Plantar pressure and shear measurement using surface stress-sensitive film

This paper describes the performance of a system for dynamic measurements of pressure and shear forces over the entire plantar surface of the foot. During human locomotion, mechanical stress at the plantar surface is 3D: pressure acting vertically and shear stress acting longitudinally and laterally. The system described here offers gait, foot deformity, diabetic ulceration, and foot-ankle kinetics researchers a means to obtain the full 3D data that is missing in most current research efforts. Surface Stress Sensitive Film (S3F) provides the sensing element for this measurement device. S3F is an incompressible elastic film that deforms in response to applied forces. Upon removal of the forces, the film returns to its original shape. The dynamic, 3D deformations are captured with a digital camera. The measured deformations can be analyzed directly, since they are proportional to the applied forces, or used as inputs to a finite element model of the film to reconstruct the pressure and shear force values. System performance is evaluated using both a standard form and human foot contact. The fundamental approach involves comparing the integrated normal and shear loads from the S3F-based sensor to the measurements provided by a force plate. The results showed high linearity (<3% nonlinearity), low uncertainty (<4%), low cross talk (<3%), high spatial resolution (1.6 mm), no loss of fidelity at walking speeds with a 45 Hz sampling rate, and good repeatability for human foot contact measurements.