Inverse dynamics for 3 D upper limb movements . A critical evaluation from electromagnetic 6 D data obtained in quadriplegic patients

C6 quadriplegic patients lose the ability to use their triceps, but the arm flexors remain intact. However, they are still able to extend the elbow. A previous kinematics study showed that their pointing movements were very similar to those of healthy subjects. We make the hypothesis that the triceps is more likely to be used to ensure elbow stiffness than to provide an active elbow extensor torque. Thus, this study aims at revealing the dynamics of arm pointing movements in quadriplegic patients. We modelled the upper limb from kinematics data acquired from an electromagnetic motion capture device. Inverse dynamics was used to calculate joint torques. Sensitivity analysis was conducted to assess model reliability. We tested the influence of systematic errors in acquisition data and reconstruction errors. Analysis showed mild sensitivity of the joint torque to errors of position or orientation of the sensors and to orientation of the reconstructed segments. However, we could not reliably decompose the total moment at the elbow into the joint coordination system in order to calculate the flexor-extensor moment. Sensitivity analysis proved that inverse dynamics models could be useful in assessing unconstrained 3D upper limb movements. This study emphasises the limits of electromagnetic devices, but suggest ways to improve the recording method, which will have to be validated by further studies evaluating the degree and shape of systematic errors. KeywordsInverse dynamics, motion analysis, upper limb model, sensitivity analysis, spinal cord injury.