Why is front crawl faster than back crawl? Kinematic differences between the two techniques during sprint swimming

The purpose of this study was to investigate why front crawl is faster than back crawl from a kinematic perspective. Ten male competitive swimmers volunteered to participate in this study. Each swimmer performed one 50 m front and back crawl trial at maximum speed, separated by at least one day. A space of 6 m length (X), 2.5 m height (Y), and 2 m width (Z) in the centre of a 25 m pool was recorded by four underwater and two above water digital video cameras and calibrated to enable calculation of three-dimensional (3D) coordinates of the digitised landmarks using the direct linear transformation method. One stroke cycle (defined as the duration between two-subsequent entries of a wrist into the water) was chosen from the videos and analysed for both techniques. Swimmers achieved faster centre of mass velocity toward the swimming direction in front than in back crawl (1.70±0.04 vs 1.54±0.06 m/s; p<0.01). There was no difference in stroke length between the two techniques (2.00±0.25 vs 2.07±0.17 m/cycle), while stroke frequency in front crawl was higher than that in back crawl (51.67±6.38 vs 44.81±4.68 cycles/min; p<0.01). Absolute duration of the release phase (from the last backward movement to the exit from the water of the wrist) and relative duration of the recovery phase were shorter in front than in back crawl (0.07±0.03 vs 0.26±0.08 s; p<0.01). In conclusion, front crawl was faster than back crawl because of its higher stroke frequency due to the shorter absolute duration of the release phase and relative duration of the recovery phase.