Sample size. Before undertaking a long term study of early intervention with one of these three devices, the investigators would like to be at least 80% sure that they have a device that is able to produce a significant improvement in lung mechanics (20% improvement in post treatment mechanics). There are over a hundred types of NMD in children and the investigators will be testing the treatment effects of three devices. Unfortunately, there is currently no reliable evidence for the magnitude (or even existence) of any treatment effect and also the standard deviation of any effect. There is also no evidence of treatment variation when used in different sub-types of NMD. There is no basic data upon which to base a calculation of sample size. The investigators would have to make so many guesses that the calculated number would have no reliability. Our only response can be to try to enroll as many children as possible. There are over 200 children in the NMD clinic. The investigators hope to be able to enroll at least 100 of them.