PROJECT SUMMARY/ABSTRACT In children, asthma is a leading cause of respiratory symptoms and missed school. By mechanisms that are unclear, obesity in children with asthma is a strong risk factor for asthma that is more symptomatic and difficult to manage. Among asthmatic children, obesity is associated with a reduced response to conventional anti-inflammatory asthma drugs. Therefore, a better understanding of the underlying mechanisms of obesity-related asthma in children is needed. Discovering the mechanisms of `obese asthma' in order to foster more appropriate phenotype-specific therapies is a pressing public health need. Obesity, regardless of underlying asthma, is known to impair respiratory system compliance and inspiratory muscle efficiency leading to increases in the effort of respiration. Our lab found in asthmatic children that obesity is associated with more frequent asthma symptoms and a higher likelihood of having dyspnea. Inspiratory training and other forms of rehabilitation are common in adults but is understudied in pediatrics. Inspiratory `rehabilitation' training (IT) recently has been shown in both children and adults to reduce dyspnea and asthma symptoms. Our research team found IT over 6 weeks was well-tolerated, and led to improved inspiratory function and consistently improved trends in clinical measures, among children with asthma. The central objective of our proposed MICA (Mechanistic Study of Inspiratory Training in Childhood Asthma) study is to measure the effects of 8-weeks of IT on inspiratory function (strength and endurance) and small airway dysfunction measured by oscillometry in 6-17 year olds with asthma - with and without obesity. Our central hypothesis is that obesity promotes inspiratory muscle fatigue and small airway dysfunction, and that IT will mitigate these mechanisms. In a masked parallel arm trial, 75 youths with asthma (half with obesity) will be randomized to either IT (low or high dose) or SHAM control for 8-weeks. Inspiratory strength and endurance, small airway dysfunction (SAD), and exercise capacity will be assessed at baseline and after IT. Aim 1 will describe the contributions of inspiratory fatigue and SAD to obesity-related asthma. Aim 2 will determine the effects of IT on inspiratory muscle fatigue. Aim 3 will determine the effects of IT on small airway dysfunction. The MICA study will provide mechanistic knowledge about pediatric `obese asthma' by comparing treatment responses by obesity status. Measuring the changes in inspiratory muscle function and small airways dysfunction following a short regimen IT in children with symptomatic obesity-related asthma will provide much needed understanding about mechanisms underlying how obesity worsens asthma in children; and will uncover mechanisms and optimal dosing intensity involved in a safe non- drug intervention that will pave the way for larger phase III trials.