Everybody has their own understanding of how they should pedal a bike, very few of these versions have anything to do with physics. Lots of people think you should pedal in circles because the pedal goes around in circles. While it is true that the pedal does go around in circles, and you do have muscles which can move the pedal in that circle, there are huge differences in muscle strength. You have two large muscle groups which fight gravity all day long – glutes and quads, and you have opposing muscle groups which can produce a fraction of the force. If you are trying to power a bike and two of the four muscle groups produce 99% of that power, you’re best off focusing on just those two.
This is where we get into the difference between perception and measurable output. The Iliopsoas is a hip flexor, it allows you to walk up stairs by lifting the femur from the hip. It’s a tiny muscle group which produces almost no power, but what you perceive while using it is muscle tension which you translate to work being done. You need only put your bike on a trainer, clip in on only one side and slowly trace the circle that the pedal goes to understand how little power there is.
The way to produce the most power and efficiency within the pedal stroke is to look at the largest, strongest muscles and apply them only when they can effectively add power. That means looking at how the muscle extends a body part from a pivot and apply that to how the pedal goes around. The largest and strongest muscle group in the body is the glutes, which extend the femur from the hip.
As you can see, extending the femur from the hip pushes down (in this diagram I never change the tibia angle – my bad!) As the glutes can only push down, the effective portion of the pedal stroke will be from about 1:00 to 4:00 (we’ll get into the math of effectiveness within the pedal stroke a bit later).
The second largest muscle group is the quads, which extend the tibia from the knee.
As the quad extends the lower leg from the knee (I left out femur movement as the tibia extends forward – oops!) the foot is pushed forward. The quads can not push down – I can’t emphasize that enough, because lots of people insist that they can push down with their quads. Looking at the knee as the pivot point, the foot cam swing forward or back – not down. The idea of the quad pushing down is learned from standing or walking where the hip is directly above the foot and extending from the knee raises the body weight. The quads are only effective from 11:00 to 2:00.