Guido, JA, Werner, SL, and Meister, K. Lower-extremity ground reaction forces in youth windmill softball pitchers. J Strength Cond Res 23(6): 1873-1876, 2009-Ground reaction forces are important in pitching given that the only external contact a pitcher has is between the foot and the ground. Windmill softball pitchers are routinely seen clinically for injuries to the lower extremities, and lower-extremity kinetics have not been well studied. The purpose of this study was to investigate the relationships between ground reaction forces and throwing mechanics in youth windmill pitchers and to provide a scientific basis for the improvement of preventive and rehabilitative protocols. Fifty-three youth softball pitchers were tested in an indoor facility. High-speed video and force plate data were collected for fastballs from each pitcher. Average ball speed was 25 m/sec. Peak vertical ground reaction force averaged 139 % body weight (BW), peak anterior force averaged 24 %BW, and the medially directed component of the ground reaction force averaged 42 %BW. Loading rates to peak force in all 3 directions were high. Preventive and rehabilitative protocols for windmill softball pitchers can begin to be improved on the basis of knowledge of the magnitudes and times to peak forces under the stride foot.