Effect of Mono- Versus Bi-Articular Ankle Foot Orthosis on Muscular Performance of the Lower Leg

Ankle foot orthoses (AFOs) are used to assist impaired gait. In this study, we investigated the effects of a mono- and a bi-articular AFO with different stiffness on the muscles’ performance in the lower extremities. The effects of both AFOs on the activation level and power of gastrocnemius, soleus and tibialis anterior muscles together with knee extensors and flexors were simulated in Opensim. The muscular performances were compared between the case without orthosis and the case with mono- and bi-articular AFO with different stiffness values (5, 10, 15, 20 and 25 kN/m). The simulations showed that increasing the stiffness of both AFOs led to reductions (up to 70%) of the activation levels of soleus and gastrocnemius in comparison to the case without orthosis. The overall stronger effects caused by the bi-articular configuration led us to investigate its effects on the knee extensors and flexors. Indeed, bi-articular AFO led to strong activation of the knee extensor muscles, suggesting a power interaction between upper and lower leg. Therefore, an intelligent design of mono- and bi-articular AFO with soft springs (<10 kN/m) may potentially allow to balance muscle activation and power consumption of the respective muscles of the leg. Experiments with customized patient-specific AFOs and with different human subjects walking at different speeds will be required to validate these conclusions.