, but unfortunately this kind of advice is alarmingly rare to hear.  Rather that conditioning the body to efficiently push a crank that's has the potential to be optimal, buying a longer or shorter crank seems to be the only solution while being unaware of the possible repercussions.

It's important for every cyclist to know the Pros and Cons of long and short crank lengths.  Too many articles and opinions are biased towards short crank arms because they only focus on the advantages without discussing the disadvantages to using short cranks.  Each crank length has its place, and it's important to know when it's advantageous to use a long or short crank length.  I always use Mark Cavendish as an example because his optimal crank length for track is 165mm, but on the road, his optimal crank length is 170mm.  By the end of this post, you will be able to answer why these lengths are optimal.

Depending on the crank length you choose, it will determine your gravitation towards hard gears that are slow to turn or easy gears that have to be spun quickly.  The relationship between crank length and optimal cadence is as follows:
As crank length increases, optimal cadence decreases.
As crank length decreases, optimal cadence increases.
The optimal cadence for power production depends on the crank length you choose.  The keyword here is "optimal."  When the cranks are long, many cyclists make the error of trying to spin it too fast.  Forcing your body to spin beyond the optimal cadence leads to compensation patterns that's both inefficient and a risk for injury.

Whether you pick a long or short crank, you must be prepared to commit yourself to training in a way that maximizes the cranks benefits and reduces its limitations.
In order to get the most out of a long crank, you must train the skeletal muscles to become efficient at a slow cadence.
In order to get the most out of a short crank, you must train the skeletal muscles to become efficient at a fast cadence.
Regardless of the gearing (compact, subcompact, standard + any cassette), the crank length you choose will produce the same optimal cadence.  If you don't own a power meter, it's difficult to perceive whether or not your effort is inadequate or excessive, and this is why I test clients on a CompuTrainer or any power enabled trainer.  In terms of perceived effort, long cranks and short cranks tend to give off different sensations:
Long cranks tend to cause riders to overshoot their efforts.  The sensation of the legs moving slowly causes many to believe that their effort isn't enough, so they tend to push the pedals harder than needed.  I often have to tell clients to "back off" in order to maintain a steady power output.
Short cranks tend to cause riders to undershoot their efforts.  The sensation of the legs moving quickly causes them to believe that they're pushing hard enough into the pedals, and the thought of shifting and losing leg speed tends to be a common reason to undershooting their target power.  I often have to tell clients to "shift" or be willing to lose a little leg speed to increase power.
The crank length you choose determines the gears you select and the range of cadences available to you.  The challenge is determining whether your physiology is more efficient at a slow cadence or a fast cadence.  This is where muscle fiber types come into play.  Do you have larger proportion of slow twitch fibers or do you have a high number of fast twitch muscle fibers?

As the name implies, fast twitch muscle fibers are recruited for fast, explosive actions or when a lot of force is needed regardless of limb speed.  Slow twitch muscle fibers are recruited for slow, less powerful actions.  In terms of metabolic waste and other characteristics, here's how they differ:
  • Lactic acid
  • Hydrogen ions
  • Fast rate of contraction 
  • Fast rate of relaxation
  • High fatigueability 
  • Optimized for muscular efficiency at 100rpm 
  • Water 
  • Carbon dioxide
  • Slow rate of contraction 
  • Slow rate of relaxation 
  • More resistant to fatigue
  • Optimized for muscular efficiency at 80rpm 
  • Fast twitch fibers which can operate aerobically and anaerobically.
The high cadences characteristic of short cranks allow for faster leg speeds and FT recruitment when the intensity lifts.  Short cranks are more suited to the rider who has more fast twitch muscle fibers.  The most optimal short crank for FT efficiency is one that naturally positions cadence at 100 rpm.  This optimizes the property of the muscle to contract and relax quickly.

On the other hand, the slower cadence of long cranks allow slow twitch muscle fibers to primarily dominate the work.  Riders who have a larger percentage of slow twitch muscle fibers benefit from a long crank.  A crank that naturally limits cadence to approximately 80 rpm optimizes for ST muscular efficiency.  The lower cadences allows ST fibers to have adequate time to relax on the return phase of the pedal stroke.

