To The (RUCK) Club® X Under Armour Built Under Load. RUCK: Training That Involves Walking or Running with a Weighted Backpack, Used in the Military, Crossfit, and Hybrid Training to Develop CardiovascularMuscular endurance. ruck: Training to walk or run with a weighted backpack. It is used in military, crossfit, and hybrid training and develops cardiopulmonary endurance and muscular endurance at the same time. The Challenge of the 45-Strong 1000cal Club Squad. This sunday, Built under load. The challenge of 45 people 1000cal Clubs. This Sunday, it is stronger under the weight. #THOUSANDCALCLUB#1000calClub#1000calories