This is something that we discuss at the Boot Camp. Essentially, its aimed at those people whose feet move at different speeds!
During a plantar pressure measurement or a slow motion video gait analysis, either a different velocity is seen in the left and right center of pressure or a difference is seen between the left and right heel coming off the ground:
- with a pressure measuring system, the differences can be seen when comparing the velocity of the center of pressure between the two feet. The assumption is that the velocity should be the same for the left and right sides (ie symmetry is good).
- with slow motion video, its a bit more difficult. What I do is move through frame by frame and look for the frame when the leading leg first gets heel contact. In that frame, note the height that the trailing leg's heel is off the ground. Then repeat for the other side. The assumption is that the amount the heel is of the ground should be the same on both sides (ie symmetry is good).
If there is an asymmetry in the timing of weight coming off the heel, then its is assumed that one foot is moving faster than the other and the person wants to keep walking in a circle! To prevent walking in a circle, there then needs to be a lot of asymmetrical activity in the pelvis and low back area, potentially resulting in problems in that area (ie asymmetry is bad).
To deal with this, its is suggested that you use a harder heel raise to speed up the slower side (I use 3mm polypro) and use a softer heel raise to slow down the faster side (I use 6mm poron). For more see this discussion: Asymmetrical density heel raises