The TANK® M4 continues the evolution of this unique push sled on wheels. The core technology that makes all TANK models unique is that unlike resistance sleds, they can be run indoors or outdoors on any surface without gouging. Also on all TANK™ models the resistance increases the faster you push it, so each user can choose their resistance on the fly, without stopping to load or unload plates. New Features on the M4: Resistance The wheel resistance or “braking” is on all 4 wheels, so if the push end rises up at high speed, the other end still applies the resistance, This  eliminates the need on earlier models to load plates to prevent rising up. Three resistance levels can be set at each end independent of each other enabling one resistance on the push and a different resistance on the pull All wheel resistance also makes TANK™ M4 bi-directional, so there is no need to turn the unit around to reverse direction. An optional weight horn is available for athletes who want to perform more intense towing exercises Features of All Tank Models  SafetyResistance that feels dangerous can be backed down instantly during the run to a level that feels safer depending on the user’s age, conditioning level, and fitness goals. The ExperienceFeeling the resistance increase with speed causes a surprise “wow” reaction and a novel exercise experience that users love H.I.I.TAll TANK™ models enable High Intensity Interval Training (H.I.I.T) in which bursts of acceleration and speed for 14 minutes of a TANK™ workout is the equivalent of an hour on a treadmill. Value PackagesValue packages are available for group training consisting of a TANK™ plus a variety of tools and accessories for imaginative exercise variety (See  TANK™ GT - Team Trainer) PortabilityA Group Anchor Station is available that houses all the weights and training tools selected for a 10 person group TANK™ workout, with portability to any location inside or outdoors. Warranty10 year warranty on frame and welds and 1 year on parts and finish on workmanship and materials U.S. patent pending