A Removable Mechanical Interface for Non-Invasive Strain Measurement in Paddle Shafts: Iterative Design and...

Accurate force measurement on paddle shafts is essential for performance assessment and technique refinement, yet existing tools typically require permanent shaft modification or recalibration after each utilization. This study presents the iterative design and characterization of a removable, non-invasive interface for strain measurement on cylindrical paddle shafts, designed for repeated installation without bonding. A total of nineteen configurations were developed through an iterative five-phase design process using fused filament fabrication (FFF) and evaluated under static and dynamic laboratory loading, including an exploratory on-water assessment of the distributed-clamping shell with one athlete. Performance was benchmarked against a bonded reference using linearity, hysteresis, repeatability, and signal fidelity. Rigid shells improved linearity but retained high hysteresis, while distributed clamping introduced placement-dependent uncertainty. Beam-based geometries reduced contact sensitivity, and the distal gauge position of an inverted U-shaped beam provided the closest agreement with the reference response under the tested conditions, reaching a coefficient of determination of 0.954-0.990, a mean absolute error of 7.8-17.7 N, and hysteresis of 3.4-9.1% of the applied load range. Its residual hysteresis was concentrated in the low-force unloading region, predominantly near the transition from the propulsive to the recovery phase. No configuration eliminated all trade-offs; acceptable performance depended on the intended measurement context. The resulting framework provides testable design indications for the future development of removable strain-sensing interfaces on cylindrical structures.