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
Authors: Kiss B. , Waterval N.F.J. , van der Krogt M.M. , Brehm M.A. , Geijtenbeek T. , Harlaar J. , Seth A. 
Published: October 2022

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.

Author Summary

Neuromuscular disorders like stroke, Charcot-Marie-Tooth disease, and poliomyelitis often lead to calf muscle weakness, which makes walking slower and more demanding. To improve walking speed and reduce energy demand, patients with calf muscle weakness are frequently provided ankle-foot orthoses (AFOs). The energy demand of walking is affected by the AFO’s stiffness and there is a stiffness that minimizes the energy demand for an individual with calf weakness. To uncover the optimal stiffness, we generated simulations of an individual with calf muscle weakness walking with an AFO over a range of stiffnesses. Stable walking patterns were generated that minimized the energy demand for a given stiffness. We found that the initial reductions in energy demand as stiffness increased, were attributed to reductions in quadriceps muscle energy. Increases in energy demand as stiffness increased above the optimum were attributed to the increased energetic cost of both hip flexor and hamstrings muscles. With further model individualization, we believe that simulations based on minimizing the energy demand of movement can sufficiently predict adaptations to an AFO. Simulations can enable the prescription of personalized AFOs for individuals with neuromuscular disorders that help them walk with sufficient speed and efficiency to keep up with their peers.