The 6MWT is highly problematic. If data were backed by actometers then we might be able to validate it. However the results were terrible for all groups. How do they rule out that one group might have been persuaded to push a little harder? How do we know this group, having decided to push harder, didn't rest up more? 6MWT is not very reliable. CPET would be better, and 2 day CPET better again. Indeed my understanding, with insufficient data being published, is that the step test they did didn't accord with improvement in fitness, which implies that improvement in distance had nothing to do with fitness. If the data shows otherwise then why was it not published?