The type of weight lifting you're most proficient at can also tell you a lot about your muscle fiber type.  If you're better at lifting heavy weights at a slow speed, then you'll be more suited to a longer crank that places you closer to 80rpm.  If you're better at lifting heavy weights explosively (Olympic lifts), then a shorter crank that places you closer to 100rpm is more ideal.

Although the crank length you choose will determine what muscle fiber type is primarily recruited, it's important to know what to expect with regards to how crank length affects venous return and heart rate.

Venous return is a term used to describe blood that's returning to the heart.  During exercise, muscle contractions press into/ squeeze the veins to help transport blood back to the heart.  This is called a "muscle pump."

When venous return is inadequate, the heart compensates by contracting harder and faster.  When this happens, cardiovascular and respiratory fatigue is more likely to limit performance.  When other muscles are allowed to assist in venous return, the heart won't need to work as hard, which helps to prolong the onset of fatigue.

Long cranks and short cranks affect venous return differently:
LONG CRANKS pump a large volume of blood per pedal stroke at a slower rate (slower cadence).  This occurs because the hip, knee and ankle travel a large distance or range of motion.  More muscles are involved, especially in standing, and they contract fully to create a more complete muscle pump.  Heart rates tend to be higher because long cranks favor slow twitch muscle fibers which produce a lot of carbon dioxide.  When carbon dioxide levels increase, this triggers the body to increase its respiratory rate and heart rate aka. you'll feel more out of breath.
SHORT CRANKS pump a small volume of blood per pedal stroke at a faster rate (high cadence).  This occurs because the hip, knee and ankle travel a smaller distance or range of motion.  Less muscles are involved, and they contract partially which contributes to a less efficient muscle pump.  This is offset by the faster cadence which is why short cranks tend to lead to lower heart rates.
Depending on the crank length you choose, it can determine the likelihood that cardiorespiratory fatigue will limit your performance.  Most sprinters (road races, crits, group rides) need to avoid going with a crank that's long, especially if they're not as aerobically fit as their rivals.  This is why having more than one crank length is beneficial.

The heart and the muscles used for breathing are just a few muscles that can limit performance.  The crank length you choose can also determine how likely the muscles at the hip and legs will limit your performance.

When lactic acid is produced, hydrogen ions are released.  When hydrogen ion concentrations increase, this is what creates the acidic environment that leads to that familiar muscle burn.

Slow twitch (ST) muscle fibers don't produce hydrogen ions, but fast twitch (FT) muscle fibers do.  Crank length determines how much fast twitch muscle fibers can contribute to power production.
Lactic accumulation is lower on long cranks.
  • Long cranks allow slow twitch (ST) muscle fibers to do most of the work because as the name implies, ST fibers contract slowly.  When cadences are lower, muscle contractions are slower which means less FT fibers get an opportunity to work and leave behind acidic hydrogen ions.  Although localized muscle burning is more avoidable on longer cranks, respiratory or cardiac fatigue tends to be the limiter.
Lactic accumulation or muscle burn is a frequent battle on short cranks. 
  • Short cranks cause fast twitch (FT) muscle fibers to contribute more often.  When cadences are high, muscle contractions are faster which means more FT fibers will be more likely to activate and leave behind acidic metabolic waste.  This explains why riders on short cranks tend to be exhausted after a single hard effort.
If you need the ability to produce multiple repeated efforts with minimal recovery time (crits, fast group rides, road races, etc.), long cranks are better.  If the type of riding you plan to do requires one steady effort (century, long duration TT, etc.), shorter cranks are better.

For overall road bike riding, the optimal crank is one that causes cardiorespiratory fatigue and localized muscle fatigue (burning) to occur at the same time.  If the muscles of your legs and hips always limit your performance before heart rate becomes a factor, then the cranks are too long for your abilities.  If being out of breath always limits your performance before legs reach fatigue, then the cranks are too short and aren't fully harnessing the power from the skeletal muscles.

All of the information before this point is only useful if your motor control is excellent.  If you're using the wrong muscles to drive the pedals, the primary muscle groups will always limit your performance, especially with long cranks.

