The SKLZ Speed Sac is an Indoor/Outdoor Variable Weight Sprint Trainer. Build explosive leg drive out 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.