# An investigation of improving validity in upper limb measurements for people with tetraplegia using construct specification equations

## TL;DR

This study improves the validity of upper limb measurements for people with tetraplegia by developing equations that explain task difficulty and person ability.

## Contribution

The paper introduces new construct specification equations for upper limb task difficulty and ability using qualitative variables.

## Key findings

- The CSE for task difficulty is largely explained by the number of joints involved.
- The CSE for person ability is dominated by grasp-related variables.
- Pearson coefficients of 0.94 and 0.73 were found between CSEs and empirical measures.

## Abstract

The aim of this paper is two-fold: to investigate development of a Construct Specification Equation (CSE) for UL task difficulty, , and a CSE for person UL ability, , in support of the validity of these two constructs. Measurements of UL task difficulty, , and person UL ability, were derived from applying the Rasch model on the Tetraplegia Upper Limb Activity Questionnaire (TUAQ). The formulations of CSEs as explanations of the two constructs were done using Principal Component Regression (PCR). The CSE for UL task difficulty, , was to a large degree explained by the number of joints involved and the CSE for person UL ability, , was dominated by grasp-related variables. Pearson coefficients of 0.94 and 0.73 were obtained between UL task difficulty and UL person ability from the CSE, respectively, when correlated with each empirical measure. The present work has both explored and extended the methodology for using more qualitative explanatory variables. Specifically, for UL measurements for people with tetraplegia a good CSE for task difficulty, , supports the validity of TUAQ when measuring person UL ability. Additionally, the CSE formulated for person ability, , can be used both for validation purposes as well as a clinical tool.

The online version contains supplementary material available at 10.1038/s41598-025-98626-4.