Minimization of metabolic cost of transport predicts changes in gait mechanics over a range of ankle-foot orthosis stiffnesses in individuals with bilateral plantar flexor weakness

• Published in: bioRxiv
• Main Authors: Kiss, Bernadett, Waterval, Niels Fj, Marjolein M Van Der Krogt, Brehm, Merel A, Geijtenbeek, Thomas, Harlaar, Jaap, Seth, Ajay
• Format: Paper
• Language: English
• Published: Cold Spring Harbor Cold Spring Harbor Laboratory Press 18.10.2022; Cold Spring Harbor Laboratory
• Edition: 1.1
• Subjects: Adaptation; Ankle; Bioengineering; Gait; Hip; Metabolism; Muscles; Neuromuscular diseases; Quadriceps muscle; Simulation; Walking; Weeds
• ISSN: 2692-8205; 2692-8205
• DOI: 10.1101/2022.10.14.512205

Neuromuscular disorders often lead to ankle plantar flexor muscle weakness, which impairs ankle push-off power and forward propulsion during gait. To improve walking speed and reduce metabolic cost of transport (mCoT), patients with plantar flexor weakness are provided dorsal-leaf spring ankle-foot orthoses (AFOs). The mCoT during gait depends on the AFO stiffness where an optimal AFO stiffness exists that minimizes mCoT. The biomechanics of why and how there exists a unique optimal stiffness for individuals with plantar flexor weakness are not well understood. To help understand why, we hypothesized that gait adaptations can be predicted by mCoT minimization. To explain how, we hypothesized that the AFO would reduce the required support moment and, hence, metabolic costs from the ankle plantar flexor and knee extensor muscles during stance and reduce hip flexor metabolic cost to initiate swing. To test these hypotheses, we generated neuromusculoskeletal simulations to represent gait of an individual with bilateral plantar flexor weakness wearing an AFO with varying stiffness. Predictions were predicated on the goal of minimizing mCoT at each stiffness level, and the motor patterns were determined via dynamic optimization. The simulation results were compared to experimental data from subjects with bilateral plantar flexor weakness walking with varying AFO-stiffness. Our simulations demonstrated that minimization of mCoT predicts gait adaptations in response to varying AFO stiffness levels in individuals with bilateral plantar flexor weakness. Initial reductions in mCoT with increasing stiffness were attributed to reductions in quadriceps metabolic cost during midstance. Increases in mCoT above optimum stiffness were attributed to the increasing metabolic cost of both hip flexor and hamstrings muscles. The insights gained from our simulations could inform clinicians on the prescription of personalized AFOs. With further model individualization, simulations based on mCoT minimization may sufficiently predict adaptations to an AFO in individuals with plantar flexor weakness. Competing Interest Statement The authors have declared no competing interest.