Statistical analysis
This was a pilot study to test feasibility of the HIIT exercise mode and inform the design and power of a larger, next phase, randomised controlled trial. Intention-to-treat analyses are presented, that is, all participants who were randomised were considered in the analysis, not only those who completed the programme together with estimates’ uncertainties as 95% CIs. This is not a hypothesis testing setting, hence the numbers followed instructions from the literature.Exploratory analyses used random effects (mixed) models to estimate average values in outcomes across the follow-up time for the four continuous longitudinal outcomes of interest, that is, 6MWD, SNIP, SGRQ-I and Quads strength which were relatively normally distributed on each occasion. Average estimates of these measurements were derived following fitting a series of a non-linear, quadratic mixed models over time allowing for a three-way interaction between time, intervention (HIIT vs MICT) and clinical group (ILD type) as well as between age, intervention and clinical group. For each of the four key exploratory outcomes of interest, a series of nested models of increasing complexity were developed and tested for their statistical improvement against the next less complex model. The complete data analysis (using missing data jargon) using mixed modelling on the four continuous longitudinal outcomes accounted for all participants and their complete observations (intention to treat analyses) and operated under ‘missing at random’ assumption for the attrition at the follow-up assumption which is not testable from the data in hand.
The average variations of outcomes through time, within and between groups, are presented as means (95% CI) and descriptive statistics as mean (SD). All analyses were conducted on STATA software (V.11).