June 2010

Investigators examine biomechanical effects of carbon composite devices

AFOs must find energy balance

The extent to which carbon composite ankle foot orthoses improve gait efficiency in stroke patients may be a function of walking speed, according to a Swedish study. And despite the fact that the devices are known for their energy storage, research from the Netherlands suggests that the relationship between energy storage and energy cost may be complicated.

Investigators from the University of Gothenburg in Sweden studied 10 patients with hemiparesis as a result of a stroke, which had occurred at least six months previously. All patients had used a carbon composite AFO for at least three months prior to the study.

Study subjects were analyzed as they walked on a treadmill for five minutes at two self-selected speeds, unbraced (speed 1) and with the AFO (speed 2). Additional analysis was performed while walking at speed 1 with the AFO.

As previously reported in the July 2004 issue of the Journal of Rehabilitation Medicine, the researchers found that the mean self-selected walking speed was 20% higher with the AFO than without. When unbraced walking at speed 1 was compared to braced walking at speed 2, energy expenditure (rate of oxygen consumption), heart rate and perceived exertion did not differ significantly between conditions. However, energy cost (oxygen consumption per meter) was 12% lower with the AFO.

New data presented in poster form at the ISPO meeting in Leipzig compared braced and unbraced conditions at speed 1, with some different results. Energy cost remained significantly lower with the AFO than without, but energy expenditure was also 4% lower with the AFO. This difference was statistically significant, but the authors noted that it may not be clinically important, since the subjects’ self-reported level of perceived exertion did not differ significantly between any of the test conditions. The increase in walking speed associated with the AFO, however, the authors believed would be clinically significant.

Stiffness is another factor that affects energy cost while walking with an energy-storing AFO. If the device is too stiff, energy is released too soon; too compliant, and the device doesn’t store enough energy. Complicating matters further is that maximal energy storage does not neccesarily correlate with maximal walking efficiency, according to research from Vrije University Medical Center in Amsterdam, the Netherlands.

Investigators created a conceptual, seven-segment computer model to analyze walking with a carbon composite, energy storing AFO. By changing the stiffness of the AFO within the model, the researchers could calculate the predicted effect on energy storage and energy cost.

A stiffness of 41 Nm/rad was associated with optimal energy cost. By comparison, a stiffness of 29 Nm/rad was associated with maximum energy storage. Optimal energy cost was also associated with the stiffness that maximized ankle push-off velocity just before contralateral foot strike, according to Daan Bregman, a doctoral student at the university, who presented the findings at the ISPO meeting. Walking speed did not affect the results.

“Optimal stiffness for energy cost is not the same as optimal stiffness for stored energy,” Bregman said. “When the most energy is stored in the AFO, gait is not the most efficient, because the timing of energy return is very important.”

Because the computer model used in the study was simplified, Bregman said it is likely that the stiffness associated with optimal energy cost will be differ in individual patients, which will be addressed in future studies.

High heeled, wedged shoes redistribute plantar pressure toward medial forefoot

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High heeled wedge shoes such as espadrilles redistribute plantar pressure away from the heel and toward the medial forefoot and great toe, similar to the effects of more conventional high heels, according to research from Slovenia.

Investigators from the University Rehabilitation Institute in Ljubljana analyzed plantar pressures in 15 female college students walking 10 meters while wearing socks only and while wearing shoes with wedge cork soles of varying heights. The shoes were designed with heel heights of 2 cm and 5 cm under the calcaneus, tapering down to a forefoot height of zero under the metatarsal joints.

They found that pressures under the big toe increased significantly with heel height on both sides; on the right side, pressures under the first metatarsal head also increased significantly with heel height. Pressures under the heel decreased significantly with wedge height on the left side. Overall, pressures under the lesser metatarsal heads decreased with heel heights, but these findings were statistically significant primarily for the fifth metatarsal joints on both sides.

The resulting pressure gradients suggest that wearers of high heels may be subject to increased shear forces, according to researcher Tomaz Stajer, who presented the findings at the ISPO meeting in Leipzig.

Although few previous studies have looked at the effect of high heels on plantar pressure, University of California-Davis researchers reported in a 1992 Foot & Ankle study that non-wedged high heeled shoes were associated with increased maximum peak pressure and rate of loading under the metatarsal heads. And in a 2005 study in Foot & Ankle International, Taiwanese researchers found that increasing heel heights to 7.6 cm shifted plantar pressure from the heel and midfoot to the medial forefoot, as well as increasing vertical and anteroposterior ground reaction force.

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