Joint kinematics estimate using wearable inertial and magnetic sensing modules

Background and aims In many applications, it is essential that the evaluation of a given motor task is not affected by the restrictions of the laboratory environment. To accomplish this requirement, miniature triaxial inertial and magnetic sensors can be used. This paper describes an anatomical calibration technique for wearable inertial and magnetic sensing modules based on the direct measure of the direction of anatomical axes using palpable anatomical landmarks. An anatomical frame definition for the estimate of joint angular kinematics of the lower limb is also proposed. Methods The performance of the methodology was evaluated in an upright posture and a walking trial of a single able-bodied subject. The repeatability was assessed with six examiners performing the anatomical calibration, while its consistency was evaluated by comparing the results with those obtained using stereophotogrammetry. Results Results relative to the up-right posture trial revealed an intra- and inter-examiner variability which is minimal in correspondence to the flex-extension angles (0.2–2.9°) and maximal to the internal–external rotation (1.6–7.3°). For the level walking, the root mean squared error between the kinematics estimated with the two measurement techniques varied from 2.5% to 4.8% of the range of motion for the flex-extension, whereas it ranged from 13.1% to 41.8% in correspondence of the internal–external rotation. Conclusion The proposed methodology allowed for the estimate of lower limb joint angular kinematics in a repeatable and consistent manner, enabling inertial and magnetic sensing based systems to be used especially for outdoor human movement analysis applications.