The O2 Trainer is made of soft, biodegradable silicone. Rutten recommends using it during exercise 4 times a week. The kit also includes 4 end caps for training lungs at rest--i.e., when you're sitting, standing, or lying down post-jackrabbiting. Beginners should affix cap Number 1 to the bottom right side of the device; at 14mm, it has the largest of the set's holes for allowing oxygen into the body. (The Trainer's left side has a static opening to conduct exhalations.) Trainer packages come with a palm-sized mouthpiece fitted with one of 10 numbered caps that alter the restriction of airflow. O2 Trainer compactness and portability allow for wearing one during most fitness pursuits, including weightlifting, interval training, running, biking, rock climbing, and screwing like a jackrabbit. The result, increased lung volume, enhances users' overall performance when game time or other athletic endeavors, such as impromptu foot races with Australians met in southwest Floridian bars, roll around. When worn during exercise, the O2 Trainer improves inspiratory (inhalation) muscle endurance by conditioning wearers' lungs and, in turn, the rest of their bodies to execute high-level activities with less oxygen. MMA master and onetime UFC Heavyweight Champion Bas Rutten developed his O2 Trainer to build strength in a part of the body many weightlifters and die hard resistance junkies neglect: the lungs. Who wants to wheeze like a fat asthmatic chasing down the ice cream man? I do! I do! Temporarily anyway.