INFLUENCE OF SPRINT ACCELERATION STANCE KINETICS ON VELOCITY AND STEP KINEMATICS IN FIELD SPORT ATHLETES ROBERT G. LOCKIE, 1 ARON J. MURPHY, 2 ADRIAN B. SCHULTZ, 1 MATTHEW D. JEFFRIESS, 1 AND SAMUEL J. CALLAGHAN 1 1 Exercise and Sport Science Department, School of Environmental and Life Sciences, University of Newcastle, Ourimbah, Australia; and 2 Sports Studies, Exercise and Sports Science, and Clinical Exercise Physiology Department, School of Science and Technology, University of New England, Armidale, Australia ABSTRACT Lockie, RG, Murphy, AJ, Schultz, AB, Jeffriess, MD, and Callaghan, SJ. Influence of sprint acceleration stance kinetics on velocity and step kinematics in field sport athletes. J Strength Cond Res 27(9): 2494–2503, 2013—The interac- tion between step kinematics and stance kinetics determines sprint velocity. However, the influence that stance kinetics has on effective acceleration in field sport athletes requires clarifi- cation. About 25 men (age = 22.4 6 3.2 years; mass = 82.8 6 7.2 kg; height = 1.81 6 0.07 m) completed twelve 10-m sprints, 6 sprints each for kinematic and kinetic assessment. Pearson’s correlations (p # 0.05) examined relationships between 0–5, 5–10, and 0–10 m velocity; step kinematics (mean step length [SL], step frequency, contact time [CT], flight time over each interval); and stance kinetics (relative ver- tical, horizontal, and resultant force and impulse; resultant force angle; ratio of horizontal to resultant force [RatF] for the first, second, and last contacts within the 10-m sprint). Relation- ships were found between 0–5, 5–10, and 0–10 m SL and 0–5 and 0–10 m velocity (r = 0.397–0.535). CT of 0–5 and 0–10 m correlated with 5–10 m velocity (r = 20.506 and 20.477, respectively). Last contact vertical force corre- lated with 5–10 m velocity (r = 0.405). Relationships were established between the second and last contact vertical and resultant force and CT over all intervals (r = 20.398 to 0.569). First and second contact vertical impulse correlated with 0–5 m SL (r = 0.434 and 0.442, respectively). Subjects pro- duced resultant force angles and RatF suitable for horizontal force production. Faster acceleration in field sport athletes involved longer steps, with shorter CT. Greater vertical force production was linked with shorter CT, illustrating efficient force production. Greater SLs during acceleration were facili- tated by higher vertical impulse and appropriate horizontal force. Speed training for field sport athletes should be tailored to encourage these technique adaptations. KEY WORDS ground reaction force, impulse, resultant force angle, step length, contact time INTRODUCTION O ne of the more important physical capacities that a field sport athlete requires is speed. Greater speed can assist with field sport–specific skill execution by allowing an athlete to become more involved in plays during a game, including those that can influence the final result (7,29). The benefit of speed for field sport athletes is further emphasized by research high- lighting that players of higher abilities from sports such as rugby league (8), soccer (7), and American football (14,15,31) tend to be faster than players of lesser ability. Many of the sprints completed in field sports are often short (i.e., less than 20 m), with efforts having a duration of 2 sec- onds or less (3,9,11,16). This places great prominence on the ability to accelerate (23), which is the capacity to generate as high a running velocity in as short a distance or time as possible. Field sport athletes must ensure that their sprint technique allows them to accomplish this, and a focus within the recent research literature is on the ground reaction force (GRF) generated during stance. However, there is conflict- ing information regarding the magnitude and direction of GRF required to increase running speed and optimize accel- eration. This has great implications for field sport and strength and conditioning coaches who wish to use certain training protocols that increase the GRF that needs to be applied when executed (e.g., plyometrics and resisted sprinting). Weyand et al. (37) suggested that faster sprint speeds are achieved by applying greater force to the ground during support. More recently, Kugler and Janshen (20) state that rather than maximal force production, there should be Address correspondence to Robert G. Lockie, robert.lockie@newcastle. edu.au. 27(9)/2494–2503 Journal of Strength and Conditioning Research Ó 2013 National Strength and Conditioning Association 2494 Journal of Strength and Conditioning Research the TM Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.