When a cyclist is able to coordinate every muscle with perfect timing and intensity, this represents a level of fine motor control.  No single muscle group limits performance, rather, the muscles of the hips and legs tend to reach fatigue as a whole unit instead.  A rider with excellent motor control will always be more likely to be limited by other physiological limitations than a single muscle group.

Other physiological limitations:

  • Glycogen depletion
  • Dehydration, electrolyte imbalances
  • Severe, but not localized acidosis
  • Hypoglycemia
  • Cardiovascular or Respiratory fatigue (accessory muscle fatigue)

Localized muscle fatigue (Ordered from most common [first] to least common [last] muscle groups):

  • Quadriceps
  • Gastrocnemius
  • Soleus
  • Intrinsic muscles of the feet
  • Hamstring
  • Gluteus Maximus

If you're able to point out a single muscle group that limits your performance, then motor control training is necessary to restoring muscular coordination.  In terms of motor control, long cranks and short cranks have different requirements.
Long cranks require greater fine motor control.
Short cranks require less refined motor control.
If a cyclists doesn't have the time or the motivation to work on dedicated motor control training, a shorter crank is necessary to reduce compensation patterns.  The most common muscle activation issue I see with clients is quad dominant pedaling.  A lot of people kick the pedal (quad) versus drive them down using the most powerful muscle of the body, the gluteus maximus.  Correcting this muscle activation can drastically improve power output across every zone.

I always like to use Peter Sagan as an example of optimal motor control and muscle activation.  It's clear that he only activates the primary muscles during the down phase and relaxes on the return phase of the pedal stroke.  Being able to contract and quickly relax the primary muscles requires excellent motor control to perform efficiently.

By now, you should have a good idea of whether or not your current crank is long, short or at the middle.  Depending on whether your crank is long or short, the focus of your training will be drastically different.
As crank length increases:
  • More flexibility/ range of motion is required.  
  • More core strength is required to resist losing a neutral spine due to the forces exerted by the lower and upper body. 
  • More upper body strength is required to counter the high force exerted by the muscles of the hip and legs.
As crank length decreases:  
  • Less flexibility/ range of motion is required. 
  • Less core strength is required because it mainly needs to resist the muscles of the lower half of the body (pelvis and below).  Don't get me wrong... this doesn't mean no core strength!
  • Less upper body strength is needed because the forces exerted on the pedals are too small to sway the bike. 
Long cranks have more physical requirements than shorter cranks.  REQUIREMENTS is the key word.  If you lack just a little bit of one of the three bullet points, you must get a shorter crank, otherwise injury is inevitable.  Keep in mind that a shorter crank is only a temporary solution that only buys more time until injuries creep back up, so be sure to correct any imbalances that you're aware of.  If you have or had an overuse injury, then you can be certain that you need corrective exercise.

People who ignore their body get caught in a counterproductive cycle of going shorter and shorter and shorter...  It's a solution that only addresses the symptoms, but not the source of the problem.  Contact me to set up a consultation for local private training or long distance coaching.

As crank length increases, sprint power output increases drastically.
As crank length decreases, sprint power output decreases drastically.
If your crank is short, you can expect an extra 70-100 watts per 2.5mm of length.  This is due to the fact that the body can more instantaneously apply maximal force than it can spin from X to Y cadence.  This extra potential power is the reward for preparing your body to push a long crank efficiently and safely.

I often get the question:  What about track cyclists?  If you look at the famous track racer, Mark Cavendish- he uses a 165mm on the track and a 170mm on the road.  This is also the case for every track racer who goes through crank optimization testing.  They tend to go up by 5mm from their track bike.

While planet internet is determined to find the holy grail of crank length, the answer is that one crank length can't do it all.
"One crank length can't do it all."
If you're not on top of your imbalances, injuries will happen.  When injuries occur, a short crank helps with rehabilitation.  If overtraining occurs, having a short crank available is beneficial to letting the heart and the entire body recover.  Progressing back to a long crank is how cycling rehabilitation should be done.  I own three cranks for this purpose and recommend my clients to have multiples too.  Consider it as an investment in preventative and rehabilitative care for your body.


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