This straight line training device creates resistance helping improve acceleration, stamina and leg drive. Simply add weight to the sledge and experience top speed resisted sprinting for unmatched power gains, explosive starts, speed increases resistance by adding additional weights can also be used on a variety of ground surfaces including grass, astroturf, and indoor floors (with a towel underneath). Resistance Sled. Top speed resisted sprinting. Unmatched power gains. Explosive starts. Maximizes stride efficiency. Explosive lower-body strength. Additional weights. Can be used on a variety of surfaces. Weights not included