Inspiratory muscle training is done against an external resistance when you breathe. There are several products (PowerLung, Ultrabreathe etc.) on the market claiming they can significantly improve your endurance. I have done some studying in Cochrane Library and Pubmed to figure out what we know about inspiratory muscle training for trained cyclists.
Inspiratory resistance training improves maximum inspiratory pressure
Nearly all studies find that it is possible to improve the maximum inspiratory pressure, but it remains uncertain whether this improvement actually affects cycling performance. Most studies have in common that they have used very small study groups with less than 10 participants in each group in the randomized placebo controlled studies. I am pretty sure that inspiratory muscle training does affect maximum inspiratory, but these respiratory muscles are also trained during hard aerobic exercise.
Respiratory muscles get exhausted during hard exercise
The first time I was introduced to exhausted inspiratory muscles was after a long race with a very high intensity. When I took a deep breath afterwards, I could feel pain in my inspiratory muscles. My conclusion was that vo2 intervals and racing probably gives your respiratory muscles a great stimulus.
Inspiratory muscle training – Yes or no?
As I have previously discussed, it remains uncertain whether strength training improves aerobic endurance. It seems like we have a similar problem with respiratory resistance training: Strength training for inspiratory muscles and skeletal muscles both increases maximum strength, but it remains unsure if there is a benefit for you in a competition.In a Cochrane review it is concluded that “Currently there is insufficient evidence to suggest that inspiratory muscle training with external resistive breathing devices provides any demonstrable clinical benefit in patients with asthma.”
So until we get more scientific evidence, I can not advice you to use respiratory resistance training regularly because there is not (enough) evidence for its benefits. On the other hand there is no doubt that instruments like PowerLung or Ultrabreath can train your respiratory muscles in a way you can’t train them during endurance training. I can’t think of any unwanted side effects to respiratory training, so if you think it sounds interesting give it a try. They are non-invasive and inexpensive, so it might be a cheap improvement of your performance.