Apparatus to measure simultaneously 14 isometric leg joint moments. Part 1: Design and calibration of six-axis transducers for the forces and moments at the ankle.

An apparatus has been developed for making isometric measurements of the joint moments corresponding to the 14 degrees of freedom of the legs, in postures ranging between sitting and near full extension. The apparatus is called the multi-moment chair system (MMCS) and is described in the companion paper. This paper describes the most critical components of the MMCS, which are the six-axis transducers for measuring the force and moment components on the plantar-flexion axis of each ankle while the feet are laced into fixed shoes. The transducers are made of steel bars, on which strain gauges are mounted, joined by clamps. The design of the transducer and methods of calibration and error estimation are described. The RMS errors are less than 2 N for the forces and 1 Nm for the moments, but these may be correlated. A method for error reduction that compensates for the finite compliance of the transducer does not reduce the measured errors.