What the hell
means freemoment?
:confused:
I am german orthosurgeon and I (63) read this word the first time, trying out to find what ist means exactly.
Sincere Gerhard Fleischner Schliersee Germany
Free moment is simply the torsional force acting on the foot and/or lower extremity that is generated by frictional forces between the foot and ground during weightbearing activities.
Free moment is determined by a force plate and the direction of the moment is within the plane of the force plate (i.e. transverse plane in vertical standing).
A free moment would tend to cause a torsional force along the long axis of the tibia/fibula and femur during weightbearing activities.
Free moment is a relatively new term in biomechanics which I just heard for the first time last month during a presentation given by Dr. Irene Davis at the PFOLA seminar in Vancouver.
Kinetic asymmetry in female runners with and without retrospective tibial stress fractures
Rebecca Avrin Zifchock, Irene Davisa,
and Joseph Hamill
Journal of Biomechanics Volume 39, Issue 15 , 2006, Pages 2792-2797
Going to take my life in my hands and try to explain freemoments.
The Free moment exists because of the unique ability of the foot to push in two direction at once by twisting about an instantaneous vertical (y) axis on the forceplate.
This causes a small moment at the centre of the force couple under the foot.
This is not possible in the horizontal axes unless the foot is adhered in some way to the foorceplate.
The freemoment is calculated as a product of the X and Z forces, the distance between strain gauge posts and the Horizontal CoP of interest (xCoP or zCoP) Typical peak values are 3 -6N/m.
This freemoment is not the same as the moment about the vertical y axis of reference of the force plate. These are produced by any force tending to rotate the force plate around its referential centre. There are no typical peak values since the moment value is determined by position and magnitude of the horizontal force on the force plate.
See attached for details.
The magnitude of the moment transmited thru the limb of interest, the Femur or Tibia for instance, is determined by the Cosine of the limb's angle of inclination to the forceplate.
So if the tibia was directly above the freemoment (magnitude 3N/m) and at an angle of 45dgs then 2.12N/m torque could be transmitted along the tibia.
If then the Femur was perpendicular to the ground then only 1.49N/m would be transmitted to the Femur. At 90dgs the transmission is zero as the Cos of 90 is zero.
Therefore freemoments probably do not have a direct correlation with torsional stress in a long bone. Rather the style of gait that produces freemoments may also produce high torsional stresses in the long bones.
So for example if you were a forefoot striking runner with a toe out gait this may produce quite high peaks of freemoments, the freemoments themselves are not pathological to the long bones but the torque that is produced by gait style IE the high posteriorly acting horizontal
GRF acting on the toe out forefoot will produce a proportionatly high moment at the tibia and incidentally a twisting action under the mets, which produce a relatively high freemoment.