Development, Calibration and Metrological Characterization of a Low-Cost 3D Printed Handgrip Dynamometer Based on Force-Sensing Resistor Technology

Handgrip strength is a widely used indicator of muscle function and it is particularly relevant for the functional assessment and rehabilitation of individuals with reduced grip capacity. This study presents the development, calibration, and short-term laboratory metrological characterization of a low-cost handgrip strength device (HGSD) manufactured by fused filament fabrication and integrating a force-sensing resistor (FSR). The device consists of two deformable half-shells designed to transfer the externally applied gripping force to an FSR integrated into a central mechanical interface. Two experimental campaigns were conducted under controlled laboratory conditions on a single HGSD assembly and its corresponding integrated FSR. The first characterized the isolated FSR under direct compression, whereas the second calibrated the complete HGSD using a dedicated mass-and-cable setup that reproduced the actions of the fingers and palm during a cylindrical grip. Increasing reference forces ranging from 0.70 kgf to 13.70 kgf (from 6.86 N to 134.35 N) were investigated. The isolated FSR exhibited a time-dependent decrease in its electrical resistance that was most pronounced at 2.45 kgf, where the mean relative variation was −2.53%. Short-term between-repetition coefficients of variation decreased from 0.59% at 2.45 kgf to 0.04% at 13.70 kgf. Power-law calibration models yielded resistance-domain goodness-of-fit metrics R2fit = 0.923 and a mean squared error (RMSE) of 33.83 kΩ for the isolated FSR, compared with R2fit = 0.994 and an RMSE of 12.22 kΩ for the complete HGSD. The different calibration exponents demonstrated that mechanical integration substantially modified the force-to-electrical resistance characteristic. An equivalent sensor-force response model was derived to quantify the overall electromechanical effect of integration. This relationship identifies the HGSD as a force-dependent nonlinear mechanical interface. Hence, the results quantitatively demonstrate that an isolated FSR calibration cannot be transferred directly to the assembled device and that the complete HGSD must be calibrated under representative mechanical integration and loading conditions. This study denotes the feasibility of the proposed measurement concept under controlled laboratory conditions but do not constitute a clinical validation.