Fast shipping, tremendous difference after one week. Does MMA 3 hours straight - was winded before, now barely breathing hard. Another great Bas Rutten product.

Multiple resistance levels work for even out-of-shape people. Started one week ago and already noticed huge difference in lung capacity while running.

Fast delivery, received updated version. Using it to improve stamina for parkour - very cool product, will update with impact results.

Using for few months - extremely helpful for cardiovascular endurance and lung capacity. Incorporating into bicycling and running maximizes cardio conditioning.

Out of all breathing resistance devices, this is the best. Can actually wear while training without tube sticking out. Suggests adding exhale resistance for faster capacity gains.

Uses with Krav Maga and Bas Rutten MMA Workout - challenging combination! Noticed remarkable stamina gains in short time with O2Trainer.

3 weeks of use with impressive results. Training is brutal but stamina and endurance increased significantly. Performance with O2Trainer improved drastically.

Chose over high altitude trainer - very impressed with quality. High-grade polymer with biodegradable rubber housing. Only trainer that works diaphragm and increases lung capacity.

Lost only 1 lb in 4-5 weeks before O2Trainer. Lost 3 lbs in first week with it. More calories burned, higher intensity, more sweat, longer heart rate recovery.

Best respiratory trainer of many tried. Simple, brilliant design. Heard about from Bas on Joe Rogan podcast. Intense workout with this device.

Had lung infections and bad asthma attacks. Uses O2Trainer daily - stamina improves, workouts last longer, lungs stronger. Haven't used inhaler since April. Unlimited stamina now.

Used during two workouts, trained without device on day 3. Breathing seemed more relaxed and efficient. Surprised by positive results after only two days.

Well-made device that works. Competes in short distance running and climbing - air processing paramount when racing. Strengthens lungs and diaphragm as advertised.

Gifted set for husband and wife. Results really started showing after adding O2Trainer. Lung stamina made world of difference - now dancing, laughing and singing simultaneously.

Professional musician noticed difference after first few uses. Like training uphill with weights, then racing downhill. Lung capacity taken to new level - more air through horn, longer notes.

15-year-old with exercise-induced asthma since age 6. After one month, no inhaler needed during 3-hour training sessions of interval training, cardio and bag work. Got life back.

Asthma ruled life since childhood, prevented organized sports. Replaced inhaler with O2Trainer - strongest cardio ever. Two months later: level 13, no inhaler, 15-minute rolls without attacks.

Several months use confirmed it works - runs farther and faster. Reached highest level and needs something stronger. Runs over 10 miles with O2Trainer easily now.