If you’re pretty serious about your fitness, you’ll know that resistance based training is essential in getting bigger muscles, which is often synonymous with better performance. But the lungs are almost always overlooked in a resistance training regimen, which is where the Bas Rutten O2 Trainer comes in. It’s a silicon/polymer device that fits in your mouth like a snorkel mouthpiece and features a variable diameter intake, and an exhaust on the side. The intake is covered by interchangeable adaptors of varying sizes, which get progressively smaller as you increase the resistance exerted to take a breath in. Unlike other similar products, only the intake of air is restricted, allowing you to completely empty your lungs with each breath and translating into a fuller range of motion for your diaphragm. The resistance your lungs encounter will make them stronger over time, giving you improved cardiovascular performance to match your increased muscle mass. The kit, which comes with adapters between 14mm and 1mm, and a carrying case, costs $49.