Kinetic Determinants of Reactive Strength in Highly Trained Sprint Athletes: Reactive strength involves the ability of an athlete to rapidly and efficiently change from an eccentric to a concentric contraction. For a sprinter, this involves the explosive action of the stretch-shortening cycle as GRF (ground reaction force) is applied each stride, during the pushing action away from the ground, to rapidly propel the body upward and forward. In team sports, movements that require a change of direction and the ability to rapidly move through the stretch shortening cycle are common throughout a competitive event.
The purpose of this study by Douglas, et. al. (2017) 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 Nkg; 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.
Coaching Application: The findings of this current research and other studies indicate that reactive strength is an important quality for acceleration, agility, and change of direction speed.
Reference: Douglas, J, Pearson, S., Ross, a, and McGuigan, M. 2017. The Kinetic Determinants of Reactive Strength in Highly Trained Sprint Athletes. J Strength Cond Res. Sept. 11.
