272 Journal of Applied Biomechanics, 2011, 27, 272-277 © 2011 Human Kinetics, Inc. Sylvain Hanneton (Corresponding Author), Svetlana Dedob- beler, Thomas Hoellinger, and Agnès Roby-Brami are with Université Paris Descartes, Paris, and with Laboratoire de Neurophysique et Physiologie, UMR 8119 CNRS, UFR Bio- médicale des Saints Pères, Paris, France. Direct Kinematic Modeling of the Upper Limb During Trunk-Assisted Reaching Sylvain Hanneton, Svetlana Dedobbeler, Thomas Hoellinger, and Agnès Roby-Brami The study proposes a rigid-body biomechanical model of the trunk and whole upper limb including scapula and the test of this model with a kinematic method using a six-dimensional (6-D) electromagnetic motion capture (mocap) device. Large unconstrained natural trunk-assisted reaching movements were recorded in 7 healthy subjects. The 3-D positions of anatomical landmarks were measured and then compared to their estimation given by the biomechanical chain fed with joint angles (the direct kinematics). Thus, the prediction errors was attributed to the different joints and to the different simplifcations introduced in the model. Large (approx. 4 cm) end-point prediction errors at the level of the hand were reduced (to approx. 2 cm) if translations of the scapula were taken into account. As a whole, the 6-D mocap seems to give accurate results, except for prono- supination. The direct kinematic model could be used as a virtual mannequin for other applications, such as computer animation or clinical and ergonomical evaluations. Keywords: trunk-assisted reaching, direct kinematics, coordination, 6-D motion capture, model During the last decade, many studies have developed the use of electromagnetic motion capture (mocap) for the kinematics of the upper limb (Meskers et al., 1998; Fayad et al., 2008; Davardhini et al., 2005). The general methodology relies on rigid bodies modeling. Experi- mentally, an electromagnetic sensor is fxed on each body segment and representative bony landmarks are located in the reference frame of each corresponding sensor. Then the bony landmarks are used for the defni- tion of the local coordinate system of each segment and for the computation of joint rotations according to the recommendations of the ISB standardization proposal (Van der Helm, 2002; Wu et al., 2005). Validation studies with bone-fxated sensors demonstrated that the method with electromagnetic sensors was reliable when arm elevation remained below 120° (Karduna et al., 2001; Ludewig et al., 2009). This method has good reliability and accuracy (Crosbie et al., 2008; Lovern et al., 2009) and is now widely used to measure the mobility of the shoulder complex during arm elevation (Amasay et al., 2009; Rundquist et al., 2009; Vermeulen et al., 2002). The use of electromagnetic sensors is also validated to measure the kinematics of the upper limb (Biryukova et al. 2000; Prokopenko et al. 2001). However, to our knowledge, such a method has still not been validated for the combined analysis of the trunk, shoulder complex, and upper limb during large functional movements, such as trunk-assisted reaching with the target beyond the ana- tomical length of the arm. The aims of our study are (1) to formalize a direct kinematic model of fve rigid body segments (trunk, scapula, upper arm, forearm, hand) built from electromagnetic mocap of the upper limb and linked by joints that have three angular degrees of freedom; (2) to estimate the errors coming from modeling simplifca- tions, that is, rigid bodies and ideal joints (Figure 1); and (3) to experimentally measure the contribution of scapular movement (rotation and translation) to the displacement of the hand during trunk-assisted reaching movements. Methods Instrumentation and Recording of Bony Landmarks Position and orientation of six sensors are recorded at a sampling frequency of 86 Hz by an electromagnetic tracking device (Motion Star, Ascension Technology Corp.). The accuracy is 0.5 mm RMS and about 0.01° for rotations (Milne et al., 1996; Poulin & Amiot, 2002). Sensors were fxed on the skin with double-sided adhesive tape over each segment respectively at the level of the sternal manubrium for the thorax; on the fat surface of the superior acromion process for the scapula; strapped to the lateral arm just below the insertion of the deltoid for the upper arm; on the posterior surface of the lower arm, roughly 8–10 cm above the wrist level; and on the posterior surface of the hand with the main axis of the sensor along the third metacarpal bone (Figure 2).