## 1 Introduction  
Judo is a grappling martial art that was created by [PERSON] in 1882. Core values of the sport include discipline, humility, and respect [1]. In practice, players attempt to either throw opponents directly onto their back from a standing position (nage-waza) or display control in groundwork via pinfall or submission (ne-waza) [2].  
Epidemiological research continues to highlight the considerable risk of injury in judo [2, 3, 4]. The shoulder is consistently recognised as a frequently injured site, and a recent publication found that almost 16% of injuries in elite judo competition over a 15-year period were to the shoulder [2, 3, 4, 5, 6]. Injuries during judo training are no less common; in fact, it is thought that more injuries occur in training (58%) than in competition (42%) [7]. A player landing after being thrown (uke) is considered the most common injury situation in judo, and two strongly implicated throws include tai-otoshi (body drop) and morote-seoi-nage (two-handed shoulder throw) [2; 8; 9]. In tai-otoshi, the throwing player (tori) places their hands on the sleeve and lapel of uke's uniform (gi) whilst pulling uke towards them. Tori widens their stance and throws uke over their shoulder with both arms. In morote-seoi-nage, tori drops their feet whilst pulling uke towards them, before loading uke onto their back and throwing them forward with both arms [10; 11].  
Previous biomechanical analyses of osoto-gari (major outer reap) and ouchi-gari (major inner reap) have helped to further our understanding of head injuries in judo [12; 13]. In osoto-gari, tori breaks uke's balance towards their rear corner, then sweeping uke's leg and causing them to be thrown backwards [14; 15; 16]. In ouchi-gari, tori first pushes uke backwards, before sweeping uke's leg from the inside with their own leg, causing them to fall onto their back [14; 15; 16]. If unable to respond adequately to an attack using osoto-gari or ouchi-gari, uke risks occipital head contact with the tatami (mat) when landing [13; 16]. The significant forces generated upon impact can cause severe head injuries [16; 17]. Risk factors for head injury from these throws have since been identified, allowing for appropriate preventative measures to be suggested and trialled in subsequent research [18; 19].  
A similar approach may help tackle the shoulder injury problem. Frequent shoulder injury diagnoses include acromioclavicular (AC) joint sprains, clavicle fractures, and dislocations [3; 4]. Current principles of shoulder injury prevention in judo include muscle group strengthening and defensive reactionary movements, in order to avoid withstanding dangerous biomechanical loads [7]. Relevant exercises and practices can be taught to players of all abilities through effective injury prevention programmes, and should be made specific where possible [20; 21]. To quantify the potential risk of certain throws, kinematic parameters such as velocity and acceleration can be measured and used to determine load and estimate impact forces [22]. An awareness of this, alongside the biomechanical sequences of high-risk throws, is necessary in order to elucidate possible shoulder injury mechanisms in judo [23]. Altogether, this affords coaches the opportunity to better manage the conditions of training and competition, to lower the risk of shoulder injury [4; 24].  
The biomechanical mechanisms of head injuries in judo have been examined extensively, although shoulder injuries have been researched in far less depth. The aim of this study was to investigate shoulder impact kinematics in tai-otoshi and morote-seoi-nage, when performed by elite adult judoka. We video-analysed and measured: (a) the peak acceleration and (b) the peak velocity of uke's shoulder during both throws to further our understanding of shoulder injury mechanisms in the sport. The null hypothesis was that there would be no difference in either variable between throws. The research hypothesis was that at least one variable would be greater in tai-otoshi than in morote-seoi-nage.