Making A-FOOTPRINT: A Musculoskeletal Foot/Ankle Model with 50 Degrees-of-Freedom for Arthroplasty and Orthotics

The human foot is structurally complicated and its basic load-carrying mechanisms remain a subject of debate in the biomechanics community. Several multisegment foot models have been proposed. Some of the most detailed models contain eight segments [1], but most are limited to two segments and it is unlikely that such a simplified model can accurately represent the biomechanics. No rigid body biomechanical foot model representing all 26 foot bones has been developed to this day. The objective of this investigation is to develop a detailed multisegment musculoskeletal model of the foot to get insights into foot biomechanics during gait for healthy and pathological feet. In particular, the model should provide detailed information on (i) individual foot joints, (ii) extrinsic and intrinsic muscles in the foot and (ii) major ligaments covering joints in the foot. The model should give an insight into the context of mechanics in the foot, but also be able to predict for subject specific cases treatment possibilities. Motion capture, EMG, force and pressure plate data, as well as CT and foot surface scans were recorded from 20 subjects. The data were used to create a musculoskeletal model in the AnyBody Modeling System. Bone geometries of 26 rigid segments representing all the bones of the human foot (except the sesamoid bones) were aligned and connected with kinematic links, Figure 1. All major foot ligaments and foot muscles are implemented in the model. Measured EMG of was compared to predicted muscle activations and ligament behavior was compared to cadaver experiments. EMG corresponded well with predicted muscle activities in the lower leg. The curves comparing the ligament stiffness from simulation and experiment show an overall good correlation. This is currently the most detailed musculoskeletal model available with an intended use to develop implants or orthotics for the foot to treat diseases like the flatfoot and metatarsalgia syndrome. This work is part of the A-FOOTPRINT project, funded by the European Commission Seventh Framework Programme (Grant Agreement NMP2-SE-2009-228893). 1. MacWilliams, B.A., M. Cowley, and D.E. Nicholson, Foot kinematics and kinetics during adolescent gait. Gait & Posture, 2003. 17(3): p. 214-224.