For more than 30 years Dr. Nigg, a professor of biomechanics and co-director of the Human Performance Lab at the University of Calgary in Alberta, has asked how orthotics affect motion, stress on joints and muscle activity. From
Dr Nigg's research, he concluded that orthotics have little effect on kinematics — the actual movement of the skeleton during a run. But they can have large effects on muscles and joints, often making muscles work as much as 50 percent harder for the same movement and increasing stress on joints by a similar amount.
As for “corrective” orthotics, he says, they do not correct so much as lead to a reduction in muscle strength.