Impact of Active Ankle Foot Orthoses on Muscle Activation and Achilles Tendon Unloading: A Musculoskeletal Modeling Study

The Triceps Surae muscles, comprising Soleus, Gastrocnemius Medialis and Lateralis, are located in the posterior compartment of the lower limb and are responsible for the plantar flexion of the foot. These muscles are linked to the calcaneus bone through the Achilles Tendon (AT), the thickest and strongest tendon in the human body. During the late-stance and push-off phases of the gait cycle, these muscles contract and generate high forces acting on the tendon that, despite its size and strength, often get injured. Injuries to the AT, like inflammation or rupture, are common, particularly among athletes, and evidence reported long-term effects and many bio-mechanical deficits in the post-surgery period. Despite the lack of extensive scientific evidence on optimized rehabilitation programs after experiencing an AT injury, traditional ankle immobilization, while intended for healing, can lead to muscle atrophy, thrombosis events, and decreased muscle-tendon units’ functionality. In this regard, active ankle-foot orthoses (AFOs) are a promising solution, facilitating gradual rehabilitation by supporting the gait, strengthening lower leg and foot muscles, and reintroducing functional movements while minimizing pain and risk of re-injury. In this context, the present study aims to evaluate the impact of active AFOs on triceps muscles activation and to provide insights into Achilles tendon unloading to eventually inform active rehabilitation protocols. The research is developed through a twofold approach: experimental trials and OpenSim musculoskeletal (MSK) modeling. Gait data were collected from four subjects walking on an instrumented treadmill at controlled speed. We measured surface electromyography (sEMG) signals, kinematic and kinetic data to investigate the effects of the AFOs on muscle activity and Achilles tendon load. We collected data under five conditions: not wearing the AFOs, wearing the AFOs without any assistance and wearing the AFOs under three increasing assistance levels—low, medium, and high. First, we evaluated the activation of the Triceps Surae and Tibialis Anterior muscles in the experimental sEMG, as they were a good indicator of the force acting on the Achilles tendon, then, we developed a MSK simulation to study the recruitment of individual muscles and, eventually, simulating the forces acting on the Achilles tendon when different assistance levels were provided by the active AFOs. Preliminary results indicated a significant reduction in muscle activity and Achilles tendon force. From the analysis of the experimental sEMG signals, we observed an average reduction across subjects of the peak of Soleus activation by 34,75% at the highest assistance level. Similarly, the Gastrocnemius Medialis showed an average decrease by 15,32%. The Gastrocnemius Lateralis experienced the highest reduction, with an average decrease by 40,25% across the analyzed subjects. The outcomes from the OpenSim MSK modeling confirmed a decrease in the Triceps Surae muscles activation and an average reduction across subjects in the force exerted on the Achilles tendon by 36,5%, when the highest assistance was provided by the active AFOs. These findings demonstrated the efficacy of powered AFOs in assisting lower limb dynamics and their integration into active rehabilitation programs targeting Achilles tendon injuries. Furthermore, our use of the musculoskeletal modeling software OpenSim highlighted the utility of such approaches in both biomechanical research and clinical applications.