Originally Posted by mattraw

When you mentioned your experiences with cramps on a 175mm crank, that's a perfect indication that 175mm was for sure long for you. Although we have to consider that you are more physically fit during the time you did the climb on a 170mm, there's no getting away with "standard" human biomechanics.

To know the reason why everybody talks about opening up the hip and the relief that comes with it, we have to understand that our Quadriceps put so much force when cycling. Although our glutes help extend our legs when pushing down the pedals, it only serves to stabilize the hip and the initial movement. Our Quadriceps need to be at an optimal angle to generate a lot of force without breaking down a lot of energy.

The length-tension (L-T) relationship of muscle basically describes the amount of tension that is produced by a muscle as a feature of its length. And in the case of the quadriceps, it is strongest at 110-120 degrees of knee extension. Given that number, shorter cranks are beneficial because it positions the Quadriceps (in relationship to the hip as well as the knee) at an angle that generates a lot of force without really breaking down a lot of energy, especially at the top of the pedal stroke where most of the force is generated to move forward.

So going back to your cramps and your relief:
175mm: Closed Hip --> Suboptimal angle for Quads during the force-generating phase of the pedal stroke (11 o'clock to 3 o'clock) --> More energy to break down for the Quads to generate force --> Tired Quads --> Cramps
170mm: Opened Hip --> Optimal angle for Quads during the force-generating phase of the pedal stroke (11 o'clock to 3 o'clock) --> Less energy to break down for the Quads to generate force (simply because of its efficiency) --> More enduring Quads --> No Cramps --> Sustained Power


One practical example for this: When you're doing a full squat, it's so much harder to overcome the force when your buttocks are all the way down to the floor with your knees fully flexed but when you reach 110-120 knee extension, or when you're almost in a full standing position, it's just so much easier to manage the weight. That's not because the weight your carrying was 100lbs heavier, but during the phase where your knee was fully flexed, the muscles that are responsible for generating the force aren't at an optimal angle.

*bow*

Human body over the years: Correction --> Adaptation --> Evolution

I love the PNW!

I'm a Med student by the way.

Sources:
Anatomy, Physiology, and Neuroanatomy by Snell
Braddom's Physical Medicine and Physical Rehabilitation
Physical Rehabilitation by Sullivan