many riders tend to force the pedal stroke through the knee (basically, a quadriceps extension) past the point where they need to transition to a calf motion through the bottom of the pedal stroke. As a result, they punch to a very heel-down position at the bottom of the stroke and lose a lot of power and cadence that way. Riders need to learn the limits of appropriate leg extension and then make use of their soleus and gastrocnemius muscles to transition through the top and bottom of the pedal stroke. You can't do that when your heel is already too low.
Thanks 11.4, always much appreciated. I think this describes very well how I used to ride, especially when fatigued or going all out. Basically "stomping" on the pedals with little ankle action at all.
But when focusing on stabilising the pedal stroke from the back, it seems much easier to get a little kick out of the ankle. Almost feels like a some muscle engagement in the lower back primes the entire "posterior chain" in the legs to be ready for ankle extension. Or is that totally off?
I can't quite visualize the action you're talking about. However, if your ankle is flexed upwards because you've extended your heel down past your pedal at the bottom of the stroke, you are at the weakest point in mobilizing your soleus and gastroc to help with crank arm movement.
To the last point, of whether it actually makes one any quicker, the answer is likely to be "sometimes." This is because when you are punching really hard, such as a seated climb or a high-geared time trial, your soleus in particular is a very strong muscle and can do a lot for you. It isn't a muscle that responds as well to very rapid cycles, so in riding at a 120 rpm cadence you can't expect it to do much. The point here is really that if one is expending energy to push one set of muscles (mostly the quads) to go through a longer range of motion than needed, one is both relying on one part of your musculature disproportionately and also making it do more than is necessary. That combination can't be ideal. And since one point of riding a shorter crank is to limit the range of leg motion, why recreate the original range by hyperextending the leg?
Every individual is going to be different and any individual is going to have to train some of these behaviors. That means there often isn't an instant benefit to a different crank arm length (and this applies to most other changes in both bike fit and personal flexibility or strength development). Creating the strength is easy. So is flexibility. Adapting to the range of movement and turning it into an efficient improved pedal stroke is what's hard.