The Biophysical study of swimming is one of the major interests of the sport sciences investigators. However, there is a lack of investigation trying to understand the relationships established between the bioenergetical and biomechanical variables, especially in butterfly stroke. Therefore, the purpose of this thesis was to conduct a bioenergetical and biomechanical characterizations of the butterfly stroke, understanding the relationships established between those two domains. In this thesis 6 independent studies are presented in order to achieve the purpose defined. The first two investigations had the purpose to obtain a general characterisation of butterfly stroke. The purpose of the first study was to compare the total energy expenditure (Ėtot) of the four competitive swimming techniques, with special reference to butterfly stroke. The freestyle was the most economic swimming technique, followed by the backstroke, the butterfly and the breaststroke. The purpose of a second study was to estimate the average resultant impulse (ARI) per stroke phase. Butterfly stroke is a swimming technique where it is possible to observe high intra-cycle variations of the ARI, due to significant reductions of this parameter during the arm’s recovery and hand’s entry. The following papers had the aim of understand the relationships established between the biomechanical and bioenergetical variables. Increases in the Ėtot were significantly related to the increase of swimming velocity (V). The energy cost (EC) increased significantly along with the increasing stroke frequency (SF) and stroke index (SI). The EC decreases with increasing stroke length (SL). The increase of the EC is significantly associated with the increase of the intra-cyclic variations of the horizontal velocity of the centre of mass (dV), in Butterfly stroke. The last papers had the aim to identify the relationships established between the biomechanical variables and the dV. The relationships between SF and V, as well as, between SI and V were positive and significant. For the relationship between V and SL, there was a slight tendency to decrease SL with the increase in V. There was a negative and significant relationship between dV and V, between dV and SL and between dV and SI. For a given swimming velocity, it is observed a positive and significant relationship between dV and SF. High segmental velocities, in the most propulsive phases of the stroke cycle, were significantly associated to decreases of dV. As a conclusion, the behavior of biomechanical variables, such as the stroke determinant, the hand’s and feet’s velocities, influence the V and the dV profile. Consequently, these parameters will affect the Ėtot and the EC of swimming. Therefore, coaches and butterfliers should conduct an exhaustive and frequent evaluation of their technique in order to reduce the EC associated to a given swimming velocity
