A 3D Neuromuscular Model of the Human Ankle-Foot Complex Based on Multi-Joint Biplanar Fluoroscopy Gait Analysis

During the gait cycle, the human ankle complex serves as a primary power generator while simultaneously stabilizing the entire limb. These actions are controlled by an intricate interplay of several lower leg muscles that cannot be fully uncovered using experimental methods alone. A combination of experiments and mathematical modeling may be used to estimate aspects of neuromusculoskeletal functions that control human gait. In this research, a three-dimensional neuromuscular model of the human ankle-foot complex based on biplanar fluoroscopy gait analysis is presented.

Biplanar fluoroscopy (BiFlo) enables three-dimensional bone kinematics analysis using x-ray videos and bone geometry from segmented CT. Hindered by a small capture volume relative to traditional optical motion capture (MOCAP), BiFlo applications to human movement are generally limited to single-joint motions with constrained range. Here, a hybrid procedure is developed for multi-joint gait analysis using BiFlo and MOCAP in tandem. MOCAP effectively extends BiFlo’s field-of-view. Subjects walked at a self-selected pace along a level walkway while BiFlo, MOCAP, and ground reaction forces were collected. A novel methodology was developed to register separate BiFlo measurements of the knee and ankle-foot complex. Kinematic analysis of bones surrounding the knee, ankle, and foot was performed. Kinematics obtained using this technique were compared to those calculated using only MOCAP during stance phase. Results show that this hybrid protocol effectively measures knee and ankle kinematics in all three body planes. Additionally, sagittal plane kinematics for select foot bone segments (proximal phalanges, metatarsals, and midfoot) was realized. The proposed procedure offers a novel approach to human gait analysis that eliminates errors originated by soft tissue artifacts, and is especially useful for ankle joint analysis, whose complexities are often simplified in MOCAP studies.

Outcomes of the BiFlo walking experiments helped guide the development of a three- dimensional neuromuscular model of the human ankle-foot complex. Driven by kinematics, kinetics, and electromyography (EMG), the model seeks to solve the redundancy problem, individual muscle-tendon contributions to net joint torque, in ankle and subtalar joint actuation during overground gait. Kinematics and kinetics from BiFlo walking trials enable estimations of muscle-tendon lengths, moment arms, and joint torques. EMG yields estimates of muscle activation. Using each of these as inputs, an optimization approach was employed to calculate sets of morphological parameters that simultaneously maximize the neuromuscular model’s metabolic efficiency and fit to experimental joint torques. This approach is based on the hypothesis that the muscle-tendon morphology of the human leg has evolved to maximize metabolic efficiency of walking at self-selected speed. Optimal morphological parameter sets produce estimates of force contributions and states for individual muscles. This research lends insight into the possible roles of individual muscle-tendons in the leg that lead to efficient gait.