A new protocol for experimental determination of human hand reference trajectories for the synthesis of upper limb prostheses

This study is a fundamental part of a wider research activity which aims at defining a procedure for the Determination of the Optimal Prosthetic Architecture (DOPA) for a given upper limb amputee. Architecture is intended here as the topology of the robotic arm, i.e. the number and type of its kinematic joints, and the geometry of the arm. The DOPA procedure comprises kinematic and kinetostatic algorithms to simulate the performance of several prosthetic architectures (with one up to six active joints) when attempting to execute Activities of Daily Living (ADLs). A series of proper indices, based on the architecture responses to the simulations, makes it possible to systematically evaluate the complexity of a prosthesis model (number of active joints and size of the actuators) with respect to its functionality (in terms of performance and accuracy). The optimal architecture is the simplest and most wearable prosthesis model (i.e. with a few active joints) that is able to execute a given set of ADLs with a sufficient accuracy. The motor tasks corresponding to the considered ADLs are implemented in the simulation algorithms by means of “Reference Trajectories” (RTs) of the terminal device, intended as the temporal succession of the hand pose (position and orientation) represented in the Cartesian Space. This paper describes how the hand Reference Trajectories were determined for a suitable use in the DOPA procedure, by means of an experimental arm motion analysis. In particular, the protocol for their determination comprises the following steps: • experimental acquisition of the hand trajectories performed by a healthy subject when executing a proper selection of ADLs; • processing of the experimental data to represent the hand trajectories in the Cartesian Space by means of Natural Coordinates. This approach, which is a novelty in biomechanical studies, consists in the use of the Cartesian coordinates of three points embedded in the hand to completely describe its pose; • definition of a novel algorithm which makes it possible to represent the RTs in a synthetic and innovative modality: particular poses of the hand in the trajectory (nodes), which can comprehensively describe the task, are automatically detected. The obtained results have proved to be suitable for the issues required by the mentioned kinematic simulations that will follow this work stage. However, it is the authors’ opinion that the stages shown in this work could be used for the determination of reference trajectories of different terminal devices. This approach could therefore be useful for the analysis and/or the synthesis of other types of machines or robotic manipulators, especially in the rehabilitation field (e.g. orthoses or rehabilitative robots).