Low-Cost Methodology for Skin Strain Measurement of a Flexed Biological Limb

Objective: The purpose of this manuscript is to compute skin strain data from a flexed biological limb, using portable, inexpensive, and easily available resources. Methods: We apply and evaluate this approach on a person with bilateral transtibial amputations, imaging left and right residual limbs in extended and flexed knee postures. We map 3-D deformations to a flexed biological limb using free-ware and a simple point-and-shoot camera. Mean principal strain, maximum shear strain, as well as lines of maximum, minimum, and nonextension are computed from 3-D digital models to inform directional mappings of the strain field for an unloaded residual limb. Results: Peak tensile strains are similar to 0.3 on the anterior surface of the knee in the proximal region of the patella, whereas peak compressive strains are similar to-0.5 on the posterior surface of the knee. Peak maximum shear strains are similar to 0.3 on the posterior surface of the knee. The accuracy and precision of this methodology are assessed for a ground-truth model. The mean point location distance is found to be 0.08 cm, and the overall standard deviation for point location difference vectors is 0.05 cm. Conclusion: This low-cost and mobile methodology may prove critical for applications such as the prosthetic socket interface where whole-limb skin strain data are required from patients in the field outside of traditional, large-scale clinical centers. Significance: Such data may inform the design of wearable technologies that directly interface with human skin.