Bike Fitting: Proper Pedal-Foot Interface
A fundamental element in bike fitting, the interface between your pedals and feet.
A cyclist contacts their bike in three places, the saddle, the bars and the pedals. How we connect to the bars and pedals is controlled by the reach from where we sit. When we are properly positioned on the bike with our center of gravity correctly located from front to back the rest of a good bike fit will fall easily into place. When we are badly positioned on the saddle, and the center of gravity is not correct the bike fit will be compromised in some fashion.
A good saddle position means it is level and set so that the sit bones are resting on the widest part of the saddle. (It's called a saddle because our legs hang over it as opposed to a seat which is something that we sit on.) Next, how a cyclist’s feet are positioned on the pedals is the next most important connection point. Ideally the balls of our feet are positioned over the pedals' spindles (their axles.)
To make this pedal foot connection work takes making the saddle height equal to about two-thirds of a rider's inseam measuring from the center of the bottom bracket (where the crank arms attach to the frame) to the top of the saddle along the seat tube. When done correctly this will yield a knee angle of 25-35 degrees when the cyclist's foot is at the bottom of the pedal stroke. (The bottom of the pedal stroke is when the crank arm is in line with the seat tube.)
This positioning means we will be on tiptoe more or less if we try to touch the ground while we are on the saddle so if a cyclist wants to be able to have their feet fully on the ground then the saddle needs to be lower and the foot pedal interface location needs to be further back toward the middle of our feet in order to maintain a healthy and efficient knee angle.