Safety in sports, especially in contact sports is an important issue. Concussions are among the most critical neurological injuries that can occur during football (or "American football" as it is called some regions of the world). Sport-related concussions (SRC) may occur alone or in combination with cervical spine (Bhamra et al., 2012; Cantu et al., 2013; Rihn et al., 2009) and/or brachial plexus injuries (Altaf et al., 2012; Cantu et al., 2013; Lee et al., 2011). In the United States of America (USA) approximately 300,000 SRC are listed every year (Marar et al., 2012). Football is the most affected sport (47% of the SRC are met in football). The overall concussion rate is 2.5 per 10,000 athletes and 6.4/10,000 in football (Marar et al., 2012). Child and adolescent reported SRC in the USA has also increased of 60% during the last decade (Gessel et al. 2007). Other severe neurological complications can occur separate from or in conjunction with concussion, such as subarachnoid hemorrhage, epidural or subdural hematomas, cerebral edema (Zuckerman et al., 2012), and could also induce neuropsychological and cognitive changes years after trauma occurred (De Beaumont et al., 2009). It has been shown that retired players present Alzheimer syndrome more precocious than a control group (Guskiewicz et al., 2005) (note, however that the prevalence was not statistically higher). Repeated mild traumatic brain injuries are major factors contributing to Parkinson disease (Lee et al., 2012). Second-impact syndrome may occur while the athlete is still symptomatic and healing from a previous concussion; such new impact can worsen the primary SRC condition (Weinstein et al., 2013). Guidelines for evaluation and management of SRC have been established (Harmon et al., 2013). Different "return-to-play" protocols are under current discussion to avoid second-impact factor (Doolan et al., 2012, Mayers et al., 2012) and allow athlete to perform sport in safety (http://www.abc.net.au/4corners/documents/concussion20 12/IRB_Concussion_Guidelines_2011.pdf.). Increasing safety in football is therefore an important question in the USA. Indeed not only the National Football League (NFL) is requesting to increase players' safety (http://www.nfl.com/news/story/0ap1000000058439/artic le/roger-goodell-on-player-safety-we-all-have-to-domore) but also the USA highest authorities previously worried about players' safety (http://www.cbssports.com/ nfl/blog/nfl-rapidreports/21619423/president-obama-forplayers-sake-reduce-violence-of-football). One issue which could lead to injuries is the initial sprint position, e.g. the so-called 'starting position in 3 points'. However, no previous study has tried to evaluate the influence of this initial position on the risk of dangerous collision.
Although motion analysis is frequently used in daily clinics, only few biomechanical studies aim to prevent sport injuries. A previous study reported a percent asymmetry for runner in order to detect injury risk when running (Rumpf et al., 2014). Another study assessed visual and sensory performance of football players trying to identify at-risk athlete (Harpham et al., 2014). This seems to show that there is an increasing awareness in the field, and that a trend is set to identify specific parameters to estimate at-risk players. This study is similar in nature and focused on analyzing three different sprint start positions by comparing their respective biomechanical parameters related to neck injury, concussion risk and head collision of players after a so-called "pre-snap" (i.e., position of the teams facing each other in one offensive versus a defensive line). Influence of learning process between players and non-players was also addressed.
Twenty five young healthy males participated in this study (height = 1.81 [+ or -] 0.09 m, weight = 80 [+ or -] 16 kg, age = 24 [+ or -] 2 years old). Twelve of them were football players (height = 1.81 [+ or -] 0.08 m, weight = 89 [+ or -] 15 kg, age = 24 [+ or -] 3 years old, mean experience in football = 5 years, 4 hours of practice per week plus one match). A population of thirteen non-player participants was selected as control group. The control group selection occurred in order to match the height player group (height = 1.80 [+ or -] 0.09 m, weight = 73 [+ or -] 13 kg, age = 23 [+ or -] 3 years old). All participants in the non-player group were students from the ULB Faculty of Motor Sciences and well-trained in athletics (all sport amateur with regular sport training during their university program: 8 to 10 hours of sport/week including athletic, swimming, gymnastic, ball sports, etc.).This study was approved by the Ethical Committee of the Erasme Hospital (B406201112048) and written informed consent was obtained from all participants prior to participation in the study.
A stereophotogrammetric system (Vicon, 8 MXT40s cameras, Vicon Nexus software, frequency: 100Hz) was used to record motions and positions parameters. 12 reflective markers (4 on the head, 4 on the thorax, 4 on the pelvis) were placed on the skin following the Plug-In-Gait model (Figure 1). Participants were in underwear and barefoot to perform the different study trials. All measurements occurred inside the authors' movement laboratory on a synthetic floor.
Participants were asked to perform 3 sprints starting from 3 different initial positions that are found in football practice. These three positions (called 2-point, 3point and 4-point starts) are schematized in Figure 2. Prior to motion capture, an experienced football player first demonstrated the different initial positions to adopt. Participants were then invited to perform the motion once as non-recorded warm-up test. Then three successive trials were recorded for each modality. A total of 9 datasets were collected for each participant (3 start positions times 3 trials).
Body segments position and global motion were statistically analyzed for each participant as following. The instantaneous head height was expressed in percent of the maximum head height (i.e., measured when subject was in standing position). Two angles were calculated: inclination of the trunk relative to the floor (Trunk inclination, in degrees), and the angle between head and trunk (Trunk head, in degrees). These two angles were further computed into a so-called "Verticality" angle (in degrees) obtained from Eq. 1:
Verticality = (180-Trunk inclination)+(180-Trunk head) Eq. 1
Verticality of the head is reached when this angle is close to 0[degrees]. It was assumed that the width of the participant visual field-of-view is optimal when Verticality is close to 0[degrees].
The height of the head was also assumed to be directly related to visual field and head injury risk minimization: the higher the head, the larger the visual field and the lower the injury risk. A ratio between the height of the head and Verticality was computed as...