Originally Posted by Kevin Kirby
If the STJ axis is medially deviated relative to the forefoot during cycling, then during the power stroke, when the pedal reaction force is at its greatest magnitude, the STJ pronation moments will also be at their greatest magnitude. Increased STJ pronation moments will tend to cause more STJ pronation motion which, with the forefoot locked into the pedal, will tend to cause adduction of the knee toward the top tube (i.e. internal rotation and adduction at the hip).
This can be demonstrated by standing on your metatarsal heads on the ground, with your knees flexed, then pronating the STJ...the knee should adduct toward the opposite knee. Very little, if any, knee adduction will occur with STJ pronation when the knee is in an extended position since the knee joint is located nearly on the same line between the hip and foot when the knee is extended. However, with the knee flexed, the knee joint is a much larger distance anterior to the line between the hip and the foot, which will cause much greater knee displacements within the transverse and frontal planes than when performing the same maneuver with the knee extended.
While seated in my chair, with my forefoot touching the ground, and creating a pronation motion of the STJ, I get a medial shift of the inferior portion of the tibia. This motion, if the top of the tibia stayed in the same place would be adduction of the tibia in the frontal plane. (This is the big if in our discussion. What causes or prevents movemnt of the proximal end of the tibia and distal end of the femur.) The motion on the bicyle that we are trying to explain is movement of the top of the tibia toward the top tube of the bicycle. This motion is either an abduction of the tibia in the frontal plane or adduction of the hip. I think we should look for moments acting on the femur and tibia to explain the medial deviation of the knee at the top of the downstroke.
I'm still having difficulty understanding how pronation moments or motions cause frontal plane motion of the tibia. If the foot was fixed in space and the tibia is alowed to move with STJ pronation, there would be lateral deviation of the knee with the down stroke. Yes this motion would be magnified by the distance from the knee to the STJ, but the motion is in the wrong direction.