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Friday, January 29, 2016

Crank Arm Length Explained

Everything you need to know about crank length in a flow chart!
It directly affects the way you produce power- it changes the amount of pressure or force you can put into the pedals and the rate that it can happen.
Power = Force (torque) x RPM (cadence)
For the same power, if force goes up, cadence goes down, and if force goes down, cadence has to go up.  Too much of either variable leads to problems.  The optimal crank length is one that doesn't require so much force that fatigue occurs too early (too long), but also doesn't make it possible to put any pressure into the pedals (too short).

Knowing what length to choose starts with understanding how crank length affects certain aspects of cycling- at the body and the bike.

As crank length increases, cadence decreases.
As crank length decreases, cadence increases.
Each crank length will cause you to naturally select a range of cadences.  It's important to stay within this range because forcing your body to spin beyond this natural range will lead to compensation patterns that's both inefficient and an injury risk.

When deciding to go shorter or longer, just make sure to not compromise venous return in a way that will interfere with the type of riding you plan to do.

Venous return is a term used to describe blood that's returning to the heart.  During exercise, muscle contractions squeeze blood through the veins to assist in its delivery.

When venous return is inadequate, the heart compensates by contracting harder and at a faster rate.  This leads to cardiovascular and respiratory fatigue, limiting performance due to being out of breath.  When other muscles assist in venous return, the heart won't need to work as hard, which helps to prolong the point 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.  This compensates for the slower cadence, but heart rates still tend to be slightly higher on longer cranks.
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 how much the heart needs to work.  This is why having more than one crank length is beneficial (I own three!).

The heart and the muscles used for breathing are just a few muscles that can limit performance.  If you often tell your legs to shut up, read on...  Crank length can also affect how quickly you produce and clear lactic acid.

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 more of the work because as the name implies, ST fibers contract slowly.  When cadences are lower, muscle contractions are slower which means FT fibers get less of an opportunity to work and leave behind hydrogen ions.
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.
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.

The next challenge is determining whether your body is conditioned to handle the demands of the crank length you need to perform well.


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 absolutely inevitable.  A shorter crank is only a temporary solution that only buys more time until injuries creep back up.  Incorrect muscle activation, muscle imbalances and body mechanics are the real problems that lead to injury.  People who ignore their body get caught in a counterproductive cycle of going shorter and shorter and shorter...  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.
For sprints, 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 as it does for the majority of elite athletes.  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.  This is why I own three cranks and recommend my clients to have multiples too.  Consider it as an investment in preventative and rehabilitative care for your body.