By: Zack Allison, BS.
Source Endurance Senior Coach
Many things go into a pedal stroke.
Consider that the daily commuter just pushes down on his flat pedals, making one
dimension of force. As this commuter pedals harder to make the red light, his
RPMs increase and he starts to bounce on the saddle and inefficiently gain
speed to make the light. On the other hand, as the elite-level racer picks up
the pace to follow an attack from the field, he shifts up and smoothly
increases power at close to the same cadence and, ideally, the same trained efficient pedal stroke.
Last week I sat in on a little
experiment with Category 1 racer Whitney Schultz. We had Whitney ride the Computrainer
and used its extensive programming capabilities to break down her pedal stroke
in all directions - left and right and up and down. We originally thought that
Whitney’s stroke was imbalanced; it looked like she was favoring her right leg.
When she was on the Computrainer it showed that she was never favoring one leg
over 2%. This is good but we need to do more experiments. She may be recruiting
other muscles to make up the difference that a back injury created. With more
fatigue she may start to favor one leg - we’ll see.
This
is Whitney Schultz on the Computrainer. The screen shows her left leg contributing
49% , while her right does 51% of the workload, all at 200 watts. The blue bars show her down-stroke and the
purple/red/yellow bars show the up- and back-strokes. Whitney, being an
accomplished racer, would not see these bars move very much between her current
200 watts and a hill at 350 watts. This pedal consistency shows efficiency. If
your stroke changes at higher watts, you are recruiting muscles not normally
used in a pedal stroke and you will fatigue faster.
Based on the principle of muscle
recruitment, your body recruits motor units at different efforts. One “motor
unit” is one muscle fiber and its motor neuron that activates that muscle. If
you are pedaling at 100 watts, your body will recruit as few slow twitch fibers
as necessary to do that action. As you pick up the pace to 200 watts, your body
will recruit just the number of motor units necessary to complete this
activity. If you take this principle all the way up to a 1300 watt maximal sprint,
your body has recruited all of the fast and slow twitch muscles it has to
perform this max effort. This principle will affect your stroke.
There are physically more motor units on
the anterior (front) side of your upper leg that, including your gluteus
muscles, perform “knee extension” or pushing down on the pedals. The diagram below shows for a full pedal stroke
where muscles are engaged and resting. As you can see, your gluteus and quadriceps
muscles are big muscle groups and have a larger range of force. This means you
can have a smooth pedal stroke but your stroke will not produce even forces all
the way around.
In any normal stroke the goal is to make
a circle and have a smooth and efficient stroke. This has many issues. Your quadriceps
are stronger than your hamstrings. If you expect your hamstrings to be as
powerful as your quadriceps, you will have an uneven stroke when your
hamstrings cramp and fatigue. The good thing is that your cranks are attached
to each other so your force on the pedals doesn’t have to make uneven speeds in
your stroke.
The goal of your pedal stroke is to be
consistent. Your quadriceps will be doing more work but they have a larger
muscle area, therefore more strength and there is no avoiding that. What you,
as a cyclist, should work on is keeping your pedal stroke consistent at every
power interval. You will change your stroke based on if you are climbing or descending
but increasing watts on a consistent grade should have as little change as
possible on your form.
This
video shows Eric Marcotte of Elbowz Racing pedaling at 300W
Eric again at 500W.
Notice Eric's stroke is the same in each video. At 500 watts, Eric is well
above his lactate threshold (LT) and is recruiting pretty much all of his muscle
fibers and has his core engaged more but his basic pedal stroke is the same,
even though he is recruiting 85-95% of his motor units as discussed above .
This is what any racer should strive for. Despite his high effort, his pedal stroke
remains efficient. As he fatigues he may start to have certain motor units’
fatigue. As his hamstrings fatigue he may start to recruit his back muscles to
compensate changing his stroke. This is why a strong core is crucial and why
lesser trained cyclists have more back fatigue on long rides.
Returning to the example of the commuter
on flat pedals, his form changes from efficient to inefficient when he doubles
his power to make the light. When you double your power from 200 to 400 watts
your pedal stroke should be the same. If it is different, you are becoming
inefficient at higher watts, which is the last thing you need. If this seems
impossible you may need to check your fit and crank length. Otherwise get out
there and do your workouts but be aware of your pedal stroke and the source of
some inefficiencies.
