For more than 30 years, Gina Kolata reports, Dr. Benno 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. His overall conclusion:
Shoe inserts or orthotics may be helpful as a short-term solution, preventing injuries in some athletes. But it is not clear how to make inserts that work. The idea that they are supposed to correct mechanical-alignment problems does not hold up.
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In his studies, he found there was no way to predict the effect of a given orthotic. Consider, for example, an insert that pushes the foot away from a pronated position, or rotated excessively outward. You might think it would have the same effect on everyone who pronates, but it does not.One person might respond by increasing the stress on the outside of the foot, another on the inside. Another might not respond at all, unconsciously correcting the orthotic’s correction.
“That’s the first problem we have,” Dr. Nigg said. “If you do something to a shoe, different people will react differently.”
The next problem is that there may be little agreement among orthotics makers about what sort of insert to prescribe.
In one study discussed in his new book, “Biomechanics of Sport Shoes,” Dr. Nigg sent a talented distance runner to five certified orthotics makers. Each made a different type of insert to “correct” his pronation.
The athlete wore each set of orthotics for three days and then ran 10 kilometers, about 6 miles. He liked two of the orthotics and ran faster with them than with the other three. But the construction of the two he liked was completely different.
Then what, Dr. Nigg asked in series of studies, do orthotics actually do?
They turn out to 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.
In one recent review of published papers, Dr. Nigg and his colleagues analyzed studies on orthotics and injury prevention. Nearly all published studies, they report, lacked scientific rigor. For example, they did not include groups that, for comparison, did not receive orthotics. Or they discounted people who dropped out of the study, even though dropouts are often those who are not benefiting from a treatment.