HajGhanbari et al. 2013, respiratory muscle training in athletes (systematic review)

Journal of Strength and Conditioning Research systematic review with meta-analyses of 21 studies of respiratory muscle training (RMT) in athletes. RMT had a significant positive effect on some sport-performance outcomes, including time trials, endurance time and Yo-Yo repetitions, and reliably increased inspiratory muscle strength and endurance. The size of the performance effect depended heavily on the type and progression of the training, so it is not a guaranteed, uniform benefit. The review confirms RMT reliably makes the breathing muscles stronger; whether that strength converts into meaningful race performance is the uncertain step. The trials were small and mostly unblinded, so this is best read as a signal rather than proof.