Certainly, f=ma, but with all else being equal, I'd say that an increase in lever length (hip joint to footprint) MIGHT explain an increase in speed. Consider:

Certainly, f=ma, but with all else being equal, I'd say that an increase in lever length (hip joint to footprint) MIGHT explain an increase in speed. Consider: an inch of shoe is probably lighter than an inch of leg. I resisted the urge to call it "an inch of man" - you're welcome. An additional factor: extra cushioning and the different shoe shape resulted in the runner not bending his knees as much. This could result in increased lever length as I described above; it could also explain the knee pain: the knee joint was experiencing increased shock during impact with the ground.