The Kinetic Determinants of Reactive Strength in Highly Trained Sprint Athletes.Douglas, Jamie; Pearson, Simon; Ross, Angus; McGuigan, MikeJournal of Strength & Conditioning Research: Post Acceptance: September 11, 2017 doi: 10.1519/JSC.0000000000002245 Original Research: PDF Only Abstract The purpose of this study was to determine the braking and propulsive phase kinetic variables underpinning reactive strength in highly trained sprint athletes in comparison to a non-sprint trained control group. Twelve highly trained sprint athletes and twelve non-sprint trained participants performed drop jumps (DJs) from 0.25m, 0.50m and 0.75m onto a force plate. One familiarization session was followed by an experimental testing session within the same week. Reactive strength index (RSI), contact time, flight time, and leg stiffness were determined. Kinetic variables including force, power and impulse were assessed within the braking and propulsive phases. Sprint trained athletes demonstrated higher RSI versus non-sprint trained participants across all drop heights (3.02 vs 2.02; ES [+/-90% CL]: 3.11 +/-0.86). This difference was primarily attained by briefer contact times (0.16 vs 0.22 s; ES: -1.49 +/-0.53) with smaller differences observed for flight time (0.50 vs 0.46 s; ES: 0.53 +/-0.58). Leg stiffness, braking and propulsive phase force and power were higher in sprint trained athletes. Very large differences were observed in mean braking force (51 vs 38 N.kg-1; ES: 2.57 +/-0.73) which was closely associated with contact time (r +/-90% CL: -0.93 +/-0.05). Sprint trained athletes exhibited superior reactive strength than non-sprint trained participants. This was due to the ability to strike the ground with a stiffer leg spring, an enhanced expression of braking force, and possibly an increased utilization of elastic structures. The DJ kinetic analysis provides additional insight into the determinants of reactive strength which may inform subsequent testing and training. Copyright (C) 2017 by the National Strength & Conditioning Association.