Amputee Locomotion: Determining the Inertial Properties of Running-Specific Prostheses

Objectives(1) To test the validity of a trifilar pendulum in estimating moments of inertia (MOIs) for running-specific prostheses (RSPs), (2) to measure inertial properties (mass, center of mass [CM] position, and MOIs) for 4 RSPs, (3) to verify the influence of the stiffness on the inertial properties of RSPs, and (4) to develop a predictive equation to estimate RSP CM positions.DesignAn aluminum block with known MOIs was used for verifying the accuracy of the trifilar pendulum MOI measurements. MOI errors were investigated by systematically misaligning the block and pendulum principal axes across a range of 1 to 10cm. Mass, CM position, and MOI were tested across 4 RSP designs with 3 stiffness categories each.SettingUniversity biomechanics laboratory.SpecimensFour different RSP designs and 3 stiffness categories per design were examined.InterventionsNot applicable.Main Outcome MeasuresMOI errors from known values and principal axis misalignments between RSPs and pendulum; mass, CM positions, and RSP principal axis MOIs; and predictive equation CM position errors.ResultsThe trifilar pendulum estimated MOIs within −6.21×10−5kg/m2 (≤1% error) for a block with known MOIs. Misalignments of 1 to 5cm between the RSPs' and pendulum's CM yielded errors from .00002 to .00113 kg/m2 (0.3%−59.2%). Each RSP's inertial properties are presented. MOIs about any axis varied <.004kg/m2 across stiffness categories; MOIs differed up to .013kg/m2 between different designs. The predictive CM equation erred between .010 and .028m when using average input values across an RSP design.ConclusionsTrifilar pendulums can accurately measure RSP MOI. The RSP inertial properties differed slightly across stiffness categories within each design, but differed more substantially across different RSP designs. Using a predictive equation to estimate RSP CM positions can provide adequate data, but directly measuring CM positions is preferable.