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 improving the maximum inspiratory pressure is possible, but it remains uncertain whether this improvement affects cycling performance. Most studies have in common that they have used tiny 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 strenuous aerobic exercise.
Respiratory muscles get exhausted during strenuous exercise
I was first introduced to exhausted inspiratory muscles after a long race with 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. We have a similar problem with respiratory resistance training: Strength training for inspiratory muscles and skeletal muscles increases strength. Still, 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 advise 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 to your performance.