## 1 Introduction  
The measurement of head kinematics during sports collisions is essential for understanding the mechanism of concussion injuries [1] and long-term brain health. However, measuring head kinematics is challenging with on-field measurement devices [2, 3]. An alternative approach is to use multibody modelling. Multibody models have been used to study head kinematics during concussive direct-head impacts in unhelmeted sports including rugby union and Australian rules football [4, 5, 6] as well as inertial head loading during legal rugby union tackles [7]. [PERSON] et al. [6] used a passive MADYMO (TASS International, Helmond, Netherlands, 2015, Version 7.6) facet human body model approach to calculate six degree-of-freedom head kinematics from concussive and non-concussive direct head impacts in unhelmeted sports. They reported that angular acceleration of the head in the coronal plane had the greatest association with a concussion, with tentative threshold values of 1747 rad/s\({}^{2}\) and 2296 rad/s\({}^{2}\) reported for a 50% and 75% chance of concussion, respectively. [PERSON] and [PERSON] [7] recently used the MADYMO passive ellipsoid multibody human body model and demonstrated that tackles to the upper body can cause greater inertial head and neck loading than tackles to the lower body, particularly for the ball carrier. They also postulated that tackles to the upper body may be a catalyst for some of the chronic head and neck symptoms exhibited by a rugby player. Although the study reported numerical values for head kinematics and neck dynamics, [PERSON] and [PERSON] [7] cautioned that their focus was on the trends reported rather than precise numerical values, as the models have not yet been validated for sporting collisions.  
These studies used MADYMO passive human body models, which have numerous input parameters (geometry, mechanical/structural properties, contact characteristics and initial conditions) and kinematic outputs, are generally validated based on staged impact tests using cadavers [8; 9; 10]. Assumptions regarding these input parameters strongly influence the results of impact simulations [11]. Although a sensitivity analysis was conducted for each of the abovementioned multibody model sports head kinematic studies, their predictive capacity in sporting impacts has not been formally assessed [7]. In particular, there is no direct validation of these passive MADYMO human models for reconstruction of head kinematics, largely because direct validation data for sporting impacts is not available [1; 7; 12]. This currently limits the potential to develop our understanding of head kinematics and concussion during sporting collisions using model-based approaches, which can then examine \"what if\" scenarios to help guide prevention strategies.  
Tackling is a highly technical, fast-paced and dynamic part of rugby union [13; 14] with certain playing positions, in extreme cases, potentially making over 30 tackles per game [15]. Unfortunately, it is also the most common cause of injury and concussion [16; 17; 18]. The biomechanics of tackling is not yet fully understood in relation to head kinematics [7]. However, reconstructing a tackle in a controlled environment such as a marker-based 3D motion laboratory enables ball carrier and tackler whole body kinematics to be recorded during an impact. Although a real-life direct head impact might not be reconstructed using such an environment (player protection requirements), this approach provides a means to assess multibody models for player-to-player sports impact reconstruction for the first time and to assess head kinematics due to inertial loading. Accordingly, the goal of this exploratory study was to record rugby tackles using human volunteers in a marker-based 3D Vicon motion analysis laboratory to assess the predictive capacity of the MADYMO human body passive ellipsoid model for head kinematic reconstruction.