Training Mask Turn Flow's industry leading 21 level (36 combination) multi-level resistance loads the respiratory muscles by adding friction to inhaled air through its patented flux valve system. Increasing your inhalation load while training is like lifting weights for your breathing muscles. More respiratory power and stronger breathing muscles will supercharge your stamina and endurance. Training Mask Turn Flow's multi-patented flux valve technology offers 21 levels of resistance which allows you to work on breathing power, inspiratory muscle strength, and breathing technique. The lightweight and comfortable design of the Training Mask makes it a perfect addition to most forms of exercise from strength workouts to cardiovascular training.