A method for calculating the joint coordinates of paraplegic subjects during the transfer movement despite the loss of reflective markers

During transfers into and out of manual wheelchairs, the upper limbs of paraplegic subjects support nearly the total body weight, which can cause musculoskeletal disorders. Nevertheless, despite the existence of experimental laboratory platforms, the specificities of this population, which cause the reflective markers to fall off the subject, makes it difficult to study these movements. To solve this problem, a two-stage method based on the use of a humanoid mannequin is introduced in this paper. The first stage aims to automatically construct a humanoid mannequin with the same anthropometry as the subject, without resorting to manual reshaping. The second stage uses this mannequin and a trajectory tracking procedure based on a global optimization of the whole body under joint constraints. The hand-supported transfer movement of a healthy subject was reconstructed firstly based on all the markers and secondly based on a reduced number of markers. These two reconstructions were then compared. The results of this comparison were good. The joint coordinates resulting from these two reconstructions were correlated at 0.98±0.03, their average difference was lower than ±7°, and the average Root Mean Square was 2.17±2.29°. This method thus constitutes a good alternative for calculating the joint coordinates in order to study the sitting pivot transfer movement.