Build explosive leg drive and speed right of the blocks with the SpeedSac. This variable weight sled allows you to strengthen your lower body muscles by resistance training similar to metal sleds. With removable weights, the SpeedSac allows you to adjust the towing resistance. You can also use the handles on the bags to perform various upper and lower body exercises such as lunges or front raises. Note: Only use on grass, turf or wood surface. Use on any other surface may damage and/or shorten the lifespan of the product.