Kinetic Sprint Asymmetries on a non-motorised Treadmill in Rugby Union Athletes

The purpose of this study was to present a potential link between sprint kinetic (vertical [F _V] and horizontal force [F _H]) asymmetries and athletic performance during acceleration and maximal velocity (v _max) sprinting. Thirty un-injured male rugby athletes performed 8-s sprints on a non-motorised treadmill. Kinetic data were divided into 'strong' and 'weak' legs based on individually averaged peak values observed during sprinting and were analysed to evaluate asymmetry. Large differences were found between the strong and weak legs in F _H during acceleration (4.3 vs. 3.5 N·kg^-1) and v _max (3.7 vs. 2.8 N·kg^-1) sprinting (both ES=1.2), but not in F _V (21.8 vs. 20.8 N·kg^-1, ES=- 0.6 for acceleration; 23.9 vs. 22.8 N·kg^-1, ES=- 0.5 for v _max, respectively). Group mean asymmetry was lower in F _V compared to F _H during acceleration (1.6 vs. 6.8%) and v _max (1.6 vs. 8.2%). The range of asymmetry was much lower in F _V (0.03-4.3%) compared to F _H (0.2-28%). In un-injured rugby athletes, the magnitude and range of asymmetry scores in F _H, occurring during acceleration and v _max phases, where much greater than those found in F _V. These findings highlight the potential for some un-injured athletes to possess kinetic asymmetries known as crucial components for acceleration performance in sprinting.