## 4 Discussion  
The main aim of this study was to describe the frequency of shoulder injuries per player position among professional South African rugby players, and this is the first study to describe the severity of each reported shoulder injury (tissue type, pathology type) in the context of momentum, intensity, and collision variables with real-time collision data over 4 years. Across the 4 years, 1 in 6 injuries was shoulder-related. Forwards sustained two-thirds of the shoulder injuries, specifically the locks (30%), and loose-forwards (16%). The ligament/joint capsule contributed two-thirds and muscle/tendon one-third. Almost half of the injuries affected the AC joint (sprains and dislocations). Player positions exposed to the highest average momentum were locks (791.45 kg\(\cdot\)m/s), loose-forwards (776.49 kg\(\cdot\)m/s), and wings (737.90 kg\(\cdot\)m/s). Injuries with the highest average momentum resulted in players suffering mostly minimal to mild severity injuries (1-7 days time-loss). Backs required significantly less (\(p\) = 0.008) momentum than forwards to suffer >28-day injury severity. The highest average collisions were recorded among centres (0.34/min), fullbacks (0.33/min), and locks (0.27/min). Backs experienced a higher number of collisions than forwards. Backs also encountered a significantly higher (\(p\) = 0.031) match intensity than forwards. The player positions that recorded the highest average match intensity were flyhalf (86.10 m/min), scrumhalf (73.50 m/min), and centre (68.79 m/min), with more than 55% of shoulder injuries sustained when the match intensity was above 60.00 m/min for the respective positions. In this study, forwards sustained two-thirds of the shoulder injuries. In rugby, forwards, particularly the back row (open-side flanker, blind-side flanker, and the eighth man), are reported to have higher incidences of all injuries than backs [38; 39]. In a study involving 140,249 tackles in 434 professional matches, flankers were significantly involved in more tackles per match [40]. Loose-forwards also spend more time in high-intensity (tackling, sprinting, and high-speed running) situations [25; 41]. In contrast, an English professional rugby union study reported a higher rate of tackle-derived shoulder injuries in backs [42]. We reported the highest number of shoulder injuries in locks. Other studies also found locks were the most injured players [43; 44]. In rugby, each player position has different roles and responsibilities as well as diverse physical game demands that may be possible reasons for players to be exposed to different forces [7; 13]. Locks are involved in many contact situations, which are not always at high speed but include tackles, scrum, line-outs, maus, rucks, and being ball carriers [5; 13; 14]. In more than 90% of cases, the main tissue type involved ligament/joint capsule (65%) followed by muscle/tendon (26%). A Super Rugby tournament study found muscle/tendon (50%) and ligament/joint (33%) injuries accounted for more than 80% of injuries [15]. In rugby, approximately 5-6% of all reported tackle injuries are shoulder ligament injuries to the tackler [42]. The main mechanism of injury during a tackle is a direct shoulder collision with the opponent or when falling onto the abducted arm, resulting in stretching and straining ligaments [45]. AC joint injuries (47%) were the most frequently occurring diagnosis, followed by shoulder dislocation and muscle contusion. The most common match injury reported in English professional rugby union was an AC joint injury (32%) [42]. In this study, the player positions exposed to the highest average momentum were forwards (locks 791.45 kg\(\cdot\)m/s, loose-forwards 776.49 kg\(\cdot\)m/s). However, backs (631.15 kg\(\cdot\)m/s) required significantly less (\(p\) = 0.0081) momentum than forwards (816.00 kg\(\cdot\)m/s) to suffer more severe (>28 days) injuries.