Respiratory muscle weakness either occurs in isolation or as part of a more generalised neuromuscular process. In the former case this is very often self limiting as, for example, after iatrogenic damage to the phrenic nerve or in the case of neuralgic amyotrophy. In the latter case patients will usually have evidence of limb muscle involvement and or a known diagnosis (notable exceptions however being Pompe’s disease and about 3% of amyotrophic lateral sclerosis, ALS, presentations). From this conjecture follow the two main reasons why clinicians request evaluation of inspiratory muscle strength; either to rule-in or rule-out inspiratory muscle weakness as a cause of symptoms or to predict prognosis (or the need for non-invasive ventilation). MIP has also frequently been used as an outcome measure for trials of inspiratory muscle training.