A biomechanical model of the human foot emphasizing the plantar soft tissue was developed. The ground reaction forces (GRFs) beneath seven areas (five submetatarsal, subhallucal and subcalcaneal) were quantified for 20 asymptomatic subjects. These data were employed to guide subsequent cadaveric plantar soft tissue testing and to validate the foot model. The normative acceleration of the calcaneus at heel strike for 12 asymptomatic subjects was measured. These data were employed in the design of a pendulum impact tester, which was used to stimulate the subcalcaneal tissue of nine asymptomatic subjects with loads similar to those experienced at heel strike. However, the measured deformations of the soft tissue were large, indicating that possibly the mechanical grounding was inadequate. Stress relaxation experiments were performed on the seven plantar soft tissue areas of ten cadaveric feet. The data were modeled with a new constitutive relationship, based on the quasi-linear viscoelastic theory (QLV), that incorporated nonlinear and frequency sensitive characteristics. Software packages (the Software for Interactive Musculoskeletal Modeling (SIMM), the Dynamics Pipeline and SD/FAST) were employed to modify an existing lower extremity model. Motion was simulated at the ankle, subtalar and metatarsophalangeal (MTP) joints while motion at the pelvis, hip and knee was prescribed. The plantar soft tissue and extrinsic muscles were incorporated. By setting input conditions and prescribing muscle excitations, preliminary forward dynamic simulations of stance phase were conducted, resulting in vertical GRFs that were larger than expected. Since no motion was possible between the subtalar and MTP joints, the foot could not flex during weight acceptance, resulting in nonphysiological, large deformations and forces in the plantar soft tissue.

The level of anatomical detail in this foot model was unprecedented, as bones, muscles and plantar soft tissue were included. The soft tissue models were unique contributions, as they represented the first descriptions of the submetatarsal and subhallucal tissue. Furthermore, the form of the QLV theory employed was the first to account for both nonlinear and frequency sensitive behavior. Finally, this work represented the first attempt at employing these software packages to study the stance phase of gait and to predict distributed GRFs.