The Effect of Dorsiflexion Resistance on Ankle and Knee Joint Kinematics Quantified with an Instrumented Ankle-Foot Orthosis During Treadmill Walking
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Orthotic management of foot drop is commonly treated with an ankle foot orthosis (AFO) that incorporates a dorsiflexion assist motion control feature. Since most AFO's are passive devices, the orthotic dorsiflexion assist mechanism may consequentially resist the natural movements that occur during the stance portion of gait cycle and instead may result in undesirable dorsiflexion and/or plantar flexion perturbations. The purpose of this study was to determine what influence dorsiflexion resistance may have on ankle movements during walking. We hypothesized that dorsiflexion resistance, if applied to the foot/ankle complex in healthy subjects during treadmill walking, will decrease the range of ankle dorsiflexion in late stance phase compared to an unconstrained (i.e.,free) ankle condition. Our pilot study for this research focused on the development of an instrumented AFO to measure both ankle motion and the dorsiflexion resist force imposed on a subject's leg and foot during treadmill walking. Bungee cords were used to apply varying magnitudes of orthotic restraint. The instrumented AFO was donned by subjects (n=10) who walked on a treadmill with five pre- randomized levels of dorsiflexion resistance and a control. The control condition was no orthotic constraint, where the instrumented AFO was worn with no bungees attached. Three trials were performed for each of the six test conditions. The results confirmed our hypothesis that increasing dorsiflexion resistance will decrease the mean peak angle dorsiflexion during late stance in treadmill walking. The findings of this study indicate that stiffness properties of an AFO can have a notable impact on gait kinematics by inducing significant perturbations to ankle motion. Therefore, clinicians should be cognizant of the motion control stiffness properties when designing an AFO. Stiffness properties greater than clinically needed to achieve toe clearance during swing, may limit the optimal functional performance and outcome potential of the AFO user .