# Relationship Between Front Crawl Trunk Incline and Lower Limbs' Biomechanics in Non-Expert Swimmers
## 1 Introduction  
Front crawl is a complex swimming technique requiring precise coordination between different body segments to generate efficient propulsion while maintaining optimum stability [1]. Due to the instability induced by the aquatic environment, specific muscular activations and kinematic adaptations help in maintaining smooth water displacement and a steady body position [2]. In fact, the position of the swimmer's body, more precisely the sagittal tilt of the torso around the medio-lateral axis, has been proven to be one of the crucial aspects for ensuring coordination between different body segments [3]. So, the ability to remain as horizontal as possible can determine a swimmer's success in generating effective propulsive force while reducing the resistance to forward motion.  
In the specific case of swimming, the trunk incline (TI) is the angle between the shoulder (acromion), hip segment (greater trochanter), and the horizontal, which is defined by the water line, and is crucial for water locomotion [3]. Several anatomical points, like the relative positional relationship between the center of mass and the center of buoyancy(i.e., when those are too far apart) [4], the lung volume [5], or the projected frontal area [6] have been found to increase the TI. The fact is that the greater the TI, the less horizontal the swimmer is and the greater are the drag forces acting on their body, which impacts the energy cost of swimming [7].  
The contribution of the lower limbs to forward displacement has attracted a great deal of interest over the years, as they account for more than 50% of the velocity in the different swimming strokes [8; 9; 10]. The segmental actions involving the lower part of the body increase swimming speed, improve the swimmer's overall stability, and reduce their TI, resulting in a more horizontal position in the water [11]. When testing competitive swimmers, the TI seems to remain quite stable, near 10\({}^{\circ}\) at a range of different velocities [3; 6; 12] or even when additional equipment (e.g., fins) is used [13]. A lower TI is expected in these swimmers as they have a high level of expertise in detailed aspects of technique, derived from a large experience in swim training. However, this is not the case for recreational swimmers, as they can take a longer time to learn, which may influence their overall technical ability.  
The teaching contents in swimming are arranged in terms of complexity, starting with work related to body position, which is then followed by the inclusion of limbs' actions and breathing [14]. Because recreational swimmers regularly show deficits in global aspects of their swimming technique, their body position and lower limbs' actions should receive greater attention from teachers. Indeed, the overall stroke mechanics and efficiency may be influenced by how they position themselves in the water and the biomechanical characteristics of their lower limbs.  
Although several studies have highlighted the importance of the lower limbs' action and its effects on the TI in front crawl, they failed to assess the relationships between TI and kicking from a deeper kinematic and kinetic standpoint. Moreover, most of the studies were conducted using competitive swimmers as participants [3; 6; 12] and not those who are more affected by TI (i.e., recreational swimmers). So, understanding the mechanisms underlying how these movements cause interactions between different segments of the swimmer's body is essential to developing effective pedagogical or training strategies for more efficient front crawl swimming.  
This study aimed to analyze the relationship between the lower limbs' kinematics or kinetics and trunk inclination in front crawl swimming. It was hypothesized that the angle of trunk inclination would be more directly related to the kinetics than the kinematics of the swimmer's lower limbs.