## 1 Introduction  
Canoeing is characterized by the paddler moving through the water on a canoe by the application of a force on the water through the use of an implement, the paddle [1]. Flat water sprint kayaking is one of the disciplines of canoeing. It is a racing sport which uses a double-bladed paddle with the kayaker sitting on the boat [2].  
The involvement of the upper limbs predominates in kayaking, although all the propulsive energy must be transferred to the boat through the lower limbs in order to move it through the water [3]. Thus, the main technical difficulty consists in transforming trunk rotation strength in the vertical axis of the paddler into a pushing or propelling force on the sagittal plane through the rapid and powerful extension of the knee on the stroke side, thereby transmitting the force to the boat through the footrest of the kayak [4, 5]. Furthermore, an increase in the time spent in the attack phase of the paddling cycle, for applying force for a longer period of time in the water and, as a consequence, to increase performance, is followed by trunk rotation and flexion [1].  
This combination of movements is repeated with each stroke during long periods of training. As a consequence of the involvement of the trunk, it has been found that 31% of paddler injuries were located on the lower back [6], even though the upper limb involvement was more pronounced. These injuries usually appear from overuse or accumulation of stress [6] from training and competition.  
Based on the theoretical aspects of paddling, it is recommended that paddlers maintain a position that respects a normal spine position [7], and increases the length of the water phase of the stroke through an anteverted pelvic tilt [8]. However, this pelvic tilt depends on hamstring extensibility. Thus, if hamstring shortness is present, the anteversion in a sitting position with legs extended is limited [9]. As a consequence, during trunk flexion, kayakers usually show a pelvic retroversion in the boat, which rectifies the lumbar lordosis and increases thoracic hyperkyphosis. Thoracic hyperkyphosis is characterized by a reduced vertebral joint mobility which must be compensated for by the nearest anatomical structures [10]. Furthermore, trunk flexion changes depend on the speed that the paddler needs [11] as a consequence of needing to increase the length of the water phase of the stroke to maximize velocity, which is achieved through higher trunk flexion. This could be more frequent at elite levels, but there are no previous studies that have analyzed this.  
There are three factors that can condition the pelvic tilt and the sagittal spine position in the boat during the action of paddling. The first is the distance between the seat and the footrest of the boat. This distance should allow trunk rotation [3] and the transmission of force to the boat [12]. The other two are the hamstring extensibility of the paddler and the anthropometric proportionality. In fact, some theories state that hamstring extensibility and anthropometric variables influence the placement of the different internal elements of the boat [13; 14]. However, no previous study has investigated this relationship.  
Consequently, this study was undertaken to analyze the differences on spine position, hamstring extensibility and anthropometric variables between kayakers from different competition levels and to determine which variables could determine the placement of the elements in the boat. It was hypothesized that differences on spine position on the boat, hamstring extensibility and anthropometric variables would be found between the different competition levels because of natural selection or a physical adaptation in high-level kayakers, and that the placement of the elements in the boat depends on lengths because other factors can be changed with training.