Originally Posted by Whiteroom
Could someone explain how
a boot can exit the heel in anything but a pure vertical plane? The boot is captured by the metal base plate, the base plate is in-line with the tibia... so the heel can rotate a bit, this doesn't help release at the toe at all and it lower the ability for the boot to exit the binding at the heel, the heel piece can actually stay engaged with the heel lug until the boot is fully ejected and out of the toe, the heel needs to come straight up to release...
I'll give it a try; not a specialist like you on bindings but know a bit about bodies. "Lateral" to an anatomist means to the outside, away from the midline of the body. In a real body, most lateral movements of the leg or arm also include a rotational component, and often abduction. If you move a limb sideways, it also describes an arc in and out of a sagittal plane. Below the hip, all our joints (yes, including the knee, ankles, and foot itself) are not simply hinges. They routinely allow some rotation; the ankle allows a lot. Ligaments take up the strain as they absorb this rotation; the ACL is particularly involved. Also leg joints do some complex sliding with both lateral and sagittal components, but that's less relevant here.
So let's assume a binding that only allows purely lateral release at the toe, 90 degrees to midline and on the horizontal plane. We'll forget about upward release at the toe. So, just like shop tests with pistons are set up for. If we apply a spiral twisting sideways movement, such as in an actual fall, the foot is also trying to rotate around an axis (well, several, but we'll just assume one) between the tibia and the toe. Assuming the heel can only move upward, until the toe releases sideways, the shaft of the tibia has to take up the portion of the force that's rotational. That movement is sent to the ankle and up to the knee. However, a modern high stiff boot cuff will prevent much rotation at the ankle, so most of the force will be transmitted up the tibia and fibula, which will rotate to the detriment of knee ligaments, particularly the ACL. Moreover, a twisting fall also typically involves downward flexion at the knee as you move from a vertical extended position to a collapsed position. That in itself will tend to create inward rotation at the knee. Which will preload the ACL, eg, stretch it taut, as it's also dealing with increasing torsion coming up the tibia. Or if the force is very high, over a briefer time interval, the tibia or fibula will fail, typically just above the boot, before you flex downward. By contrast, if the heelpiece allows some rotation, as the toe rotates outward, the heel can rotate inward (think propeller). So there is less rotational force that the tibia has to deal with before the toe is out of the binding; the heel "helps" in the counter direction. The classic Look turntable allows some limited rotation, regardless of whether it ultimately releases upward or at all, and regardless of whether the sides allow lateral displacement of the boot. Rotation not equal to lateral, in short.
Other advantages of classic Looks (and to a lessor degree the PX's) have to do with the location of the binding axis relative to the tibia, as it relates to fractures, but that's not your question. And let's not get going on actual lateral release designs, or Chris from RI will feel obligated to chime in with how many days he's been injury free, and someone will have to remind us that Tyrolias only release at 150 degrees, and we'll need to replicate several hundred posts over the years.
Now back to our regular scheduled trashing of Markers...Edited by beyond - 4/4/10 at 10:07pm