Neuromuscular rehabilitation research targets powered AFOs

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November 2006 Biomechanics Magazine

http://www.biomech.com/showArticle.jhtml?articleID=193700258

Neuromuscular rehabilitation research targets powered AFOs

By: Jordana Bieze

Even as studies continue to support the concept of ankle foot

orthoses powered by pneumatic muscle actuators for patients with

impaired plantar flexion, research also suggests that practitioners

should consider an AFO's effect on neuromuscular control when

weighing rehabilitation options.

Powered AFOs (alternatively known as " actuated AFOs " or " active

AFOs " ), were first described by Danish scientists in 1995. Although

they were originally designed for studying reflex responses during

gait, more recent models are being developed with the potential for

neuromuscular rehabilitation following stroke or spinal cord injury.

Using a powered AFO design pioneered at the University of Michigan,

researchers from the collaborative Virginia Tech-Wake Forest

University School of Biomedical Engineering created a computer

algorithm to trigger a pneumatic plantar-flexion contraction in an

ambulating subject 60 ms after the subject's heel began to lift off

the ground. The contraction was discontinued after 200 ms,

consistent with toe-off.

When two young adult volunteers wore the powered AFOs while walking

overground, activation of the pneumatic system increased walking

speed by 0.14 m/s in one subject and by 0.07 m/s in the other,

compared to wearing the device while inactivated. Average net ankle

power for the two subjects increased from 2.45 W/kg to 3.2 W/kg.

AFO activation also increased mean walking speed in nine young adult

volunteers who wore the devices while walking on a treadmill while

decreasing metabolic cost of transport (see table). The findings

were presented in September at the annual meeting of the American

Society of Biomechanics.

The researchers, however, are finding that the powered AFO is not

associated with significant increases in walking speed in their

target population of older adults, said A. Norris, a graduate

student in the school's Musculoskeletal Biomechanics Laboratory, who

presented his group's findings at the ASB meeting.

" Maybe older adults are more interested in being more stable, and

maybe this model actually makes them less stable, " Norris said.

Meanwhile, research from the University of Delaware suggests that

even the use of passive AFOs should take into account the devices'

effects on muscle activation. After developing a computer model to

predict these effects, the investigators found that passive AFO use

decreased tibialis anterior activation in late stance. Between 15%

and 40% of the gait cycle, soleus activation was less than nominal,

but during preswing soleus activation was slightly higher than

nominal. Timing of both ankle muscles was not different between the

two conditions.

" If you allow the patient to walk continuously with less tibialis

anterior use, then you're not training the tibialis anterior, " said

A. Crabtree, a graduate student in mechanical engineering at

the university, who presented his group's findings at the ASB

meeting. " Ultimately we hope this technique can provide therapists

with an extra tool to help tune the AFO. "

The potential for reduced muscle activation was a point of interest

for the Michigan researchers (whose powered AFO design was featured

in the Virginia Tech-Wake Forest study) when they tested the

feasibility of using powered AFOs in five patients with chronic

incomplete spinal cord injury. They found that use of the devices

decreased soleus activation by 13%; activation of the medial and

lateral gastrocnemius decreased by 7% and 5%, respectively, but

those differences were not statistically significant. Tibialis

anterior activation did not change.

The results were published in the February issue of the Journal of

NeuroEngineering and Rehabilitation.