September 2009

In the Moment: Footwear 3338991 3338991

by Jordana Bieze Foster

Kids’ loads shift underfoot

Midfoot contact area changes over time

Loading patterns during running change dramatically over time in young children, according to research from the Nike Sport Research Laboratory that could significantly affect the way children’s footwear is designed.

As part of the larger Kids Global Research Project, Nike investigators analyzed running data for 61 children who were tested once a year for three consecutive years. The children ran barefoot at a self-selected speed (the speed at which they would be running on a playground), on a runway covered with a medium-density foam material similar to that of a running shoe midsole.

Pressure data indicated a significant decrease in medial midfoot contact area between the ages of three and seven, which is suggestive of longitudinal arch development during that period. The findings were surprising, in light of a 1994 study from Essen University in Germany that found no evidence during walking of continuing logitudinal arch development in children between the ages of six and 10, published in Foot & Ankle International.

The Nike study also found that midfoot contact area continued to change even after longitudinal arch development was complete, as children between the ages of nine and 11 demonstrated significant increases in lateral midfoot contact area. This may be related to the concurrent finding of increased normalized (to body weight) peak loading under the lateral midfoot during the same age range. The increased loading may be the result of an increase in self-selected running speed, increasing muscle strength, or changes in coordination as children age, according to Nike researcher Martine I.V. Mientjes, PhD, who presented the findings in late August at the annual meeting of the American Society of Biomechanics.

Interestingly, the researchers found that midfoot contact area in children may be influenced not just by age but also by gender. Across all ages within their study population, they found that boys consistently had a larger midfoot contact area than girls of the same age, although the trend was not statistically significant. This trend was not seen once the data were normalized to body weight, and boys were not significantly heavier than girls, suggesting that the finding was not simply reflective of differences in body weight. It is also consistent with a 2004 FAI study from University Hospital Munster in Germany, which reported a significantly broader midfoot in boys than in girls during the first year of walking.

In a separate analysis of the same population, the Nike researchers also found that a surprisingly high percentage of children run using a heel-strike pattern, even in the earliest stages of running development. The researchers had hypothesized that initially the youngest children would tend to run flatfooted or landing on the forefoot, and that the heel-strike pattern would become more common as children got older. What they found, however, was that more than 80% of children were already heel strikers at age three, a patten adopted by more than 93% of those over the age of six. This was in spite of the fact that the children were tested while running barefoot, which is typically associated with more of a forefoot-strike pattern.

“A large percentage of young kids are already landing on their heel, even though they are running barefoot,” Mientjes said.

A more expected finding was that peak forces under the entire foot increased with age, but that finding remained true only for the forefoot after the force data were normalized to body weight.

The Nike group’s findings suggest that future designs for children’s footwear might incorporate targeted cushioning, age-specific footbed designs with regard to midfoot loading, and gender-specific widths, Mientjes said.

Stair descent study adds to evidence that high heels increase knee OA risk

Research from Iowa State University adds to the evidence that wearing high-heeled shoes increases the risk of medial compartment knee osteoarthritis.

Investigators analyzed seven young adult women as they descended a three-step wooden staircase at a self-selected pace while wearing either low-heeled (heel of less than 2 cm) or high-heeled (average 7 cm) footwear.

Peak knee varus torques, which are thought to predispose the knee to medial degeneration, were significantly higher during stance phase for the high-heeled condition than the low-heeled condition. However, the researchers observed no significant between-condition differences in center of pressure excursion, possibly because subjects walked 8% more slowly while wearing high heels, which might have compensated for increased instability. The findings were presented in August at the annual ASB meeting.

Previous studies have found similar increases in knee varus torque associated with high heeled shoes while ascending stairs or walking on level ground. In 2005, University of Virginia researchers observed the same type of effect even in shoes with moderately high (1.5 inch) heels.

Data suggest preschool children run differently in flip-flops than sneakers

Flip-flops may be appealing to parents of growing children as an inexpensive shoe option, but research suggests the flimsy footwear has some of the same adverse effects on gait in young children as in adults.

In 12 preschool-aged children, investigators at Auburn University found that stride length during running was significantly shorter while wearing flip-flops than while wearing standardized sneakers. The same group found a similar effect of flip-flops in adults during walking in a study presented at the 2008 meeting of the American College of Sports Medicine, which is now in press. Decreased stride length could be the result of altered mechanics, however, it could also reflect the difference in mass between the two shoe types.

In adults, the researchers also found that flip-flops were associated with decreased ankle dorsiflexion, thought to stem from an effort to keep the sandal from falling off. But in the more recent study, which was presented in August at the annual ASB meeting, no significant differences in ankle dorsiflexion were reported.

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