A research team studied the interaction of AFO bending stiffness with neutral angle and footplate stiffness on the effect of bending stiffness on walking energy cost, gait kinematics, and kinetics in people with bilateral plantarflexor weakness. The researchers concluded that AFO bending stiffness and neutral angle substantially interact regarding their effect on energy cost of walking. This suggests that these properties should be tuned together to achieve maximal treatment outcomes.
Bilateral weakness of the plantarflexors is a common symptom in neuromuscular diseases like Charcot-Marie-Tooth disease. In case of bilateral plantarflexor weakness, the forward rotation of the tibia increases when the center of pressure moves anterior of the ankle joint during gait. Consequently, when the center of pressure progresses forward during the stance phase of gait, the ankle moves into excessive ankle dorsiflexion, which forces the knee into flexion, reducing walking speed and increasing the energy cost of walking.
The simulation framework consisted of a planar 11 degrees of freedom model, containing 11 muscles activated by a reflex-based neuromuscular controller. The controller was optimized by a comprehensive cost function, predominantly minimizing walking energy cost. The dorsal leaf AFO bending and footplate stiffness were modelled as torsional springs around the ankle and metatarsal joint. The neutral angle of the AFO was defined as the angle in the sagittal plane at which the moment of the ankle torsional spring was zero.
Simulations without the AFO and with the AFO for nine bending stiffnesses (0-14 Nm/degrees), three neutral angles (0-3-6 degrees dorsiflexion) and three footplate stiffnesses (0–0.5–2.0 Nm/degrees) were performed.
When changing the neutral angle towards dorsiflexion, the researchers found that a higher AFO bending stiffness minimized energy cost of walking and normalized joint kinematics and kinetics. Footplate stiffness mainly affected metatarsophalangeal joint kinematics and kinetics, while no systematic and only marginal effects on energy cost were found, according to the study.
The open-access study, “Interacting effects of AFO stiffness, neutral angle and footplate stiffness on gait in case of plantarflexor weakness: A predictive simulation study,” was published in the Journal of Biomechanics.