Advanced pedal stroke

Pedal stroke 101 is about identifying the large muscle groups used in powering the bike and restricting their use to where that muscle has mechanical advantage. Advanced pedal stroke is about working around problems and maximizing power or efficiency within the pedal stroke.

Power or efficiency?

Mechanical efficiency and biomechanical efficiency are not the same thing. Mechanical efficiency is a matter of always pushing in the direction of travel of the pedal. Biomechanical efficiency has to do with muscles used, their system of leverage and where in their range of motion they are used.   Producing power over the top of the pedal stroke is the perfect example, both the hip flexor and the quad must be used to produce the correct direction of force, and there is a lot of power to be gained. On the other hand, the biomechanical cost is very high, the hip flexor is a very small muscle operating at it’s end range of motion.

Is it better to be a dragster (lots of power, terrible gas mileage) or an economy car (great gas mileage, not so much power)?  Why not be both? The ability to only use muscles where they are biomechanically efficient means you can ride for long periods of time. Using muscles where they can generate more power takes more of a toll, but you’ll be going faster. The trick is learning the differences and how to use them.

The advanced pedal stroke class focuses on the same two large muscle groups (glutes and quads), but it addresses the issues that most people have in generating more power – timing and muscular reciprocal inhibitions.

Timing:

Studies of elite level riders have shown that they simply push down harder – at least that was their conclusion. Looking at the raw data from wheel based power meters (PowerTap) and the newer raw data from pedal based power meters (Garmin Vector) shows something very interesting, they’re not the same.   Power meter pedals use strain gauges to measure force, when the raw data is looked at independently of position of the crank, you see the true force produced. If that downward force is produced when the crank is at 3:00, the power meter in the rear hub sees the force applied x the lever arm (crank length). If the pedal is at 6:00 there is still force on the pedal, but the power meter at the rear hub sees none of it.

Elite level riders push down harder at the right time.

Pedal stroke 101 was about learning to recruit the right muscle group to push down. To do that we used gravity – the idea of “falling into the pedals”  This produces a constant force on the pedal, based on your body weight and the gravitational pull of your planet.  Useful in teaching, but not as effective as it could be.

Make hay when the sun is shining…

In terms of pedal stroke, that means push down the hardest when mechanical efficiency is at it’s highest. So let’s define mechanical efficiency as it applies to just pushing down:

If we use a model where the crank at 12:00 is 0 degrees, the crank at 3:00 is 90 degrees…   then mechanical efficiency = SIN(crank angle).   So, if you’re pushing down when the crank is at the top, the mechanical efficiency is 0, and you’re wasting energy. Same thing when the crank is pointing straight down. At 1:30 (45 degrees up) the mechanical efficiency is around 70%, at 3:00 it’s 100% and at 4:30 (45 degrees down) it’s back to 70%.

Our method from pedalstroke 101 isn’t looking so bad here, constant force from 1:30 to 4:30 means mechanical efficiency is somewhere around 85% on average – pretty good!   If that’s the case, why do you have good days and bad days on the bike?  Math doesn’t change, gravity doesn’t change, it must be user error – your timing isn’t what you think it is.

 

Advertisements