A novel method for tracking movements of backpack’s centre of mass in dynamic activities

Postural compensations with backpack may cause considerable body strains, resulting in fatigue, pain, and injury [1]. Backpack’s position can influence carrier’s posture and dynamic performance [2]. Characterizing the backpack’s position, namely the position of its centre of mass (COM) with respect to the carrier’s body, allows modelling its dynamic loading towards revealing the moment and moment of inertia it renders on the carrier. These knowledges will provide novel insights into the carrier’s postural compensations and musculoskeletal injury [3]. Despite of the importance, there is a lack of an easy approach that can determine and track the movement of a backpack’s COM during dynamic activities. How to determine the position of a backpack’s COM and track its movements in walking with the backpack? A backpack was tightly filled with sandbags, resulting in a total weight of 10 kg. Using a 3D motion capture system (Vicon, UK), we created the backpack’s local coordinate system (CS) with the three reflective markers attached on it. A directional cosine matrix was established for coordinate transformations between the backpack’s and the lab’s CS. A mannequin was then placed on an integrated force plate (Kistler, Switzerland), and its weight and centre of pressure were measured. This measurement was repeated after placing the backpack on the mannequin (two positions, Fig. 1a), and the horizontal coordinates of the backpack’s COM were calculated according to the Varignon's Theorem. Fig. 1. Experiments and outcomes: a) Measuring centre of pressure in backpack’s two postures; b) Displacement of backpack’s and subject’s COM during walking. Download : Download high-res image (81KB)Download : Download full-size image As the coordinates of the backpack’s COM in the backpack’s local CS remained unchanged, an equation could be established to calculate the vertical coordinate of the backpack’s COM with its horizontal coordinates. Finally, the coordinates of the backpack’s COM in the backpack’s local CS were determined through coordinate transformation. Afterwards, a healthy young subject was instrumented with full-body marker set, and then performed walking with the backpack at 5 km/h. Using Visual 3D (C-Motion, USA), a virtual marker was created according to above outcomes, and the marker’s movements were computed from the gait trials. The results indicated that the vertical displacement magnitude of backpack’s and subject’s COM was similar (Fig. 1b), with a small temporal difference. In the mediolateral direction, the displacement of the backpack’s COM was much greater than that of the subject’s COM. A clear lag effect was observed in their mediolateral displacement during walking, where the backpack’s COM reached its ultimate mediolateral positions later than the subject’s COM did. Our approach can be applied to easily determine a backpack’s COM in 3D motion analysis, towards quantifying backpack’s loading effects and studying carrier’s postural adaptation and control strategy.