Not because they float but because they might be irritating your metatarsal area. If your foot isn't happy you will change your pedal stroke to compensate and that might put more pressure on your knee. Speedplays seem to irritate the metatarsal area more than other pedals with a larger base to push off of, like Time and Look pedals. Having said that, more float is not always better. Some people actually do better with less float as long as the cleat position is properly aligned.