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Front crawl (FC) and back crawl (BC) are similar in terms of having alternating contributions of the arms combined with a six beat kick. However, the reason for the faster swimming times of FC than BC has not yet been established. There have been several studies in which the energy expenditure (Ė) of FC and BC were investigated. However, few researchers have compared Ė between the strokes. Also, although there have been some studies for FC using 3D motion analysis, few researchers have applied 3D motion analysis for BC. There have also been some studies in which the relationship between isokinetic torque produced on an isokinetic dynamometer and FC performance has been investigated, however, the relationship between isokinetic torque and BC performance is unclear. Therefore, the aim of this study was to determine why FC is faster than BC by investigating physiological and biomechanical differences between FC and BC. Ten Portuguese male national level swimmers were recruited for this study. Three studies were conducted to achieve the aim. In the first study, Ė of FC and BC at the same testing speed below the anaerobic threshold were investigated by measuring swimmers’ oxygen uptake. Kinematic variables of FC and BC below the anaerobic threshold were also measured by 3D motion analysis in the first study. In the second study, 3D motion kinematics of FC and BC at the same selected speeds were investigated. In the third study, kinematic differences between FC and BC at the same exercise intensities, and correlations between the kinematics and isokinetic muscular torques of the swimmer in FC and BC and their differences were assessed. Below the anaerobic threshold, Ė of the swimmers in BC was significantly greater than that in FC at the same speed although there were no differences in stroke frequency (SF), stroke length (SL) and stroke index (SI). Swimmers also had significantly higher Froude efficiency (ηF) in FC than in BC. Differences in several kinematic variables (range of motion of the foot, duration of non-propulsive phases, and intra-cycle velocity variation) suggested that swimmers expended greater energy in BC than in FC. Differences in other kinematic variables (body roll angle, hand speed/acceleration, yaw angle fluctuation, centre of mass displacement, and hand/foot displacements) suggested the possibility of resistive impulse being larger in BC than in FC during the stroke cycle. Thus, FC is more economical and efficient than BC because swimmers lose less energy to the water during the non-propulsive phase, and possibly have smaller resistive impulse in FC than in BC at speeds below the anaerobic threshold. At the same selected speeds above the anaerobic threshold, ηF in BC was significantly lower than that in FC, which was due to faster mean 3D hand speed during the stroke cycle in BC than in FC. The faster mean hand speed in BC than in FC was due to the faster 3D hand speed during the pull phase, and longer relative duration of the release and above-water phases in BC than in FC. SI was also larger in FC than in BC, which was due to longer SL in FC than in BC. The longer SL in FC than in BC was due to the longer duration of propulsive phases and probably smaller resistive impulse during the stroke cycle in FC than in BC. At the same selected exercise intensities, FC was faster than BC because of higher SF. The higher SF in FC than in BC was due to the longer duration of the above-water phase in BC than in FC, longer hand path distance during non-propulsive phases in BC than in FC, earlier timing of the hand entry in relation to the underwater phase of the other hand in FC than in BC. SF in both FC and BC was significantly correlated with shoulder adduction isokinetic torque of the swimmers, however, the effect of shoulder isokinetic torque on the difference in swimming performance between FC and BC required further investigation. In conclusion, FC is faster than BC because swimmers can achieve higher SF in FC than in BC, and FC is more economical and efficient than in BC with indirect evidence that resistive force are greater in BC than in FC.