friction syndrome by Kevin C. Farrella, Kim D. Reisingera, and Mark D. Tillman.
It's pretty instructive. The vertical axis is relative force. The horizontal axis is crank angle. Zero degrees is the top of the pedal stroke, 90 degrees is horizontal, 180 is the bottom, 270 is the foot at the back of the stroke.
The geometry of the knees, hips, ankle and crank that determine the shape of this chart. There are a few cardinal points around the cycle.
Force acts along a line between the hip and the pedal (more or less) through the first half of the stroke. When this line is directed along the pedal path (at about 68 degrees for me), all of the force of the straightening leg goes into propelling the bike. But at that point, the quads and glutes aren't producing maximum force. That happens at the peak of the chart, just past horizontal.
As the leg straightens out past the 120-130 degree crank angle, the force of straightening the leg is no longer tangent to the pedal path, so the hamstrings and glutes kick in to swing the cranks around and the force drops of rapidly.
There's a big potential for wasted effort, so it seem plausible that a cyclist could gain quite a bit of power from just developing a more efficient pedal stroke.
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