Tuesday, August 28, 2012

Pedaling Efficiency at Low vs High intensity

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.